EE-6351 ELECTRICAL DRIVES AND CONTROL

1. 1
A COURSE MATERIAL ON
EE-6351 ELECTRICAL DRIVES AND CONTROL
By
Mr.G.JAYABASKARAN
ASSISTANT PROFESSOR
DEPARTMENT OF ELECTRICAL AND ELECTRONICS ENGINEERING
KIT - KALAIGNARKARUNANIDHI INSTITUTE OF TECHNOLOGY
Kannampalayam Post, Coimbatore - 641 402

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EE6351 ELECTRICAL DRIVES AND CONTROL
Unit-I Introduction
Basic elements-types of electric drives-factors influencing electric drives-heating and cooling
curves-loading conditions and classes of duty-Selection of power rating for drive motors with
regard to thermal overloading and load variation factors
Unit-II Drive motor characteristics
Mechanical characteristics- speed- torque characteristics of various types of load and drive
motors - braking of electrical motors-dc motors: shunt, series, compound motors-single phase
and three phase induction motors
Unit-III Starting methods
Types of d.c motor starters-typical control circuits for shunt and series motors-three phase
squirrel and slip ring induction motors
Unit-IV Conventional and solid state speed control of D.C Drives
Speed control of DC series and shunt motors-Armature and field control, ward-leonard control
system-using controlled rectifiers and DC choppers –applications
Unit-V Conventional and solid state speed control of AC drives
Speed control of three phase induction motor-Voltage control, voltage/frequency control, slip
power recovery scheme-using inverters and AC voltage regulators-applications
TEXT BOOKS
1. VEDAM SUBRAMANIAM ―Electric drives (concepts and applications)‖, Tata McGraw-
Hill.2001
2. NAGARATH.I.J & KOTHARI .D.P,‖Electrical machines‖, Tata McGraw-Hill.1998
REFERENCES
1. PILLAI.S.K ―A first course on Electric drives‖, Wiley Eastern Limited, 1998
2. M.D. SINGH, K.B.KHANCHANDANI,‖Power electronics,‖ Tata McGraw-Hill.1998
H.Partab,‖Art and science and utilization of electrical energy,‖Dhanpat Rai and sons, 1994.

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UNIT I
INTRODUCTION
Definition for an electric drive:
An electric drive can be defined as an electromechanical device for converting electrical
energy into mechanical energy to impart motion to different machines and mechanisms for
various kinds of process control.
Functions performed by Electric Drives:
1. Driving fans, ventilators, compressor and pumps, etc.,
2. Lifting goods by hoists and cranes
3. Imparting motion to conveyors in factories mines and warehouses
4. Running excavators and escalators, electric locomotives, trains, cars, trolley buses, lifts
and drum winders, etc.,
BASIC ELEMENTS OF ELECTRICAL DRIVES
Motion control is required in large number of industrial and domestic applications like
transportation systems, rolling mills, paper machines, textile mills, machine tools, fans, pumps,
robots, washing machines etc. Systems employed for motion control are called drives and may
employ any of the prime movers such as, diesel or petrol engines, gas or steam turbines, steam
engines, hydraulic motors and electric motors, for supplying mechanical energy for motion
control. Drives employing motors are known as electrical drives.
Source
Electrical sources or power supplies provide the energy to the electrical motors. For high
efficiency operation, the power obtained from the electrical sources need to be regulated using
power electronic converters. Power sources can be AC or DC in nature and them normally
uncontrollable, i.e their magnitudes or frequencies are fixed or depend on the sources of energy
such as solar or wind. AC source can be either three phase or single phase. Three phase sources
are normally for high power applications.
Power processor or power modulator
Since the electrical sources are normally uncontrollable, it is therefore necessary to be
able to control the flow of power to the motor-this is achieved using power processor or power
modulator. With controllable sources( i.e. output of power processor), the motor can be
reversed, brake or can be operated with variable speed. Classical method used, for example,

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variable impedance or relays, to shape the voltage or current that is supplied to the motor-these
methods however are inflexible and inefficient.
Increasing number of power processors use power electronic converters which has
advantages over classical methods such as:
More efficient-since ideally no losses occur in power electronic converters
Flexible-voltage and current can be shaped by simply controlling switching functions of
the power converter
Fig 1.1 block diagram of an electric drive
Power electronic converters
Converters are used to convert and possibly regulate (i.e using closed-loop control) the available
sources to suit the load i.e motors.
DC to AC,
AC to DC
DC to DC
AC to AC
These converters are efficient because the switches operate in either cut-off or saturation modes.
Motors
Motors obtain power from electrical sources. They convert energy from electrical to
mechanical therefore can be regarded as energy converters. There are several types of motors
used in electric drives-choice of type used depending on applications and electrical sources
available. Broadly, they can be classified as either DC or AC motors:

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DC motors (wound or permanent magnet)-DC voltage
AC motors
Induction motors-squirrel cage, wound rotor-AC voltage
Synchronous motors-wound field, permanent magnet-AC voltage
Brushless DC motor-AC voltage
Stepper motor-require power electronic converters
Synchronous reluctance motors or switched reluctance motor-require power
electronic converters.
Control unit
Complexity depends on drive performance
- Analog-noise, nonflexible, infinite bandwidth
- Digital-immune to noise, configurable, bandwidth depends on sampling frequency
- DSP/microprocessor-flexible, lower bandwidth compared to above. DSPs perform
faster operation than microprocessors (multiplication in single cycle). With
DSP/microprocessor complex estimations and observers can be easily implemented.
TYPES OF ELECTRICAL DRIVES:
Electrical drives are normally classified into three groups, based on their development,
namely group, individual and multimotor electric drives.
Group drive
If several groups of mechanisms or machines are organized on one shaft and driven or
actuated by one motor, the system is called group drive or shaft drive. The various mechanisms
connected may have different speeds. Hence the shaft is equipped with multistepped pulleys and
belts for connection to individual loads. In this type of drive a single machine whose rating is
smaller than the sum total of all connected loads may be used, because all the loads may not
appear at the same time. This makes the drive economical, even though the cost of the shaft with
stepped pulleys may seem to be high.
This method is rarely used in modern drive systems and has become of historical interest,
because of the following disadvantages:
1. The efficiency of the drive is low, because of the losses occurring in several transmitting
mechanisms.
2. The complete drive system requires shutdown if the motor requires servicing or repair.

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3. The location of the mechanical equipment being driven depends on the shaft and there is
little flexibility in its arrangement.
4. The system is not very safe to operate.
5. The noise level at the work spot is high.
Individual drive
If a single motor is used to drive or actuate a given mechanism and it does all the jobs
connected with this load, this drive is called individual drive. For example, all the operations
connected with operating have to be performed at different speeds, transmission devices may be
required. The efficiency may become poor over several applications, due to power loss. In some
cases it is possible to have the drive motor and driven load in one unit.
Multimotor drive
In a multimotor drive each operation of the mechanism is taken care of by a separate
drive motor. The system contains several individual drives, each of which is used to operate its
own mechanism. This type of drive finds application in complicated machine tools, travelling
cranes, rolling mills, etc. Automatic control methods can be employed and each operation can be
executed under optimum conditions.
Fig 1.2 types of electrical drives
FACTORS INFLUENCING THE CHOICE OF ELECTRICAL DRIVES

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Choice of an electrical drive depends on a number of factors. Some of the important factors
are:
1. Steady state operation requirements: Nature of speed torque characteristics, speed
regulation, speed range, efficiency, duty cycle, quadrants of operation, speed fluctuation
if any, ratings.
2. Transient operation requirements: Values of acceleration and deceleration, starting,
braking and reversing performance.
3. Requirements related to the source: Type of source, and its capacity, magnitude of
voltage, voltage fluctuations, power factor, harmonics and their effect on other loads,
ability to accept regenerated power.
4. Capital and running cost, maintenance needs, life.
5. Space and weight restrictions if any.
6. Environment and location
7. Reliability.
HEATING AND COOLING CURVES
In a D.C machines the various energy losses which occur are finally converted into heat,
this causes an increase in temperature which depends upon:
(i) The heat-absorbing capacity of the various parts of the machines, and
(ii) The facility which heat is conducted away or radiated or otherwise dissipated from
the surface of the machine.
In the beginning the temperature rise will be steep but after some time steady conditions
would prevail. At that time there would be no further increase in temperature because the rate of
heat production would become equal to rate of heat dissipation.
Let Q = Power loss or heat developed, J/s or W
m = mass of active parts of machine, ke
s = Specific heat, J/kf-C
A = area of the cooling surface, m2
 = specific heat dissipation or emissivity, W/m2
-C difference
the surface and the ambient medium
 = 1/ = cooling co-efficient
 = temperature rise at any time t, C
m = final steady temperature rise while heating, C

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n = final steady temperature while cooling, C
i = initial temperature rise over ambient medium, C
Th = heating time constant, s
Tc = cooling time constant, s
t = time, s
Heating of the machine
Let us consider the conditions at any time t from start.
Heat produced in the body during an infinitely small time dt = heat produced per
second x dt
= Qdt----------(1)
If during this period dt the time temperature of the body rises by d, then
Heat stored in the body = mass of the body x specific heat x temperature difference
= m s d-----------------(2)
If in the process of heating, the temperature of the surface rises by  over the ambient medium,
instant considered, the heat dissipated by the body into ambient medium due to radiation and
conduction,
= surface in m2 x watt dissipated per m2 per C x temperature rise x time
= Adt--------------------------------------------(3)
Since, heat produced in machine = heat stored in parts + heat dissipated
( )
( )
Solving the differential equation (5), we get
( ) ( )
Where C is the constant of integration,

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The value of C is found by using the boundary conditions at t=0, =i
Putting this in equation (6), we get
( )
Or ( )
Substituting the value of C in equation (6), we get
( ) ( )
( ) ( )
When t =  the machine reaches a final steady temperature rise m. Under this condition there is
no further temperature rise and the rates of heat production and dissipation are equal. This
means d=0 or msd=0
When the machine attains final steady temperature rise.
When  = m in equation (4)
( )
Putting in equation (7), we get

( ) ( )
The term

is called the heating time constant Th

( )
Putting this value of

in equation (10), we get

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( ) ( )
( )
( )
( ) ( )
i.e.,
If the machine starts from cold conditions, i=0., no temperature rise over the ambient
medium.
( ) ( )
The above relation is the equation of temperature rise with time. The temperature rise—
time curve is exponential in nature as shown in figure.1.3
Figure 1.3 Heating and cooling curves
Heating time constant, considering the equation (12), we get
( )
Putting t=Th , in the above expression we get,

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( )
[Because e-1
= 0.368]
Thus the heating time constant can be defined as follows:
The heating time constant: ― It is the time taken by the machines to attain 0.632 of its final steady
temperature rise‖.
The heating time constant of conventional electrical drives is usually within the range of
½ to 3 to 4 hours.
Cooling of the machine
Considering the equation (4),
After solving the above equation, we get
( )
The value of C is obtained by using the boundary conditions, when t=0, =I , we get
( )
( )
Substituting the value of C and proceeding as in the case of heating of the machine, we get
( ) ( )
( )

( )
Where Tc is called the cooling time constant.
If the machine is shut down, no heat is produced and so its final steady temperature rise
when cooling is zero or . Under these conditions equation (13) reduced to
( )

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From above expression it is obvious that the cooling curve is also exponential in nature
(Fig 1.3)
Equation (13) is applied to machines allowed to cool owing to partial removal of load.
Equation (16) is applicable to machines which are shut down.
Cooling time constant
Considering the equation (16), we get
Putting t = Tc, we have
Thus we can define the cooling time constant as follows:
“The cooling time constant is the time taken by the machine for its temperature rise to
fall to 0.368 of its initial value”
The cooling time constant is usually larger due to poorer ventilation conditions when the
machines cool.
LOADING CONDITIONS AND CLASSES OF DUTY
An electric motor, under steady state operation, develops electromagnetic torque
of such a magnitude which can counterbalance the actual load torque TLof the connected
equipment and an opposing torque Tmech corresponding to the losses that take place in gear and
transmission mechanisms. Under transient conditions, the motor torque has to overcome the
inertia torque Tdyn also. Hence, in general, the torque developed by the motor should be
expressed as:
( )
The combines load torque ( ) is determined from the torque time plot of the connected
load. A typical example is shown in fig 1.4 (a) In order to determine the variation of inertia
torque with respect to time, the speed-time curve, and example of which is shown in fig 1.4 (b)
and the moment of inertia of the rotating masses J must be known. Now, with the help of
equation 15 it will be possible to obtain the torque-time curve of the driving motor fig 1.4 (c),
which is called the duty cycle of the motor.

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Fig 1.4 (a) torque time curve of connected load
(b) speed-time curve
(c) torque-time curve of the driving motor
Since, initially J remains unknown, the torque-time graph can be plotted by taking into
account only TL and Tmech. After determining the power rating of the machine using the above
time-torque graph it can be corrected for the presence of inertia torque by increasing the rating
obtained by 15-20 percent. Knowing J of the chosen motor, the exact torque-time curve can be
plotted and a more correct estimation of the rating of the driving motor can be made.
CLASSES OF MOTOR DUTY
IS:4722-1968 categories various load time variations encountered in practice into eight standard
classes of duty:
1. Continuous duty
2. Short time duty
3. Intermittent periodic duty
4. Intermittent periodic duty with starting
5. Intermittent periodic duty with starting and braking
6. Continuous duty with intermittent periodic loading
7. Continuous duty with starting and braking
8. Continuous duty with periodic speed changes

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Fig 1.5 some classes of duty
Continuous duty
It denotes the motor operation at a constant load torque for a duration long enough for the
motor temperature to reach steady-state value. This duty is characterized by a constant motor
loss. Paper mill drives, compressors, conveyers, centrifugal pumps and fans are some examples
of continuous duty. Fig(a)
Short time duty
In this, time of drive operation is considerably less than the heating time constant and
machine is allowed to cool off to ambient temperature before the motor is required to operate
again. In this operation, the machine can be overloaded until temperature at the end of loading
time reaches the permissible limit. Some examples are: crane drives, drives for household
appliances, tuning bridges, sluice-gate drives, valve drives and many machine tool drives for
position control.Fig(b)
Intermittent periodic duty
It consists of periodic duty cycles, each consisting of a period of running at a constant
load and a rest period. Neither the duration of running period is sufficient to raise the
temperature to a steady-state value, nor the rest period is long enough for the machine to cool off
to ambient temperature. In this duty, heating of machine during starting and braking operations is
negligible. Some examples are pressing, cutting and drilling machine drives. Fig (c)
Intermittent periodic duty starting

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This is the intermittent periodic duty where heat losses during starting cannot be ignored.
Thus, it consists of period of starting, a short period of operation at a constant load and a rest
period; with operating and rest periods being too short for the respective steady state temperature
to be attained. In this duty, heating of machine during braking is considered to be negligible,
because mechanical brakes are used for stopping or motor is allowed to stop due to its own
friction. Few examples are metal cutting and drilling tool drives, drives for fork lift trucks, mine
hoist etc.Fig(d)
Intermittent periodic duty with starting and braking
This is the intermittent periodic duty where heat losses during starting and braking cannot
be ignored. Thus, it consists of a period of starting, a period of operation with a constant load, a
braking period with electrical braking and a rest period; with operating and rest periods being too
short for the respective steady state temperatures to be attained. Billet mill drive, manipulator
drive, ingot buggy drive, schrewdown mechanism of blooming mill, several machine tool drives,
drives for electric suburban train and mine hoist are examples of this duty. Fig (e)
Continuous duty with intermittent periodic loading
It consists of periodic duty cycles, each consisting of a period of running at a constant
load and a period of running at no load, with normal voltage across the excitation winding.
Again the load period and no load period being too short for the respective temperatures to be
attained. This duty is distinguished from the intermittent periodic duty by the fact that a period
of running at a constant load is followed by a period of running at no load instead of rest.
Pressing, cutting, shearing and drilling machine drives are the examples.
Continuous duty with starting and braking
Consists of periodic duty cycle, each having a period of starting, a period of running at a
constant load and a period of electrical braking; there is no period of rest. The main drive of a
blooming mill is an example
Continuous duty with periodic speed changes
Consists of periodic duty cycle, each having a period of running at one load and speed,
and another period of running at different speed and load; again both operating periods are too
short for respective steady-state temperatures to be attained. Further there is no period of rest.
SELECTION OF POWER RATING FOR DRIVE MOTORS WITH REGARD TO
THERMAL OVERLOADING AND LOAD VARIATION FACTORS
The power rating of a motor for a specific application must be carefully chosen to
achieve economy with reliability. Use of a motor having insufficient rating, either fails to drive

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the load at its normal productive level or lowers the productivity and reliability through frequent
damages and shut-downs due to overloading of the motor and power modulator.
When the motor operates, heat is produced due to losses (copper, iron and friction) inside
the machine and its temperature rises. As the temperature increases beyond ambient value, a
portion of heat produced flows out to the surrounding medium. The amount of outflow of heat is
a function of temperature rise of motor above the ambient value. As motor temperature rises, the
heat outflow increases and equilibrium ultimately sets in when the heat generated becomes equal
to heat dissipated into the surrounding medium. Motor temperature then reaches a steady state
value. Steady state temperature depends on power loss, which in turn depends on the output
power of the machine. Since temperature rise has a direct relationship with the output power, it
is termed as thermal overloading on the machine.
When operating for a specific application, motor rating should be carefully chosen to
ensure that the insulation temperature never exceeds the prescribed limit, otherwise either it will
lead to its immediate thermal breakdown causing short circuit and damage to winding, or it will
lead to deterioration of its quality results into thermal breakdown in near future.
Table 1.1 insulation temperature limits
Y
A
E
B
F
H
C
90
105
120
130
155
180
Above 180
Continuous duty with constant load:
 For most of the applications, the rating can be determined from the equation given as
under:
( )
Where, T = Load torque, kg-m,
N = speed r.p.m., and
 = product of the efficiency of the driven equipment and that of transmitting device.
 In case of linear motion, the rating of the motor required is given by,
( )

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Where, F = Force caused by the load, kg.m and
v = velocity of motion of the load, m/s
Equation (2) is directly applicable, in case of hoisting mechanisms. It is also suitable for
the lifts or elevators; but since a counterweight which balances the weight of the cage or car as
well as one-half of the useful load will be always present, it should be modified as follows:
( )
The velocity of normal passenger lift cabins vary from 0.5 to 1.5 m/s.
 In case of pumps, the rating can be determined form the following relation:
( )
Where,  = Density of liquid pumped, kg/m3
,
Q = Delivery of pump, m3
/s, and
H = Gross head (static head +friction head), m
varies from 0.8 to 0.9 for reciprocating pumps and from 0.4 to 0.8 for centrifugal
pumps.
 The rating of a fan motor is given by,
( )
Where , Q = Volume of air or any other gas, m3
/s and
h = pressure in mm of motor or kg/m2
For small power fans, the efficiency  may be taken as 0.6 and for large power ones it
may reach a value upto 0.8.
 The rating of a motor used in metal shearing lathes can be found from the relation,
( )
Where, F = Shearing force, kg;
V = velocity of shearing, m/min, and
= Mechanical efficiency of the lather.
Motor Rating for Variable Load:
The following are the commonly used methods for determination of motor rating for
various load drives:
(i) Method of average losses
(ii) Equivalent current method
(iii) Equivalent torque method
(iv) Equivalent power method

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(i) Method of average losses (Qavg).
This method consists of finding average losses Qavg in the mtor when it operates according to
the given load diagram. These losses are then compared with Qnom, the losses corresponding to
the continuous duty of the machine when the operated at its nominal rating. This method
presupposes that when Qavg =Qnom, the motor will operate without temperature rise going above
the maximum permissible for the particular class of insulation.
Fig 1.6 shows a simplified load diagram for a certain drive
The loss diagram (loss Q plotted versus time) of the electric motor is shown. The rating
of the electric motor can be found from method of successive approximations. To start with a
motor is selected in accordance with the required capacity calculated by multiplying average
power load a factor of safety of 1.1 to 1.3. The losses of the motor are calculated for each portion
of the load diagram by referring to the efficiency curve of the motor. The average losses are
given by
( )

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The average losses as found from equation (7) are compared with losses of selected
motor at rated frequency. In case the two losses are equal or differ by a small amount the motor
is selected. However, in case the losses differ considerably, another motor is selected and the
calculations repeated till the motor having almost the same losses or the average losses is found.
It should be checked that the motor selected has a sufficient overload capacity and starting
torque.
 This method does not take into account the maximum temperature rise under variable
load conditions. However, this method is accurate and reliable for determining the
average temperature rise of the motor during one work cycle.
The disadvantage of the method of equal losses is that it is tedious to work with and also
many a times the efficiency curve is not readily available and the efficiency has to be calculated
by means of empirical formulae which may not be accurate to work with.
(ii) Equivalent current method
This method is based on the assumption that the actual variable current may be
replaced by an equivalent current Ieq which produced the same losses in the motor as
the actual current.
√ ( )
The heating and cooling conditions in self ventilated machines depend upon its speed. At
slow speeds the cooling conditions are poorer than at normal speeds. Therefore, if the work
cycle involves slow speed operation, it must be taken into consideration when using equation (8)
The equivalent current as found from equation (8) should be compared with the rated
current of the motor selected and the conditions should be met (Inom is the rated
current of the machine).
The machine selected should also be checked for its overload capacity.
In case the overload capacity of the motor selected is not sufficient it becomes necessary
to select a motor of high power rating.

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It may not be easy to calculate the equivalent current especially in cases where the
current load diagram is irregular as shown in 1.7. The equivalent current in such cases is
calculated from the following expression
√
∑
∫
∑
( )
An easier way is to break up the current load diagram into a series of straight line
geometrical figures.
For a triangular shaped diagram:
√ ( )
For a trapezoidal diagram:
√ ( )
The current values obtained by this method are sufficiently accurate for practical purposes.
(iii) Equivalent torque and power method
For the selection of suitable capacity of the motor it often becomes necessary to
use torque or power load diagrams. The equivalent torque or power is found in the
same manner as the equivalent current
Assuming constant flux and constant power factor, the torque is directly
proportional to current and, therefore, the equivalent torque is:
√ ( )
The equation for equivalent power follows directly from equation (12) as power is
directly proportional to the torque. At constant speed or where the changes in speed are small,
the equivalent power is given by:
√ ( )

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 The “equivalent current method” is the most accurate out of all the above methods
discussed above.
This method may be used to determine the motor capacity for all uses except where it is
necessary to take into account the changes in so called ‗constant losses‘ i.e., the iron and
mechanical losses.
 The “equivalent torque method” cannot be used for cases where equivalent current
method cannot be applied.
The “equivalent torque method” cannot be used for selection of motor rating for cases in
which the field flux does not remain constant like D.C series motors and for squirrel cage
induction motors under starting and braking condition.
 The disadvantages of the “equivalent power method” is that it cannot be used for motors
whose speed varies considerably under load, especially when dealing with starting and
running conditions.
Load Equalisation
In several industrial drives (e.g., electric hammer, presses, rolling mills, reciprocating
pumps, planning machines etc) the load fluctuates between wide limits over a span of few
seconds. It is very essential to smooth out the fluctuating load, otherwise during intervals of
peak load it will draw a heavy current from the supply either producing large voltage drop in the
system or requiring cables and wires of heavy section. The process of smoothing out the
fluctuating load is known as load equalisation.
The process of load equalization involves the storage of energy during light load period
which can be given out during the peak load period, so that the demand for the system is more or
less constant.
Load equalization is often achieved by means of a flywheel, which is mounted on the
motor-shaft, if the speed of the motor is not to be reversed. In case of reversing rolling mills, the
flywheel is mounted on the motor-generator sett feeding the driving motor.
In order that the flywheel operates effectively, the driving motor should have a drooping
speed characteristic. During intervals of heavy load, speed of the motor decreases. This enables
the flywheel to release a portion of the stored kinetic energy, which together with the energy
drawn from supply, will meet the demand of stored kinetic energy, which together with the
energy drawn from supply, will meet the demand of load.
During off peak load period, the motor draws more energy than require by the load. The
surplus energy taken is again stored as kinetic energy in the flywheel, whose speed, then
increases. The load torque required and motor torque developed as well as speed variation with
time are shown in Fig 1.7

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Fig 1.7 Variations of speed, load torque and motor torque against time
Flywheel calculations:
Let,
Tload = Load torque (assumed constant during the period), Nm
Tfw = Flywheel torque, Nm,
To = No-load torque, Nm,
Tmotor = Motor torque at any instant, Nm,
ωo = Motor speed on no-load, rad/s,
ω = (ωo-ω) = Motor slip,
J = Moment of inertia of flywheel, kg-m2, and
t = Time, s

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Case 1. Load increasing (flywheel decelerating):
During the period the flywheel decelerates and gives up a part of stored energy in it, the
torque required to be supplied by the motor,
( )
Energy given out by the flywheel when its speed is reduced from
( ) ( ) ( )
( ) ( ) ( )
Therefore energy given out =Jωs
The power given out by the flywheel = Rate of energy given up
( )
( )
Equation (1) thus becomes
As for value of slip upto 10%, the slip is proportional to the torque, i.e., s=KTmotor
( )
( )
From initial conditions, when t=0; Tmotor =To
( )

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( ) ( )
( )
( ) ( )
Case 2. Load Decreasing (Flywheel accelerating):
When load decrease, the motor accelerates the flywheel to its normal speed, slip is
therefore decreased and ds/dt is –ve
( )…… since ds/dt is –ve
By solving the above expression, we get
( ) ( )
UNIT II
DRIVE MOTOR CHARACTERISTICS

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2.1 MECHANICAL CHARACTERISTICS
The quadrantal diagram and speed-torque characteristics depicting the four quadrant
operation are described below for different types of dc motors-separately excited (or shunt).
Series and compound- and induction motors.
2.1.1 TYPES OF LOAD
Fig 2.1 speed-torque characteristics of fan load
A positive load torque is opposite in direction to the positive motor torque. The load
torque may remain constant, and be practically independent of speed or vary with some power of
speed. Besides load torque, there may be other types of loads are enumerated below:
Load for which torque varies as Power of speed: there are some loads for which the torque
increases with some power of speed. Such is the case, for example, with fans and centrifugal
pumps for which the torque varies as square of the speed.
Constant friction type of load: Friction torque opposes rotation. It is independent of speed, but
passive. It is very small during a normal operation. Sticking friction at the time of starting may
be considerable. But almost negligible, if the drive is started frequently. It is also negligible, if
the machine is fitted with some form of ball or roller bearings.
Dynamic or inertia load: When the speed of a rotating mass is changing, dynamic torque or
inertia torque acts against the motor torque (Fig 2.2). This torque is given by J , where J is the
equivalent inertia of all rotating masses.
2.2 DC MOTORS AND THEIR PERFORMANCE
The commonly used dc motors are shown in the figure 2.3. In a separately excited motor,
the field and armature voltages can be controlled independent of each other. In a shunt motor,
field and armature are connected to a common source. In case of a series motor, field current is

26. 26
same as armature current and therefore field flux of armature current. In a cumulatively
compound motor, the mmf of the series field is a function of armature current and is in the same
direction as mmf of the shunt field.
Figure 2.3 commonly used dc motors
The steady state equivalent circuit of armature of a dc machine is shown in the figure 2.4.
Resistance Ra is the resistance of the armature circuit. For separately excited and shunt motors it
is equal to the resistance of armature winding and for series and compound motors it is the sum
of armature and field winding resistances. Basic equations applicable to all dc motors are,
Figure 2.4 steady state equivalent circuit of the armature
( )
( )
( )
Where is the flux/pole, Webers; Ia the armature current A; V the armature voltage V;
Ra the resistance of the armature circuit, ohms; wm the speed of armature, rad/sec; T the torque
developed by the motor, N-m; and Ke the motor constant.
From equations 2.1 to 2.3
( )
( )
( )
2.2.1 Shunt and Separately excited motors
In case of shunt and separately excited motors, with a constant field current, the flux can
assumed to be constant, Let
( ) ( )
me wKE 

27. 27
Then from equations 2.1,2.3 and 2.4 to 2.6
( )
( )
( )
The speed torque and torque-current characteristics of a separately excited motor for
rated terminal voltage and full field are shown in the figure 2.5. The speed-torque curve is a
straight line. The no load speed ωm0 is determined by the values of armature voltage and field
excitation. Speed decreases as torque increases and speed regulation depends on the armature
circuit resistance (Eq 2.9). The usual drop in speed from no load to full load, in case of a
medium size motor is of the order of 5%. Separately excited motors are employed in application
requiring good speed regulation and adjustable speed.
Figure 2.5 Performance curves of dc motors
2.2.2 Series Motor
In series motors, the flux is a function of armature current. In unsaturated region of
magnetization characteristic, Ф can be assumed to be proportional to Ia, thus
( )
Substituting in Equations 2.3, 2.4 and 2.5 gives
( )
( )
√ √
( )
Where armature circuit resistance Ra is now the sum of armature and field winding
resistances. The speed torque and torque-current characteristics of a series motor at rated
terminal voltage and full field are shown in fig 2.5. Series motors are suitable for applications
requiring high starting torque and heavy torque overloads. Since torque is proportional to the
armature current squared, for the same increase in torque, increase in motor current is less
compared to that in a separately excited motor where torque is proportional to armature current.
Thus, during heavy torque overloads and starting, power overload on the source and
thermal overloading of the motor are kept limited to reasonable values. According to Eq.2.13, as

28. 28
speed varies inversely as the square root of torque, machine runs at a large speed at light load.
Generally, mechanical strength of a dc motor permit it to operate upto about twice rated speed.
Hence, the series motor should not be used in those drives where there is a possibility of the load
torque being dropped to the extent that the speed may exceed twice rated value.
2.2.3 Compound motor
Speed-torque and torque-current characteristics of cumulative compound motor are also
shown. The no load speed depends on the strength of the shunt field and slope of the
characteristic on the strength of series field. Cumulative compound motors are used in those
applications where a drooping characteristic similar to that of a series motor is required and at
the same time the no load speed must be limited to a safe value; typical examples are lifts and
winches. It is also used in intermittent load applications, where the load varies from almost no
load to very heavy loads. In these applications a fly-wheel may be mounted on the motor shaft
for load equalization. This apart from equalizing load on the supply, permits the use of a smaller
size motor. Pressing machine is a typical example of this type of application.
The characteristics of fig 2.5, which are obtained at rated terminal voltage and full field
are known as natural speed-torque characteristics. Rated (or full load) speed is known as the
base speed.
2.3. BRAKING OF ELECTRICAL MOTORS
Need for electrical Braking
While operating electrical drives it is often necessary to stop the motor quickly and also
reverse it. In applications like cranes or hoists the torque of the drive motor may have to be
controlled so that the load does not have any undesirable acceleration, e.g. in the case of
lowering of loads under the influence of gravity. The speed and accuracy of stopping or
reversing operations improve the productivity of the system and quality of the product.
Electric braking is of two kinds, depending upon the application of the drive:
1. To bring the driving motor completely to rest in a given amount of time and at exactly
specified points. While doing so, the K.E is fed back to the mains.
2. To restrict the speeds to safe values. This arises normally while lowering loads using a
hoist or crane. While maintain constant speed the excess of K.E and P.E of the load
tending to accelerate the motor is fed back to the mains.
Methods of electrical braking in the following three ways,
a. Regenerative braking
b. Dynamic or rheostatic braking
c. Counter current braking
Regenerative braking

29. 29
Figure 2.6 Regenerative braking characteristics of a separately excited motor
In regenerative braking, generated energy is supplied to the source. For this to happen
(fig 2.4), following condition should be satisfied:
( )
Field flux cannot be increased substantially beyond rated because of saturation.
Therefore, according to eqs 2.1 to 2.14, for a source of fixed voltage or rated value regenerative
braking is possible only for speeds higher than rated and with a variable voltage source it is also
possible below rated speeds. The speed-torque characteristics can be calculated from equations
2.1 to 2.5 and are shown in fig 2.6 for a separately excited motor.
In series motor as speed increases, armature current, and therefore, flux decreases.
Consequently, condition of equation 2.14 cannot be achieved. Thus regenerative braking is not
possible.
In actual supply system when the machine regenerates its terminal voltage rises.
Consequently the regenerated power flows into the loads connected to the supply and the source
is relieved from supplying this much amount of power. The regenerative braking is therefore
possible to the regenerated power. When the capacity of the loads is less than the regenerated
power, all the regenerated power will not be absorbed by the loads.
The remaining power will be supplied to capacitors (including stray capacitances) in line
and the line voltage will rise to dangerous values leading to insulation breakdown. Hence,
regenerative braking should only be used when there are enough loads to absorb the regenerated
power.
Alternatively an arrangement is made to divert the excess power to a resistance bank
where it is dissipated as heat. Such a braking is known as composite braking because it is a
combination of regenerative braking and dynamic braking. When the source is a battery, the
regenerated energy can be stored in the battery.
Dynamic braking
In dynamic braking, motor armature is disconnected from the source and connected
across a resistance Rb. The generated energy is dissipated in Rb and Ra. For motoring
connections of figs.2.1 (a) and (c), the braking connections are shown in Fig 2.7. Since series

30. 30
machine works as a self-excited generator, the field connection is reversed so that field assists
the residual magnetism. Figures 2.8 show speed-torque curves and transition from motoring and
braking. These characteristics are obtained from equations 2.10 to 2.13 for V=0.
When fast braking is desired, Rb consists of a few sections. As the speed falls, sections
are cut-out to maintain a high average torque, as shown in figure 2.8 for a separately excited
motor. During braking, separately excited motor can be converted as a self-excited generator.
This permits braking even when supply fails.
Figure 2.7 Dynamic braking of dc motors
Figure 2.8 Dynamic braking speed-torque curves
Plugging
For plugging, the supply voltage of a separately excited motor is reversed so that it assists
the back emf in forcing armature current in reverse direction. (fig 2.9). A resistance Rb is also
connected in series with armature to limit the current. For plugging of a series motor armature
alone is reversed.
Speed-torque curves can be calculated from equations 2.9 and 2.13 by replacing V by –V
and are shown in the fig 2.10. A particular case of plugging for motor rotation in reverse
direction arises, when a motor connected for forward motoring, is driven by an active load in the
reverse direction. Here again back emf and applied voltage act in the same direction.
However, the direction of torque remains positive (fig 2.11). This type of situation arises
in crane and hoist applications and the braking is then called counter-torque braking.

31. 31
Figure 2.9 Plugging operation of dc motors
Plugging gives fast braking due to high average torque, even with one section of braking
resistance Rb. Since torque is not zero at zero speed, when used for stopping a load, the supply
must be disconnected when close to zero speed. Centrifugal switches are employed to
disconnect the supply. Plugging is highly inefficient because in addition to the generated power,
the power supplied by the source is also wasted in resistances.
Figure 2.10 Plugging speed-torque curves
Figure 2.11 Counter-torque braking
2.4 TORQUE-SPEED CHARACTERISTICS OF INDUCTION MOTOR
Induction motors have been used in the past mainly in applications requiring a constant
speed because conventional methods of their speed control have either been expensive or highly
inefficient. Variable speed applications have been dominated by dc drives.

32. 32
Availability of thyristors, power transistors, IGBT and GTO have allowed the
development of variable speed induction motor drives.
The main drawbacks of dc motors is the presence of commutator and brushes, which
require frequent maintenance and make them unsuitable for explosive and dirty environments.
On the other hand, induction motors, particularly squirrel cage are rugged, cheaper,
lighter, smaller, more efficient, require lower maintenance and can operate in dirty and explosive
environments.
Although variable speed induction motor drives are generally expensive than dc drives,
they are used in a number of applications such as fans, blower, mill run-out tables, cranes,
conveyers, traction etc. because of the advantages of induction motors.
Other dominant applications are underground and underwater installations, and explosive
and dirty environments.
2.4.1 Torque equation, typical starting performance
Three-phase induction motors are of two types:
 Squirrel cage induction motor
 Slip ring or wound rotor induction motor
In squirrel cage the rotor consists of longitudinal conductor-bars shorted by circular
connectors at the two ends while in wound-rotor motor, the rotor also has a balanced
three-phase distributed winding having same poles as stator winding. However, in both,
stator carries a three-phase balanced distributed winding.
Analysis and performance
Per-phase equivalent circuit of a three-phase induction motor is shown in Fig.2.12 Rr`
and Xr` are the stator referred values of rotor resistance Rr and rotor reactance Xr. Slip is defined
by
( )
( )
Where ωm and ωms are rotor and synchronous speeds, respectively. Further
( )
Where f and p are supply frequency and number of poles, respectively. Since, stator impedance
drop is generally negligible compared to terminal voltage V, the equivalent circuit can be
simplified to that shown in Fig. 2.12(b)

33. 33
Figure 2.12 Per phase stator referred equivalent circuits of an induction motor
Also from Eq.(2.15)
( ) ( )
From Fig. 2.12(b) the rotor current referred to the stator is,
( ) ( )
( )
Power is transferred to rotor (or air-gap power)
( )
Rotor copper loss is
( )
Electrical power converted into mechanical power,
( ) ( )
Torque developed by motor

34. 34
( )
Substituting from Eq.(2.17) and (2.21) yields
( )
Substituting from Eq.(2.18) gives
[
( )
( ) ( )
] ( )
A comparison of equations (2.19) and (2.23) suggests that
( )
Motor output torque at the shaft is obtained by deducing friction windage and core-loss
torques from the developed torque.
The developed torque is a function of slip only (Eq.(2.24)). Differentiating T in (2.24)
with respect to s and equating to zero gives the slip for maximum torque
√ ( )
( )
Substituting from Eq. (2.26) into (2.24) yields an expression for maximum torque
[
√ ( )
] ( )
Maximum torque is also known as breakdown torque. While it is independent of rotor
resistance sm is directly proportional to rotor resistance.
The nature of speed-torque and speed-rotor current characteristics are shown in Fig.2.13.
Both rotor-current and torque are zero at synchronous speed. With decrease in speed, both
increase. While torque reduces after reaching breakdown value, the rotor-current continues to
increase, reaching a maximum value at zero speed.

35. 35
Fig 2.13 Speed torque and speed rotor current characteristics of an induction motor
Drop in speed from no load to full load depends on the rotor resistance. When rotor
resistance is low, the drop is quite small, and therefore, motor operates essentially at a constant
speed. The breakdown torque is a measure of short-time torque overload capacity of the motor.
Motor runs in the direction of the rotating field. Direction of rotating field, and therefore,
motor speed can be reversed by reversing the phase sequence. Phase sequence can be reversed
by interchanging any two terminals of the motor.
Sometime, torque is expressed in terms of sm and Tmax, which not only facilitates
calculations, but also enables a quick appreciation of nature of speed-torque characteristics.
Dividing equation 2.23 by 2.27 and then substituting from equation 2.26 yields
( ( ) )
( )
The nature of speed-torque characteristics (Fig 2.13) can now be readily explained from
Equation 2.28.
For slips much smaller than sm, second term of the denominator dominates. Therefore,
speed-torque relation from 0 to rated torque is approximately represented by a straight line. For
slips much large than sm, first term of the denominator dominates and speed-torque relation
takes a hyperbolic shape in this region.
In the whole region of motor operation, term (RsSm/Rr`) is small compared to 1 and
dominating term in the denominator. There, it can be dropped from equation 2.28
( )

36. 36
Typical Torque-Speed characteristics of a poly phase induction motor
The general shape of the torque speed or torque slip-curve with the motor connected to a
constant voltage, constant frequency source is shown in Fig 2.14
Figure 2.14 typical torque-speed characteristic of three phase induction motor.
(i) Motoring region: 0≤s≤1)
For this range of slip, the mechanical power is outputted or torque developed is in the
direction in which the rotor rotates. Also
(a) Torque is zero at s=0
(b) The torque has a maximum value, called the breakdown torque, (TBD), at slip, Sm.
The motor would decelerate to a halt if it is loaded with more than the breakdown
torque.
(c) At s=1, ie when the rotor is stationary the torque corresponds to the starting
torque, Ts. In a normally designed motor Ts is much less than TBD.
(d) The normal operating point is located well below TBD. The full load slip is
usually 2-8%
(e) The torque-slip characteristics from no-load to somewhat beyond full load is
almost linear.
(ii) Generating region: S<0
Negative slip- implies rotor running at super synchronous speed (n>ns). The load
resistance is negative, which means that mechanical power must be put in while
electrical power is put out at the machine terminals.
(iii) Braking Region : S>1

37. 37
The motor runs in opposite direction to the rotating field ( ie n is negative),
absorbing mechanical power (braking action ) which is dissipated as heat in the rotor
copper.
It may be noted that the values of sm are identical for both motoring and generating
operations. It is also clear from equation 2.20 that during generating operation, the maximum
torque is greater than that during motoring.
Induction motors with special designs
A general purpose induction motor is designed to operate at low slip at full load in order
to have good running performance. Depending on the rating, full load slip varies from 2 to 7%.
Such a motor has high starting current (5-8 times) and low starting torque ( full load torque to
twice full load torque). Certain application require motor to be designed differently. Some of
these are:
High slip induction motors
For intermittent load applications, involving frequent start and stop and/or running at low
speeds for prolonged periods, inductions motors are designed with high rotor resistance. Such
motors have low starting current and high starting torque, but low full load efficiency due to high
rotor copper loss.
Because these motors operate at a large slip (between 10 to 40% at full load) they are
called high slip motors. High slip motors are also suitable for fan drives where speed is
controlled by stator voltage control and are found among both---squirrel cage and wound rotor.
The nature of speed-torque characteristics of such motors is shown in Fig.2.15
Fig 2.15 Speed torque curves
In squirrel-cage induction motors, good starting performance (low starting current and
high starting torque) is realized without appreciably affecting full load performance by the use of
deep-bar rotor or double-cage rotor motors.
Rotor frequency changes from 50Hz to 1-3Hz as the speed changes from standstill to full
load. Variation of rotor frequency is utilized in these motors to vary rotor resistance from a large

38. 38
value at standstill to a very small value at full speed. Thus, while starting and low speed
performance is improved, full load performance is not appreciably effected.
Deep-Bar Squirrel Cage rotor induction motor
Stator of the machine is identical to a general purpose induction motor. Rotor has deep
and narrow conductor bars as shown in Fig. 2.16 Slot leakage fluxes produced by the current in
bar are also shown in figure. One can imagine that the bar is made of number of arrow layers
connected in parallel.
Fig 2.16 Deep-rotor bar
Let us compare the behavior of top and bottom layers. More leakage flux links with
bottom layer than the top layer. Consequently, bottom layer has a much higher leakage
inductance than the top layer.
As rotor frequency is high at low speeds, the reactance and impedance of bottom layer
are much higher than the top layer. Therefore, at low speeds highest amount of current is carried
by topmost layer and lowest by the bottommost.
Because of unequal distribution of current across cross section of the bar, effective
resistance of rotor is high and starting and low speed performance is improved. At near full load
speed both—frequency of rotor current and leakage reactance are low.
Therefore, current gets equally distributed across cross section of the bar and effective
rotor resistance has a low value. This , full load performance is not appreciably affected. The
nature of speed-torque curve is shown in Fig.2.15
Double squirrel cage rotor induction motor
Rotor consists of two layers of conductor bars in each slot (Fig. 2.17) short circuited by
end rings. Top bar has smaller cross section than the bottom. Therefore, it has higher resistance.
Bottom bar links to higher amount of leakage flux than the top bar and therefore has higher
inductance.

39. 39
Fig 2.17 Double cage-rotor
At low speeds, for which rotor frequency is high, bottom bar has higher impedance.
Consequently, more current flows through the top bar. As the resistance of the top bar is high
good starting performance is obtained.
At high speeds, for which rotor frequency is low, bottom bar has much smaller
impedance than the top one. Hence, rotor current is carried mainly by bottom bar and full load
performance remains good as it has a low resistance. The nature of speed-torque characteristics
is shown in Fig 2.15
Torque motor
Motors designed to run for long periods in a stalled or low speed condition are known as
torque motors. They are designed to develop desired torque with low current at low speeds.
Their speed torque characteristics are shaped to have negative slope so that they provide stable
operation with most loads at low speeds. They can be squirrel cage or wound rotor type. Both
three and single phase motors are available.
2.5 OPERATION WITH UNBALANCED SOURCE VOLTAGES AND SINGLE-
PHASING
As supply voltage may sometimes become unbalanced, it is useful to know the effect of
unbalanced voltages on motor performance. Further, motor terminal voltage may be unbalanced
intentionally for speed control of starting .
A three-phase set of unbalanced voltages (Va, Vb and Vs) can be resolved into three-phase
balanced positive sequence (Vp), negative sequence (Vn) and zero sequence (Vo) voltages, using
symmetrical component relations:
( ) ( )
( ) ( )

40. 40
( ) ( )
Where
( )
Positive sequence voltages have the same phase sequence as original system, and
negative sequence voltages have opposite phase sequence. It will be assumed here that machine
does not have a neutral connection. In the absence of a neutral connection, zero sequence line
voltage becomes zero.
Motor performance can be calculated for positive and negative sequence voltages
separately resultant performance is obtained by the principle of superposition by assuming motor
to be a linear system.
Positive sequence voltages produce an air-gap flux wave which rotates at synchronous
speed in the forward direction. For a forward rotor speed ωm, slip s is given by equation (2.15).
For positive sequence voltages, equivalent circuits are same as shown in Fig.2.12, except that V
is replaced by Vp. The positive sequence rotor current and torque are obtained by replacing V by
Vp in Equations (2.18) and (2.24). Thus
( ) ( )
[
( )
( ) ( )
] ( )
Negative sequence voltages produce an air-gap flux wave which rotates at synchronous
speed in the reverse direction. The slip is
Substitution from equation (2.17) gives
( ) ( )
Again equivalent circuits of Fig 2.12 are applicable when s is replace by (2-s) or sn, and
V are replace by Vn. Proceeding as in previous section, following expressions are obtained for
rotor current and torque:
( ) ( )

41. 41
[
( )
( ) ( )
] ( )
Torque has a negative sign because for negative sequence voltages the synchronous speed
is (-ωms).
The rms rotor current and torque are given by
( ) ( )
[
( )
( ) ( )
( )
( ) ( )
] ( )
Positive sequence, negative sequence and the resultant speed-torque characteristics are
shown in Fig. (2.18a).
Fig. 2.18 Speed-torque curves of an induction motor with unbalance stator voltages
Single phasing (when supply to any one phase fails) is the extreme case of unbalancing,
when Vp=Vn. At zero speed, s is equal to sn, consequently starting torque is zero. Speed-torque
curves for single phasing are shown in Fig 2.18b.

42. 42
For analysis of star connected machine under single phasing, method of analysis
described in forth coming topic for ac dynamic braking with two lead connection and equivalent
circuit must be used.
Interaction between positive sequence air-gap flux wave and positive sequence rotor
currents produce positive sequence torque Tp. Negative sequence torque Tn is produced due to
interaction between negative sequence flux wave and negative sequence rotor current.
Interaction between positive sequence flux wave and negative sequence rotor currents,
and negative sequence flux wave and positive sequence rotor current, also produce torques.
However, these torques are pulsating in nature with zero average values. The pulsating torques
cause vibrations which reduce the life of motor and produce hum.
Equations (2.35) and (2.36) suggest that while the torque is reduced, copper losses ( and
also core losses) are increases. Thus, the unbalanced operation substantially reduces the motor
torque capability and efficiency.
To prevent burning of the motor, it is not allowed to run for a prolonged period when the
unbalance in voltages in more than 5%. For the same reason, motor is disconnected from the
source whenever single phasing occurs, unless the single phasing is always accompanied by a
light load.

43. 43
UNIT-III
STARTING METHODS
STARTING OF D.C. MACHINES:
For the machine to start, the torque developed by the motor at zero speed must exceed that
demanded by the load. Then TM _ TL will be positive so also is di=dt, and the machine
accelerates. The induced emf at starting point is zero as the i = 0 The armature current with rated
applied voltage is given by V=Ra where Ra is armature circuit resistance.
Normally the armature resistance of a d.c. machine is such as to cause 1 to 5 percent drop
at full load current. Hence the starting current tends to rise to several times the full load current.
The same can be told of the torque if full flux is already established. The machine instantly picks
up the speed. As the speed increases the induced emf appears across the terminals opposing the
applied voltage. The current drawn from the mains thus decreases, so also the torque. This
continues till the load torque and the motor torque are equal to each other. Machine tends to run
continuously at this speed, as the acceleration is zero at this point of operation. The starting is
now discussed with respect to specific machines.
DC shunt motor
If armature and field of d.c. shunt motor are energized together, large current is
drawn at start but the torque builds up gradually as the field flux increases gradually. To improve
the torque per ampere of line current drawn it is advisable to energize the field first. The starting
current is given by V=Ra and hence to reduce the starting current to a safe value, the voltage V
can be reduced or armature circuit resistance Ra can be increased. Variable voltage V can be
obtained from a motor generator set. This arrangement is called Ward-Leonard arrangement. A
schematic diagram of Ward-Leonard arrangement is shown in Fig. By controlling the field of the
Ward-Leonard generator one can get a variable voltage at its terminals, which is used, for
starting the motor. The second method of starting with increased armature circuit resistance can
be obtained by adding additional resistances in series with the armature, at start. The current and
the torque get reduced. It can be readily seen from this graph that the unloaded machine reaches
its final speed but a loaded machine may crawl at a speed much below the normal speed. Also,
the starting resistance wastes large amount of power. Hence the starting resistance must be
reduced to zero at the end of the starting process. This has to be done progressively, making sure
that the current does not jump up to large values. Starting of series motor and compound motors
are similar to the shunt motor. Better starting torques are obtained for compound motors as the
torque per ampere is more. Characteristics for series motors are given in fig.
Grading of starting resistance for a shunt motor
If the starting resistor is reduced in uniform steps then the current peaks reached as we
cut down the resistances progressively increase. To ascertain that at no step does
Starting of D.C shunt motor
1. Problems of starting with full voltage
We know armature current in a d.c motor is given by
At the instant of starting, rotor speed n = 0, hence starting armature current is Since,
armature resistance is quite small, starting current may be quite high (many times larger than the

44. 44
rated current). A large machine, characterized by large rotor inertia (J), will pick up speed rather
slowly. Thus the level of high starting current may be maintained for quite some time so as to
cause serious damage to the brush/ commutator and to the armature winding. Also the source
should be capable of supplying this burst of large current. The other loads already connected to
the same source, would experience a dip in the terminal voltage, every time a D.C motor is
attempted to start with full voltage. This dip in supply voltage is caused due to sudden rise in
voltage drop in the source's internal resistance. The duration for which this drop in voltage will
persist once again depends on inertia (size) of the motor. Hence, for small D.C motors extra
precaution may not be necessary during starting as large starting current will very quickly die
down because of fast rise in the back emf. However, for large motor, a starter is to be used
during starting.
2. A simple starter
To limit the starting current, a suitable external resistance Rext is connected in
series (Figure (a)) with the armature so that At the time of starting, to have
sufficient starting torque, field current is maximized by keeping the external field resistance Rf,
to zero value. As the motor picks up speed, the value of Rext is gradually decreased to zero so
that during running no external resistance remains in the armature circuit. But each time one has
to restart the motor, the external armature resistance must be set to maximum value by moving
the jockey manually.Imagine, the motor to be running with Rext = 0 (Figure (b)).
Now if the supply goes off (due to some problem in the supply side or due to load shedding),
motor will come to a stop. All on a sudden, let us imagine, supply is restored. This is then
nothing but full voltage starting. In other words, one should be constantly alert to set the
resistance to maximum value whenever the motor comes to a stop. This is one major limitation
of a simple rheostatic starter.
i) TWO POINT STARTERS: (Refer fig a )
It is used for starting D.C. series motors which has the problem of over speeding due to the loss
of load from its shaft. Here for starting the motor the control arm is moved in clock-wise
direction from its OFF position to the ON position against the spring tension. The control arm is

45. 45
held in the ON position by the electromagnet E. The exciting coil of the hold-on electromagnet E
is connected in series with the armature circuit. If the motor loses its load, current decreases and
hence the strength of the electromagnet also decreases. The control arm returns to the OFF
position due to the spring tension, Thus preventing the motor from over speeding. The starter
also returns to the OFF position
Figure (a) Shows the Two-Point Starter
3. 3-point starter
A ―3-point starter‖ is extensively used to start a D.C shunt motor. It not only overcomes the
difficulty of a plain resistance starter, but also provides additional protective features such as

46. 46
over load protection and no volt protection. The diagram of a 3-point starter connected to a shunt
motor is shown in figure b Although, the circuit looks a bit clumsy at a first glance, the basic
working principle is same as that of plain resistance starter The starter is shown enclosed within
the dotted rectangular box having three terminals marked as A, L and F for external connections.
Terminal A is connected to one armature terminal Al of the motor. Terminal F is connected to
one field terminal F1 of the motor and terminal L is connected to one supply terminal as shown.
F2 terminal of field coil is connected to A2 through an external variable field resistance and the
common point connected to supply (-ve). The external armatures resistances consist of several
resistances connected in series and are shown in the form of an arc. The junctions of the
resistances are brought out as terminals (called studs) and marked as 1,2,.. .12. Just beneath the
resistances, a continuous copper strip also in the form of an arc is present.
There is a handle which can be moved in the clockwise direction against the spring tension.
The spring tension keeps the handle in the OFF position when no one attempts to move it. Now
let us trace the circuit from terminal L (supply +ve). The wire from L passes through a small
electro magnet called OLRC, (the function of which we shall discuss a little later) and enters
through the handle shown by dashed lines. Near the end of the handle two copper strips are
firmly connected with the wire. The furthest strip is shown circular shaped and the other strip is
shown to be rectangular. When the handle is moved to the right, the circular strip of the handle
will make contacts with resistance terminals 1, 2 etc. progressively. On the other hand, the
rectangular strip will make contact with the continuous arc copper strip. The other end of this
strip is brought as terminal F after going through an electromagnet coil (called NVRC).
Terminal F is finally connected to motor field terminal Fl.

47. 47
4.Working principle
Let us explain the operation of the starter. Initially the handle is in the OFF position.
Neither armature nor the field of the motor gets supply. Now the handle is moved to stud number
1. In this position armature and all the resistances in series gets connected to the supply. Field
coil gets full supply as the rectangular strip makes contact with arc copper strip. As the machine
picks up speed handle is moved further r to stud number 2. In this position the external resistance
in the armature circuit is less as the first resistance is left out. Field however, continues to get full
voltage by virtue of the continuous arc strip. Continuing in this way, all resistances will be left
out when stud number 12 (ON) is reached. In this position, the electromagnet (NVRC) will
attract the soft iron piece attached to the handle. Even if the operator removes his hand from the
handle, it will still remain in the ON position as spring restoring force will be balanced by the
force of attraction between NVRC and the soft iron piece of the handle. The no volt release coil
(NVRC) carries same current as that of the field coil. In case supply voltage goes off, field coil
current will decrease to zero. Hence NVRC will be deenergised and will not be able to exert any
force on the soft iron piece of the handle. Restoring force of the spring will bring the handle back
in the OFF position.The starter also provides over load protection for the motor. The other
electromagnet, OLRC overload release coil along with a soft iron piece kept under it, is used to
achieve this. The current flowing through OLRC is the line current IL drawn by the motor. As
the motor is loaded, Ia hence IL increases. Therefore, IL is a measure of loading of the motor.
Suppose we want that the motor should not be over loaded beyond rated current. Now gap
between the electromagnet and the soft iron piece is so adjusted that for the iron
piece will not be pulled up. However, if rated I I force of attraction will be sufficient
to pull up iron piece. This upward movement of the iron piece of OLRC is utilized to de-energize
NVRC. To the iron a copper strip (Ä shaped in figure) is attached. During over loading
condition, this copper strip will also move up and put a short circuit between two terminals B
and C. Carefully note that B and C are nothing but the two ends of the NVRC. In other words,
when over load occurs a short circuit path is created across the NVRC. Hence NVRC will not
carry any current now and gets deenergised. The moment it gets deenergised, spring action will
bring the handle in the OFF position thereby disconnecting the motor from the supply.
Three-point starter has one disadvantage. If we want to run the machine at higher speed
(above rated speed) by field weakening (i.e., by reducing field current), the strength of NVRC
magnet may become so weak that it will fail to hold the handle in the ON position and the spring
action will bring it back in the OFF position. Thus we find that a false disconnection of the motor
takes place even when there is neither over load nor any sudden disruption of supply.

48. 48
5. 4-point starter
FOUR POINT STARTER: ( refer fig c )
The connection diagram of the four point starter is shown in fig 3. In a four point starter
arm touches the starting resistance, the current from the supply is divided into three paths. One
through the starting resistance and the armature, one through the field circuit, and one through
the NVR coil. A protective resistance is connected in series with the NVR coil. Since in a four
point starter the NVR coil is independent of the of the field ckt connection, the d.c motor may
over speed if there is a break in the field circuit. A D.C motor can be stopped by opening the
main switch. The steps of the starting resistance are so designed that the armature current will
remain within the certain limits and will not change the torque developed by the motor to a great
extent.

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DIFFERENT TYPES OF STARTERS FOR 3 PHASE INDUCTION MOTOR (IM)
INSTRUCTIONAL OBJECTIVES
• Need of using starters for Induction motor
• Two (Star-Delta and Auto-transformer) types of starters used for Squirrel cage Induction
motor
• Starter using additional resistance in rotor circuit, for Wound rotor (Slip-ring)
Induction motor
Introduction
In the previous, i.e. fourth, lesson of this module, the expression of gross torque developed, as a
function of slip (speed), in IM has been derived first. The sketches of the different torque-slip
(speed) characteristics, with the variations in input (stator) voltage and rotor resistance, are
presented, along with the explanation of their features. Lastly, the expression of maximum torque
developed and also the slip, where it occurs, have been derived. In this lesson, starting with the
need for using starters in IM to reduce the starting current, first two (Star-Delta and Auto-
transformer) types of starters used for Squirrel cage IM and then, the starter using additional
resistance in rotor circuit, for Wound rotor (Slip-ring) IM, are presented along with the starting
current drawn from the input (supply) voltage, and also the starting torque developed using the
above starters.
Keywords:
Direct-on-Line (DOL) starter, Star-delta starter, auto-transformer starter, rotor resistance
starter, starting current, starting torque, starters for squirrel cage and wound rotor induction
motor, need for starters.
Direct-on-Line (DOL) Starters
Induction motors can be started Direct-on-Line (DOL), which means that the rated voltage is
supplied to the stator, with the rotor terminals short-circuited in a wound rotor (slip-ring) motor.
For the cage rotor, the rotor bars are short circuited via two end rings. Neglecting stator
impedance, the starting current in the stator windings.

50. 50
The input voltage per phase to the stator is equal to the induced emf per phase in the
stator winding, as the stator impedance is neglected (also shown in the last lesson (#32)). In the
formula for starting current, no load current is neglected. It may be noted that the starting current
is quite high, about 4-6 times the current at full load, may be higher, depending on the rating of
IM, as compared to no load current. The starting
torque is which shows that, as the starting current increases, the starting
torque also increases. This results in higher accelerating torque (minus the load torque and the
torque component of the losses), with the motor reaching rated or near rated speed quickly.
Need for Starters in IM
The main problem in starting induction motors having large or medium size lies mainly in the
requirement of high starting current, when started direct-on-line (DOL). Assume that the

51. 51
distribution line is starting from a substation (Fig.), where the supply voltage is constant. The
line feeds a no. of consumers, of which one consumer has an induction motor with a DOL starter,
drawing a high current from the line, which is higher than the current for which this line is
designed. This will cause a drop (dip) in the voltage, all along the line, both for the consumers
between the substation and this consumer, and those, who are in the line after this consumer.
This drop in the voltage is more than the drop permitted, i.e. higher than the limit as per ISS,
because the current drawn is more than the current for which the line is designed. Only for the
current lower the current for which the line is designed, the drop in voltage is lower the limit. So,
the supply authorities set a limit on the rating or size of IM, which can be started DOL. Any
motor exceeding the specified rating, is not permitted to be started DOL, for which a starter is to
be used to reduce the current drawn at starting.
Starters for Cage IM
The starting current in IM is proportional to the input voltage per phase
to the motor (stator), i.e. , where, as the
voltage drop in the
stator impedance is small compared to the input voltage, or if the stator impedance
is neglected. This has been shown earlier. So, in a (squirrel) cage induction motor, the starter is
used only to decrease the input voltage to the motor so as to decrease the starting current. As
described later, this also results in decrease of starting torque.

52. 52
This type is used for the induction motor, the stator winding of which is nominally delta-
connected (Fig. 33.2a). If the above winding is reconnected as star (Fig. 33.2b), the voltage per
phase supplied to each winding is reduced by )1/√3(.577). This is a simple starter, which can be
easily reconfigured as shown in Fig. 33.2c. As the voltage per phase in delta connection is Vs,
the phase current in each stator winding is (Vs/Zs), where is the impedance of the motor per
phase at standstill or start (stator impedance and rotor impedance referred to the stator, at
standstill). The line current or

53. 53
to the motor is which is the current, if the motor is started direct-
on-line (DOL). Now, if the stator winding is connected as star, the phase or line current drawn
from supply at start (standstill) is which is of the
starting current, if DOL starter is used. The voltage per phase in each stator winding is now (. 3 /
s V ). So, the starting current using star-delta starter is reduced by 33.3%. As for starting torque,
being proportional to the square of the current in each of the stator windings in two different
connections as shown earlier, is also reduced by ( 2 ) 3 / 1 ( 3 / 1 = ), as the ratio of the two
currents is ( 3 / 1 ), same as that (ratio) of the voltages applied to each winding as shown earlier.
So, the starting torque is reduced by 33.3%, which is a disadvantage of the use of this starter.
The load torque and the loss torque, must be lower than the starting torque, if the motor is to be
started using this starter. The advantage is that, no extra component, except that shown in Fig.
33.2c, need be used, thus making it simple. As shown later, this is an auto- transformer starter
with the voltage ratio as 57.7%. Alternatively, the starting current in the second case with the
stator winding reconnected as star, can be found by using star-delta conversion as given in lesson
#18, with the impedance per phase after converting to delta, found as ( s Z · 3 ), and the starting
current now being reduced to (1/3 ) of the starting current obtained using DOL starter, with the
stator winding connected in delta.
Auto-transformer Starter

54. 54

55. 55
An auto-transformer, whose output is fed to the stator and input is from the supply (Fig.
33.3), is used to start the induction motor. The input voltage of IM is, which is the output
voltage of the auto-transformer, the input voltage being V s. The output voltage/input voltage
ratio is x , the value of which lies between 0.0 and 1.0
(0.0<x<1.0) Let be the starting current, when the motor is started using DOL starter, (i.e.,)
applying rated input voltage. The input current of IM, which is the output current of auto-
transformer, when this starter is used with input voltage. The input current of auto-transformer,
which is the starting current drawn from the supply, is obtained by equating input and output
volt-amperes, neglecting losses and assuming nearly same power factor on both sides. As
discussed earlier, the starting torque, being proportional to the square of the input current to IM
in two cases, with and without auto-transformer (i.e. direct), is also reduced by , as the ratio of
the two currents is same as that (ratio) of the voltages applied to the motor as shown earlier. So,
the starting torque is reduced by the same ratio as that of the starting current.
If the ratio is , both starting current and torque are %) 80 ( 8 . 0 = x %) 64 ( 64 . 0 ) 8 . 0 (
2 2 = = x times the values of starting current and torque with DOL starting, which is nearly 2
times the values obtained using star-delta starter. So, the disadvantage is that starting current is
increased, with the result that lower rated motor can now be started, as the current drawn from
the supply is to be kept within limits, while the advantage is that the starting torque is now
doubled, such that the motor can start against higher load torque. The star-delta starter can be
considered equivalent to an autotransformer starter with the ratio, %) 7 . 57 ( 577 . 0 = x . If %)
70 ( 7 . 0 = x , both starting current and torque are times the values of starting current and torque
with DOL starting, which is nearly 1.5 times the values obtained using star delta starter.

56. 56
Rotor Resistance Starters for Slip-ring (wound rotor) IM
In a slip-ring (wound rotor) induction motor, resistance can be inserted
in the rotor circuit via slip rings (Fig. 33.4), so as to increase the starting torque.
The starting current in the rotor winding is
Where, Rest= Additional resistance per phase in the rotor circuit.
The input (stator) current is proportional to the rotor current as shown
earlier. The starting current (input) reduces, as resistance is inserted in the rotor
circuit. But the
starting torque increases, as the total resistance in the
rotor circuit is increased. Though the starting current decreases, the total resistance increases,
thus resulting in increase of starting torque as shown in Fig. 32.2b, and also obtained by using
the expression given earlier, for increasing values of the resistance in the rotor circuit. If the
additional resistance is used only for starting, being rated for intermittent duty, the resistance is
to be decreased in steps, as the motor speed increases. Finally, the external resistance is to be
completely cut out, i.e. to be made equal to zero (0.0), thus leaving the slip-rings short-circuited.
Here, also the additional cost of the external resistance with intermittent rating is to be incurred,
which results in decrease of starting current, along with increase of starting torque, both being
advantageous. Also it may be noted that the cost of a slip-ring induction is higher than that of IM
with cage rotor, having same power rating. So, in both cases, additional cost is to be incurred to
obtain the above advantages. This is only used in case higher starting torque is needed to start IM
with high load torque. It may be observed from Fig. 32.2b that the starting torque increases till it
reaches maximum value, the external resistance in the rotor circuit is
increased, the range of total resistance being [r2<r2+Rext)<x2] The range of external resistance is
between zero (0.0) and
(x2-r2)2rx-). The starting torque is equal to the maximum value, i.e. ((T0)st=(T0)m), if the external
resistance inserted is equal to (x2 – r2) if the external resistance in the rotor circuit is increased

57. 57
further, i.e., [Rext>(x2 – r2)] the starting torque decreases. This is, because the
((T0)st<(T0)m),
This is, because the starting current decreases at a faster rate, even if
the total resistance in the rotor circuit is increased.
In this lesson - the fifth one of this module, the direct-on-line (DOL) starter used for IM,
along with the need for other types of starters, has been described first. Then, two types of
starters - star-delta and autotransformer, for cage type IM, are presented. Lastly, the rotor
resistance starter for slip-ring (wound rotor) IM is briefly described. In the next (sixth and last)
lesson of this module, the various types of single-phase induction motors, along with the starting
methods, will be presented.
STARTING METHODS FOR SINGLE-PHASE INDUCTION MOTOR
Introduction
In the previous, i.e. fifth, lesson of this module, the direct-on-line (DOL) starter used in three-
phase IM, along with the need for starters, has been described first. Two types of starters - star-
delta, for motors with nominally delta-connected stator winding, and autotransformer, used for
cage rotor IM, are then presented, where both decrease in starting current and torque occur.
Lastly, the rotor resistance starter for slip-ring (wound rotor) IM has been discussed, where
starting current decreases along with increase in starting torque. In all such cases, additional cost
is to be incurred. In the last (sixth) lesson of this module, firstly it is shown that there is no
starting torque in a single-phase induction motor with only one (main) winding in the stator.
Then, the various starting methods used for such motors, like, say, the addition of another
(auxiliary) winding in the stator, and/or capacitor in series with it.
Keywords:
Single-phase induction motor, starting torque, main and auxiliary windings, starting
methods, split-phase, capacitor type, motor with capacitor start/run.
Single-phase Induction Motor
The winding used normally in the stator (Fig.) of the single-phase induction motor (IM) is a

58. 58
distributed one. The rotor is of squirrel cage type, which is a cheap one, as the rating of this type
of motor is low, unlike that for a three- phase IM. As the stator winding is fed from a single-
phase supply, the flux in the air gap is alternating only, not a synchronously rotating one
produced by a poly- phase (may be two- or three-) winding in the stator of IM. This type of
alternating field cannot produce a torque if the rotor is stationery
so, a single-phase IM is not self-starting, unlike a
three-phase one. However, as shown later, if the rotor is initially given some torque in either
direction then immediately a torque is produced in the motor. The motor then
accelerates to its final speed, which is lower than its synchronous speed. This is now explained
using double field revolving theory.
Double field revolving theory
When the stator winding (distributed one as stated earlier) carries a sinusoidal current
(being fed from a single-phase supply), a sinusoidal space distributed mmf, whose peak or
maximum value pulsates (alternates) with time, is produced in the air gap. This sinusoidally

59. 59
varying flux is the sum of two rotating fluxes or fields, the magnitude of which is equal to
half the value of thealternating flux and both the fluxes rotating synchronously at the
speed, in opposite directions. This is shown in Fig. The first set of figures
(Fig. 34.1a (i-iv)) show the resultant sum of the two rotating fluxes or fields, as the time axis
(angle) is changing from Fig. shows the alternating or pulsating flux
(resultant) varying with time or angle.
The flux or field rotating at synchronous speed, say, in the anticlockwise direction, i.e.
the same direction, as that of the motor (rotor) taken as positive induces emf (voltage) in the
rotor conductors. The rotor is a squirrel cage one, with bars short circuited via end rings. The
current flows in the rotor conductors, and the electromagnetic torque is produced in the same
direction as given above, which is termed as positive (+ve). The other part of flux or field rotates
at the same speed in the opposite (clockwise) direction, taken as negative. So, the torque
produced by this field is negative (-ve), as it is in the clockwise direction, same as that of the
direction of rotation of this field. Two torques are in the opposite direction, and the resultant
(total) torque is the difference of the two torques produced (Fig. 34.3). If the rotor is stationary
the slip due to forward (anticlockwise) rotating field is 0 . 1 = sf . Similarly, the
slip due to backward rotating field is also sb = 0 .1. The two torques are equal and opposite, and
the resultant torque is 0.0 (zero). So, there is no starting torque in a single-phase IM.
But, if the motor (rotor) is started or rotated somehow, say in the anticlockwise
(forward) direction, the forward torque is more than the backward torque, with the resultant

60. 60
torque now being positive. The motor accelerates in the forward direction, with the forward
torque being more than the backward torque. The resultant torque is thus positive as the motor
rotates in the forward direction. The motor speed is decided by the load torque supplied,
including the losses (specially mechanical loss).
Mathematically, the mmf, which is distributed sinusoidally in space, with its peak
value pulsating with time, is described as (space angle) measured from
the winding axis. Now, FPeak =FmaxCosᵚt
. so the mmf is disturbed both in space and time,i.e.,
i.e. This can be expressed as,
which shows that a pulsating field can be considered as the sum of two synchronously rotating
fields The forward rotating field is and
the
Backward rotating field is, Both the fields have the same
amplitude equal to where is the maximum value of the pulsating mmf along
the axis of the winding. When the motor rotates in the forward (anticlockwise) direction with
angular speed ( r=2 ), the slip due to the forward rotating field is
Similarly the slip due to
backward rotating field the speed of which is (- s) is,
The torques produced by the two fields are
in opposite direction.
The resultant torque is,
T = Tf - Tb It was early shown that, in the rotor is stationary, Tf =T b, with both Sf = Sb = 1.0 or
nr = 0.0 therefore the resultant torque is 0.0 (zero).
STARTING METHODS
The single-phase IM has no starting torque, but has resultant torque, when it rotates at any other
speed, except synchronous speed. It is also known that, in a balanced two-phase IM having two
windings, each having equal number of turns and placed at a space angle of 90o
(electrical), and
are fed from a balanced two-phase supply, with two voltages equal in magnitude, at an angle of
90o
, the rotating magnetic fields are produced, as in a three-phase IM. The torque-speed
characteristic is same as that of a three-phase one, having both starting and also running torque
as shown earlier. So, in a single-phase IM, if an auxiliary winding is introduced in the stator, in
addition to the main winding, but placed at a space angle of 90o
(electrical), starting torque is
produced. The currents in the two (main and auxiliary) stator windings also must be at an angle
of 90o
, to produce maximum starting torque, as shown in a balanced two-phase stator. Thus,
rotating magnetic field is produced in such motor, giving rise to starting torque. The various

61. 61
starting methods used in a single-phase IM are described here.
Resistance Split-phase Motor
The schematic (circuit) diagram of this motor is given in Fig. . As detailed earlier, another
(auxiliary) winding with a high resistance in series is to be added along with the main winding in
the stator. This winding has higher resistance to reactance ratio as compared to
that in the main winding, and is placed at a space angle of from the main winding as
given earlier. The phasor diagram of the currents in two windings and the input voltage is shown
in Fig. 34.4b. The current (Ia ) in the auxiliary winding lags the voltage (V ) by an angle, a,
which is small, whereas the current (Im ) in the main winding lags the voltage (V ) by an angle
m which is nearly 90 . the phase angle between the two current is (90 - a). which is should be
atleast 30. This result in a small amound of starting torque The switch, S (centrifugal switch) is
in series with the auxiliary winding. It automatically cuts out the auxiliary or starting winding,
when the motor attains a speed close to full load speed. The motor has a starting torque of 100-
200% of full load torque, with the starting current as 5-7 times the full load current. The torque-
speed characteristics of the motor with/without auxiliary winding are shown in Fig. The change
over occurs, when the auxiliary winding is switched off as given earlier. The direction of rotation
is reversed by reversing the terminals of any one of two windings, but not both, before
connecting the motor to the supply terminals. This motor is used in applications, such as fan,
saw, small lathe, centrifugal pump, blower, office equipment, washing machine, etc. The motor
described earlier, is a simple one, requiring only second (auxiliary) winding placed at a space
angle of 90o from the main winding, which is there in nearly all such motors as discussed here. It
does not need any other thing, except for centrifugal switch, as the auxiliary winding is used as a
starting winding. But the main problem is low starting torque in the motor, as this torque is a

62. 62
function of, or related to the phase difference (angle) between the currents in the two windings.
To get high starting torque, the phase difference required is 90°(Fig. 34.5b), when the starting
torque will be proportional to the product of the magnitudes of two currents. As the current in
the main winding is lagging by the current in the auxiliary winding has to lead the input
voltage by with a, is taken as negative (-ve), while is positive
(+ve). This can be can be achieved by having a capacitor in series with the auxiliary winding,
which results in additional cost, with the increase in starting torque,
The two types of such motors are described here
Capacitor-start Motor
The schematic (circuit) diagram of this motor is given in Fig. It may be
observed that a capacitor along with a centrifugal switch is connected in
series with the auxiliary winding, which is being used here as a starting
winding. The capacitor may be rated only for intermittent duty, the cost
of which decreases, as it is used only at the time of starting.
The function of the centrifugal switch has been described earlier. The phasor
diagram of two currents as described earlier, and the torque-speed
characteristics of the motor with/without auxiliary winding, are shown in
Fig. respectively. This motor is used in applications, such as compressor,
conveyor, machine tool drive, refrigeration and air-conditioning
equipment, etc.
Capacitor-start and Capacitor-run Motor

63. 63

64. 64
In this motor (Fig. 34.6a), two capacitors -Cs for starting, and Cr for running, are used.
The first capacitor is rated for intermittent duty, as described earlier, being used only for starting.
A centrifugal switch is also needed here. The second one is to be rated for continuous duty, as it
is used for running. The phasor diagram of two currents in both cases, and the torque-speed
characteristics with two windings having different values of capacitors, are shown in Fig. 34.6b
and Fig. 34.6c respectively. The phase differencebetween the two currents is
in the first case (starting), while it is for 90° second case (running). In the second case, the motor
is a balanced two phase one, the two windings having same number of turns and other conditions
as given earlier, are also satisfied. So, only the forward rotating field is present, and the no
backward rotating field exists. The efficiency of the motor under this condition is higher. Hence,
using two capacitors, the performance of the motor improves both at the time of starting and then
running. This motor is used in applications, such as compressor, refrigerator, etc.
Beside the above two types of motors, a Permanent Capacitor Motor (Fig.) with the same

65. 65
capacitor being utilised for both starting and running, is also used. The power factor of this
motor, when it is operating (running), is high. The operation is also quiet and smooth. This motor
is used in applications, such as ceiling fans, air circulator, blower, etc.
Shaded-pole Motor
A typical shaded-pole motor with a cage rotor is shown in Fig. This is a singlephase
induction motor, with main winding in the stator. A small portion of each pole is covered
with a short-circuited, single-turn copper coil called the shading coil. The sinusoidally
varying flux created by ac (single-phase) excitation of the main winding induces emf in the
shading coil. As a result, induced currents flow in the shading coil producing their own flux
in the shaded portion of the pole.
Let the main winding flux be
Where,
As per the above equations, the shading coil current (Isc ) and flux ( sc) phasors lag behind the
induced emf (Esc ) by angle sc ; while the flux phaser leads the induced emf (Esc ) by 90o.

66. 66
Obviously the phasor m is in phase with m
sc
The resultant flux in the shaded pole is given by
the phasor sum ( sp = m
sc
+ sc) as shown in Fig. and lags the flux m of the remaining pole
by the angle The two sinusoidally varying fluxes m and sp are displaced inspace as well as
have a time phase difference ( ) thereby producing forward and backward rotating fields,
which produce a net torque. It may be noted that the motor is self-starting unlike a single-phase
single-winding motor. It is seen from the phasor diagram (Fig. 34.8b) that the net flux in the
shaded portion of the pole ( sp) lag the flux ( m) in the unshaded portion of the pole resulting
in a net torque, which causes the rotor to rotate from the unshaded to the shaded portion of the
pole. The motor thus has a definite direction of rotation, which cannot be reversed. The reversal
of the direction of rotation, where desired, can be achieved by providing two shading coils, one
on each end of every pole, and by open-circuiting one set of shading coils and by short-
circuiting the other set. The fact that the shaded-pole motor is single-winding (no auxiliary
winding) self- starting one, makes it less costly and results in rugged construction. The motor
has low efficiency and is usually available in a range of 1/300 to 1/20 kW. It is used for
domestic fans, record players and tape recorders, humidifiers, slide projectors, small business
machines, etc. The shaded-pole principle is used in starting electric clocks and other single-
phase synchronous timing motors.
In this lesson - the sixth and last one of this module, firstly, it is shown that, no starting torque
is produced in the single-phase induction motor with only one (main) stator winding, as
the flux produced is a pulsating one, with the winding being fed from single phase
supply. Using double revolving field theory, the torque-speed characteristics
of this type of motor are described, and it is also shown that, if the motor is initially given some
torque in either direction, the motor accelerates in that direction, and also the torque is produced
in that direction. Then, the various types of single-phase induction motors, along with the
starting methods used in each one are presented.
Two stator windings - main and auxiliary, are needed to produce the starting torque. The
merits and demerits of each type, along with their application area, are presented. The process of
production of starting torque in shade-pole motor is also described in brief. In the next module
consisting of seven lessons, the construction and also operation of dc machines, both as generator
and motor, will be discussed.

67. 67

68. 68
UNIT IV
CONVENTIONAL AND SOLID STATE SPEED CONTROL OF DC DRIVES
Conventional Speed Control of DC Motors:
The nature of the speed control requirement for an industrial drive depends upon its type.
Some drives may require continuous variation of speed for the whole of the range i.e from zero
to full speed, or over a portion of this range, while others may require two or more fixed speeds.
Some machines may require a creeping speed for adjusting or setting up the work. For most of
the drives, however, a control of speed is within the range of ±20% may be suitable.
Factors Controlling Motor Speed
The speed of a d.c. motor is given by:

RIVE
N ab 
 ……………. (1)
where R = Ra for shunt motor
= Ra + Rse for series motor
From exp. (i), it is clear that there are three main methods of controlling the speed of a dc
motor,
(i) By varying the flux per pole (). This is known as flux control method.
(ii) By varying the resistance (R) in the armature circuit. This is known as armature
control
method.
(iii) By varying the applied voltage V. This is known as voltage control method.
These methods as applied to shunt, series and compound motors.
Field Control Method
Field control is the most common method. In this method speed can be varied above the
rated or base speed of the motor. The speed is inversely proportional to the field current. In this

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method of speed control back emf kept constant and varying field current so, flux can be varied
and hence the speed can be varied.

1
N or
fI
N
1
 ( fI )
The advantages of this method are:
1. Good working efficiency
2. Accurate speed control
3. Speed control can be performed effectively even at light loads
4. Inexpensive
5. Provides smooth and stepless control of speed
6. Very less losses
7. Speeds above rated speed an be obtained by this method
8. Since voltage across the motor remains constant, it continues to deliver constant
output. This
characteristics, makes this method more suitable for fixed output loads.
The drawbacks of this method are listed below:
1. Inability to obtain speeds below the basic speed.
2. Instability at high speeds because of-armature reaction.
3. The highest speed is limited electrically by the effects of armature reaction under weak
field conditions in causing motor instability and poor commutation. There is possible
commutator damage at high speeds.
Armature Rheostatic Control
This method consists of obtaining reduced speeds by including external series resistance
in the armature circuit. It can be used with series, shunt and compound motor. This method is
used when speeds below the no load speed is required. It is common method of speed control for