electric poiwer drives

1. Module-II
Dynamics of electrical drives
1
Reference: Gopal K. Dubey, “ Fundamentals of Electrical Drives” Second Edition, Narosa Publishing House.

2. Fundamental torque equation
• A motor generally drives a load (machine) through some transmission system.
While motor always rotates, the load may rotate or may undergo a translational
motion.
• Load speed may be different from that of motor, and if the load has many parts,
their speeds may be different and while some may rotate, others may go through
a translational motion.
• It is, however, convenient to represent the Torque Equation of Motor Load
System by an equivalent rotational system shown in Fig. 2.1
2

3. • Various notations used are:
J = Polar moment of inertia of motor-load system referred to the motor shaft, kg-m2
ωm = Instantaneous angular velocity of motor shaft, rad/sec.
T = Instantaneous value of developed motor torque, N-m.
Tl = Instantaneous value of load (resisting) torque, referred to motor shaft, N-m.
• Load torque includes friction and windage torque of motor.
Torque Equation of Motor Load System of Fig. 2.1 can be described by the following
fundamental torque equation:
Equation (2.1) is applicable to variable inertia drives such as mine winders, reel drives,
industrial robots.
For drives with constant inertia, (dJ/dt) = 0. Therefore
3
Contd..

4. • Equation (2.2) shows that torque developed by motor is counter balanced by a load
torque Tl and a dynamic torque J(dωm/dt).
• Torque component J(dωm/dt) is called the dynamic torque because it is present only
during the transient operations.
• Drive accelerates or decelerates depending on whether T is greater or less than Tl.
During acceleration, motor should supply not only the load torque but an additional
torque component J(dωm/dt) in order to overcome the drive inertia.
• In drives with large inertia, such as electric trains, motor torque must exceed the load
torque by a large amount in order to get adequate acceleration.
• In drives requiring fast transient response, motor torque should be maintained at the
highest value and Torque Equation of Motor Load System should be designed with a
lowest possible inertia.
• Energy associated with dynamic torque J(dωm/dt) is stored in the form of kinetic
energy given by (ω2
m /2).
• During deceleration, dynamic torque J(dωm/dt) has a negative sign. Therefore, it
assists the motor developed torque T and maintains drive motion by extracting energy
from stored kinetic energy.
4

5. Speed – torque characteristics of various types of loads and drives:
• Classification of load torques:
1. Active Load torques
2. Passive Load torques
 Active Load Torques: Load torques which have the potential to drive the motor
under equilibrium conditions are called active load torques. Load torques
usually retain sign when the drive rotation is changed.
 Passive Torque: Load torques which always oppose the motion and change their
sign on the reversal of motion are called passive load torques. Torque due to
friction cutting – Passive torque.
5

6. Components of Load Torques
• Components of Load Torques Tl can be further divided into following components:
(i) Friction torque TF :
Friction will be present at the motor shaft and also in various parts of the load. TF is equivalent
value of various friction torques referred to the motor shaft.
(ii) Windage torque, Tw :
When a motor runs, wind generates a torque opposing the motion. This is known as windage
torque.
(iii) Torque required to do the useful mechanical work, TL:
Nature of this Components of Load Torques depends on particular application. It may be constant
and independent of speed; it may be some function of speed; it may depend on the position or path
followed by load; it may be time invariant or time-variant; it may vary cyclically and its nature
may also change with the load’s mode of operation. 6

7. • Variation of friction torque with speed is shown
in Fig. 2.6(a).
• Its value at standstill is much higher than its
value slightly above zero speed.
• Friction at zero speed is called stiction or static
friction.
• In order for drive to start, the motor torque
should at least exceed stiction.
• Friction torque can be resolved into three
components (see Fig. 2.6(b)).
• Component Tv which varies linearly with speed is
called viscous friction and is given by:
Windage torque Tw, which is proportional to speed squared, is given by
where C is a constant. 7
Contd..

8. From the above discussion, for finite speeds,
In many applications (Tc + Cω2
m) is very small compared to Bωm and negligible compared to TL.
In order to simplify the analysis, term (Tc + Cω2
m) is approximately accounted by updating the value of
viscous friction coefficient, B. With this approximation, from Eq. (2.2)
If there is a torsional elasticity in shaft coupling the load to the motor, an additional components of Load
Torques, known as Coupling Torque, will be present. Coupling torque (Te) is given by
where θe is the torsion angle of coupling (radians) and Ke the rotational stiffness of the shaft (Nm/rad).
8
Contd..

9. • In most applications, shaft can be assumed to be perfectly stiff and coupling
torque Te can be neglected.
• Its presence in appreciable magnitude has adverse effects on motor. There is
potential energy associated with coupling torque and kinetic energy with the
dynamic torque.
• Exchange of energy between these two energy storage’s tends to produce
oscillations which are damped by viscous friction torque B ωm . When B is
small, oscillations occur producing noise.
• Further, shaft may also break when the drive is started.
9
Contd..

10. Contd..
• Nature of the torque depends on type of load.
It may be constant and independent of speed, some function of speed, may be time
invariant or time variant.
• The nature of the torque may change with the change in the mode of operation of loads.
Characteristics of different types of load:
• In electric drives the driving equipment is an electric motor.
• Selection of particular type of motor driving a machine is the matching of speed-torque
characteristics of the driven unit and that of the motor.
• Different types of loads exhibit different speed torque characteristics.
• Most of the industrial loads can be classified into the following 4 general categories:
1. Constant torque type load.
2. Torque proportional to speed (generator type load).
3. Torque proportional to square of the speed (fan type load).
4. Torque inversely proportional to speed (constant power type load).
10

11. 1. Constant Torque Characteristic:
A constant torque load implies that the torque required to keep the load running is
the same at all speeds. A good example is a drum-type hoist, where the torque
required varies with the load on the hook, but not with the speed of hoisting.
Fig: Constant Torque characteristics. 11

12. 2. Torque proportional to speed:
The characteristics of the charge imply that the torque required increases with the
speed. This particularly applies to helical positive displacement pumps where the
torque increases linearly with the speed.
Fig: Torque proportional to speed. 12

13. 3. Torque proportional to square of the speed (fan type load):
Quadratic torque is the most common load type. Typical applications are centrifugal
pumps and fans. The torque is quadratically, and the power is cubically proportional to
the speed.
Fig: Torque proportional to square of the speed. 13

14. 4. Torque inversely proportional to speed ( constant power type load):
A constant power load is normal when material is being rolled and the diameter
changes during rolling. The power is constant and the torque is inversely proportional to
the speed.
Fig: Constant power type load. 14

15. Speed torque conventions and Multi quadrant operation
• For consideration of Multi quadrant operation of drives it is useful to establish suitable
converters about the signs of torque and speed .
• Motor speed is considered the when rotating in the forward direction in load involving
up and down motions the speed of motor which causes upward motions is consider
forward motion and as the +ve .
• For reversing drives forward speed is chosen arbitrarily .
• Positive motor torque is defined as the torque which produces acceleration or the +ve
rate of change of speed in forward direction.
• The load torque is opposite in the direction to the +ve motor torque.
• Motor torque is considered -ve if it produces deceleration.
15

16. • A motor operates in two modes, i.e., motoring and braking.
• In motoring, it converts electrical energy to mechanical energy, which supports its motion.
• In braking, it works as a generator converting mechanical energy to electrical energy, and
thus, opposes the motion.
• Motor can provide motoring and braking operations for both forward and reverse
directions.
Figure 2.2 shows the torque and speed coordinates for both forward
(positive) and reverse (negative) motions of Four Quadrant
Operation of Motor Drive.
• Power developed by a motor is given by the product of speed and
torque. In quadrant I, developed power is positive. Hence,
machine works as a motor supplying mechanical energy.
Operation in quadrant I is, therefore, called forward motoring.
• In quadrant II, power is negative. Hence, machine works under
braking opposing the motion. Therefore, operation in quadrant II
is known as forward braking. Similarly, operations in quadrant
III and IV can be identified as reverse motoring and braking
respectively. Figure 2.2: Multi-quadrant
operation of drives
16

17. Contd..
• Let us consider operation of a hoist in
Four Quadrant Operation of Motor
Drive as shown in Fig. 2.3. Directions
of motor and load torques, and
direction of speed are marked by
arrows.
• A hoist consists of a rope wound on a
drum coupled to the motor shaft. One
end of the rope is tied to a cage which
is used to transport man or material
form one level to another level.
• Other end of the rope has a counter
weight. Weight of the counter weight is
chosen to be higher than the weight of
an empty cage but lower than of a fully
loaded cage.
• 17

18. • Forward direction of motor speed will be one which gives upward motion of the cage.
Speed-torque characteristics of the hoist load are also shown in Fig. 2.3. Though the
positive load torque is opposite in sign to the positive motor torque, according to Eq.
(2.2), it is convenient to plot it on the same axes. Load-torque curve drawn in this
manner is, in fact, negative of the actual.
• Load torque has been shown to be constant and independent of speed. This is nearly
true with a low speed hoist where forces due to friction and windage can be considered
to be negligible compared to those due to gravity. Gravitational torque does not change
its sign even when the direction of driving motor is reversed.
• Load torque line Tl1 in quadrants I and IV represents speed-torque characteristic for the
loaded hoist. This torque is the difference of torques due to loaded hoist and counter
weight.
• The load torque line Tl2 in quadrants II and III is the speed-torque characteristic for an
empty hoist. This torque is the difference of torques due to counter weight and the
empty hoist. Its sign is negative because the weight of a counter weight is always
higher than that of an empty cage.
18
Contd..

19. • The quadrant I operation of a hoist requires the movement of the cage upward,
which corresponds to the positive motor speed which is in anticlockwise
direction here. This motion will be obtained if the motor produces positive
torque in anticlockwise direction equal to the magnitude of load torque Tl1.
Since developed motor power is positive, this is forward motoring operation.
• Quadrant IV operation is obtained when a loaded cage is lowered. Since the
weight of a loaded cage is higher than that of a counter weight, it is able to come
down due to the gravity itself. In order to limit the speed of cage within a safe
value, motor must produce a positive torque T equal to Tl2 in anticlockwise
direction. As both power and speed are negative, drive is operating in reverse
braking.
• Operation in quadrant II is obtained when an empty cage is moved up. Since a
counter weight is heavier than an empty cage, it is able to pull it up. In order to
limit the speed within a safe value, motor must produce a braking torque equal
to Tl2 in clockwise (negative) direction. Since speed is positive and developed
power negative, it is forward braking operation. 19

20. • Operation in quadrant III is obtained when an empty cage is lowered. Since an
empty cage has a lesser weight than a counter weight, the motor should
produce a torque in clockwise direction. Since speed is negative and
developed power positive, this is reverse motoring operation.
20

21. Motor Design Parameters
• Different parts of a load may be coupled through different mechanisms, such
as gears, V-belts and crankshaft.
• These parts may have different speeds and different types of Motions such as
rotational and translational.
21

22. Loads with Rotational Motion:
• Let us consider a motor driving two loads, one coupled directly to its shaft and other
through a gear with n and n1 teeth as shown in Fig. 2.4(a).
where a1 is the gear tooth
ratio.
Let the moment of inertia, speed and torque of
the load coupled through a gear be J1, ωm1 and
Tl1 respectively. Now,
Let the moment of inertia of motor and load
directly coupled to its shaft be J0, motor speed
and torque of the directly coupled load be
ωm and Tl0 respectively.
22

23. From Eqs. (2.3) and (2.4)
Power at the loads and motor must be the same. If transmission efficiency of the gears
be η1, then
where Tl is the total equivalent torque referred to motor shaft.
From Eqs. (2. 3) and (2. 6)
• If the losses in transmission are neglected, then the kinetic energy due to equivalent
inertia must be the same as kinetic energy of various moving parts. Thus
23

24. If loads are driven through a belt drive instead of gears, then, neglecting slippage, the
equivalent inertia and torque can be obtained from Eqs. (2.8) and (2.9) by considering a1, a2, . .
. , am each to be the ratios of diameters of wheels driven by motor to the diameters of wheels
mounted on the load shaft.
If in addition to load directly coupled to the motor with inertia J0 there are m other loads with
moment of inertias J1, J2, . . . , Jm and gear teeth ratios of a1, a2, . . . am then
24

25. Loads with Translational Motion:
• Let us consider a motor driving two loads, one coupled directly to its shaft and
other through a transmission system converting rotational motion to linear
motion (Fig. 2.4(b)).
• Let moment of inertia of the Motor Design Parameters and load directly
coupled to it be J0, load torque directly coupled to motor be Tl0, and the mass,
velocity and force of load with translational motion be M1 (kg), υ1 (m/sec) and
F1 (Newtons), respectively.
25

26. If the transmission losses are neglected, then kinetic energy due to equivalent
inertia J must be the same as kinetic energy of various moving parts. Thus
Similarly, power at the motor and load should be the same, thus if efficiency of
transmission be η1
26

27. If, in addition to one load directly coupled to the motor shaft, there are m other
loads with translational motion with velocities υ1,υ2, . . . υm and masses M1,M2, . .
. , Mm, respectively, then
27

28. Measurement of Moment of Inertia:
• Moment of inertia can be calculated if dimensions and weights of various parts
of the load and Motor Design Parameters are known. It can also be measured
experimentally by retardation test.
• In retardation test, the drive is run at a speed slightly higher than rated speed and
then the supply to it is cut off.
• Drive continues to run due to kinetic energy stored in it and decelerates due to
rotational mechanical losses.
• Variation of speed with time is recorded.
• At any speed ωm , the power P consumed in supplying rotational losses is given
by
28

29. • From retardation test dωm/dt at rated speed is obtained. Now drive is reconnected to the
supply and run at rated speed and rotational mechanical power input to the drive is measured.
• This is approximately equal to P. Now J can be calculated from Eq. (2.14).
• Main problem in this method is that rotational mechanical losses cannot be measured
accurately because core losses and rotational mechanical losses cannot be separated.
• In view of this, retardation test on a dc separately excited motor or a synchronous motor is
carried out with field on.
• Now core loss is included in the rotational loss, which is now obtained as a difference of
armature power input and armature copper loss.
• In case of a wound rotor induction motor, retardation test can be carried out by keeping the
stator supply and opening the rotor winding connection.
29

30. • J can be determined more accurately by obtaining speed time curve from the
retardation test as above and also rotational losses versus speed plot as shown in Fig.
2.5.
• Using these two plots, rotational losses versus time plot can be obtained, e.g. for time
t1, ωm1 is found from the retardation plot.
• Then for this speed rotational loss P1 is obtained from the plot of rotational loss versus
speed and plotted against t1.
• Area A enclosed between the rotational loss
versus t plot and the time axis (shaded area), is
the kinetic energy dissipated during retardation
test.
• If initial speed of the drive during retardation
test was ωm0 then
30

31. Steady state stability, dynamic stability, load equalization
 Steady State Stability of Drive:
 Equilibrium speed of a motor-load system is obtained when motor torque equals
the load torque. Drive will operate in steady-state at this speed, provided it is the
speed of stable equilibrium.
 In most drives, the electrical time constant of the motor is negligible compared
to its mechanical time constant.
 Therefore, during transient operation, motor can be assumed to be in electrical
equilibrium implying that steady-state speed-torque curves are also applicable to
the transient operation.
31

32. • The equilibrium point will be termed as stable when the operation will be
restored to it after a small departure from it due to a disturbance in the motor
or load.
• Let the disturbance causes a reduction of Δωm in speed as shown in Fig.
2.9(a). At new speed, motor torque is greater than the load torque,
consequently, motor will accelerate and operation will be restored to A. 32

33. • Similarly, an increase of Δωm in speed caused by a disturbance will make load
torque greater than the motor torque, resulting into deceleration and restoration
of operation to point A. Hence the drive is steady-state stable at point A.
• Let us now examine equilibrium point B which is obtained when the same
motor drives another load. A decrease in speed causes the load torque to become
greater than the motor torque, drive decelerates and operating point moves away
from B.
• Similarly, when working at B an increase in speed will make motor torque
greater than the load torque, which will move the operating point away from B.
Thus, B is an unstable point of equilibrium. 33

34. • Above discussion suggests that an equilibrium point will be stable when an
increase in speed causes load-torque to exceed the motor torque, i.e. when at
equilibrium point following condition is satisfied:
34

35. Load Equalization in Electrical Drives:
• Load Equalization in Electrical Drives – In some drive applications, load torque
fluctuates widely within short intervals of time
• Fluctuating loads are overcome by mounting a flywheel on the motor shaft in non-
reversible drives.
• Motor speed-torque characteristic is made drooping. Alternatively, by closed loop
current control torque is prevented from exceeding a permissible value.
• During high load period, load torque will be much larger compared to the motor torque.
• Deceleration occurs producing a large dynamic torque component (J dωm/dt). Dynamic
torque and motor torque together are able to produce torque required by the load.
35

36. • Because of Fig. 2.10 shapes of motor
speed torque curves for deceleration, the
motor speed falls.
 During light load period, the motor
torque exceeds the load torque causing
acceleration speed is brought back to
original value before the next high load
period.
36

37. • Variation of motor and load torques, and speed for a periodic load and for a
drooping motor speed-torque curve are shown in Fig. 2.11.
• It shows that peak torque required from the motor has much smaller value than the
peak load torque.
• Hence, a motor with much smaller rating than peak
load can be used and peak current drawn by motor
from the source is reduced by a large amount.
• Fluctuations in motor torque and speed are also
reduced. Since power drawn from the source
fluctuates very little, this is called load equalization 37

38. • In variable speed and reversible drives, a flywheel cannot be mounted on the
motor shaft, as it will increase transient time of the drive by a large amount.
• If motor is fed from a motor-generator set (Ward-Leonard Drive), then flywheel
can be mounted on the shaft of the motor-generator set.
• This arrangement of Load Equalization in Electrical Drives on the source, but not
the load on motor.
• Consequently, a motor capable of supplying peak-load-torque is required.
38

39. Basic principles of closed-loop control
• If open-loop control fails to provide the desired speed regulation, drive is
operated as a closed-loop speed control system.
• Feedback loops in an electrical drive may be provided to satisfy one or more of
the following requirements.
(i) Protection
(ii) Enhancement of speed of response.
(iii) To improve steady-state accuracy.
39

40. Current Limit Control of Drives:
• Current Limit Control of Drives scheme of Fig. 3.3 is employed to limit the converter
and motor current below a safe limit during transient operations.
• It has a current feedback loop with a threshold logic circuit.
• As long as the current is within a set maximum value, feedback loop does not affect
operation of the drive.
40
• During a transient operation in
Current Limit Control of Drives, if
current exceeds the set maximum
value, feedback loop becomes active
and current is forced below the set
maximum value, which causes the
feedback loop to become inactive
again.

41. • The current fluctuates around a set maximum limit during the transient
operation until the drive condition is such that the current does not have a
tendency to cross the set maximum value, e.g. during starting, current will
fluctuate around the set maximum value.
• When close to the steady-state operation point, current will not have tendency to
cross the maximum value, consequently, feedback loop will have no effect on
the drive operation.
41

42. Closed Loop Torque Control of Drives:
• Closed Loop Torque Control of Drives scheme of Fig. 3.4 finds application in
battery operated vehicles, rail cars and electric trains. Driver presses the
accelerator to set torque reference T*.
• Through Closed Loop Torque Control of Drives, the actual motor torque T
follows torque reference T*.
• Speed feedback loop is present through the driver.
• By putting appropriate pressure on the accelerator, driver adjusts the speed
depending on traffic, road condition, his liking, car condition and speed limit.
42

43. Closed Loop Speed Control:
• Figure 3.5 shows a closed loop speed control scheme which is widely used in
electrical drives.
• It employs an inner current control loop within an outer speed-loop.
• Inner current control loop is provided to limit the converter and motor current or
motor torque below a safe limit.
• In some schemes the current is controlled directly. In others it may be controlled
indirectly.
43

44. 44
• For example, in a variable frequency induction motor drives the current is
controlled by controlling the slip.
• Inner current loop is also beneficial in reducing the effect on drive performance of
any non-linearity present in converter-motor system.
Drive of Fig. 3.5 operates as follows:
• An increase in reference speed ω*
m produce a positive error Δωm. Speed error is
processed through a speed controller and applied to a current limiter which
saturates even for a small speed error.
• Consequently, limiter sets current reference for inner current control loop at a
value corresponding to the maximum allowable current.
• Drive accelerates at the maximum allowable current (and in some cases at the
maximum torque).
• When close to the desired speed, limiter desaturates. Steady-state is reached at the
desired speed (with some steady-state error) and at current for which motor torque
is equal to the load torque.

45. • A decrease in reference speed ω*
m produces a negative speed error. Current limiter
saturates and sets current reference for inner current loop at a value corresponding to
the maximum allowable current.
• Consequently, drive decelerates in braking mode at the maximum allowable current.
• When close to the required speed, current limiter desaturates. The operation is
transferred from braking to motoring. Drive then settles at a desired speed and at
current for which motor torque equals the load torque.
• In those drives where the current I does not have to reverse for braking operation,
current limiter will have the input-output characteristic shown in Fig. 3.5(b).
• In those drive applications where the load torque is able to provide enough
decelerating torque, electric braking need not be used. Then also current limiter has the
characteristic shown in Fig. 3.5 (b).
• Current and speed controllers may consist of proportional and integral (PI),
proportional and derivative (PD) or proportional, integral and derivative (PID)
controller, depending on steady-state accuracy and transient response requirements.
45

46. Selection of motor power rating
• The Selection of Motor 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 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.
• On the other hand, if power rating is decided liberally, the extra initial cost and extra
loss of energy due to operation below rated power makes the choice uneconomical.
• When a 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. 46

47. • 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
thermal loading on the machine.
• Steady state temperature is not the same at various parts of the machine. It is usually
highest in the windings because loss density in conductors is high and dissipation is
slow; since the conductors which are wrapped in insulating material are partly
embedded in slots and thus are not directly exposed to the cooling air.
• The insulation has lowest temperature limit. Depending on the temperature limits,
insulating materials employed in electric machines are divided into classes γ, A, E, B, F,
H, C.
47

48. 48
• 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 resulting into thermal
breakdown in near future.

49. • For loads which operate at a constant power and speed, determination of motor
power rating is simple and straightforward. But only a few loads operate at a
constant speed and power.
• Most loads operate at variable power and speed, and the patterns of these
variations are different for different applications.
49

50. Thermal model of motor for heating and cooling
• An accurate prediction of Heating and Cooling Curves of Electrical Drives rise
inside an electrical motor is very difficult owing to complex geometrical shapes
and use of heterogeneous materials.
• Since conductivities of various materials do not differ by a large amount, a
simple thermal model of the machine can be obtained by assuming machine to
be a homogeneous body.
• Although inaccurate, such a model is good enough for a drive engineer whose
job is only to select the motor rating for a given application ensuring that
temperatures in various parts of motor body do not exceed the safe limits. 50

51. • In self cooled motors, where cooling fan is mounted on motor shaft, the velocity
of cooling air varies with motor speed, thus varying cooling time constant τ′.
• Cooling time constant at standstill is much larger than when running.
• Therefore, in high performance, and medium and high power variable speed
drives, motor is always provided with separate forced cooling, so that motor
cooling be independent of speed.
51
• Figure 4.1 shows the variation of motor temperature
rise with time during Heating and Cooling Curves of
Electrical Drives. Thermal time constants of a motor
are far larger than electrical and mechanical
time constants.
• While electrical and mechanical time constants have a
typical ranges of 1 to 100 ms and 10 ms to 10 s, the
thermal time constants may vary from 10 min to
couple of hours.

52. Classes of motor duty
• IS: 4722-1968 categorizes various load time variations encountered in practice
into eight standard Classes of Motor Duty in Electrical Drives:
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.
52

53. 53
a. Continuous duty.
b. Short time duty.
c. Intermittent periodic
duty.
d. Intermittent periodic
duty with starting.
e. Intermittent periodic
duty with starting and
braking.

54. 1. Continuous Duty (Fig. 4.2(a)):
• 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 Classes of Motor Duty in Electrical Drives.
2. Short Time Duty (Fig. 4.2(b)):
• 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, turning
bridges, sluice-gate drives, valve drives, and many machine tool drives for
position control.
54

55. • 3. Intermittent Periodic Duty (Fig. 4.2(c)):
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 Classes of Motor Duty in Electrical Drives, heating of machine during
starting and braking operations is negligible.
Some examples are pressing, cutting and drilling machine drives.
4. Intermittent Period Duty with Starting (Fig. 4.2(d)):
• This is intermittent periodic duty where heat losses during starting cannot be
ignored. Thus, it consists of a period of starting, a period of operation at a
constant load and a rest period; with operating and rest periods, being too short
for the respective steady-state temperatures to be attained.
• In this duty, heating of machine during braking is considered to be negligible.
• Few examples are metal cutting and drilling tool drives, drives for fork lift
trucks, mine hoist etc.
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56. 5. Intermittent Periodic duty with Starting and Braking (Fig. 4.2(e)):
• This is the intermittent periodic duty where heat losses during starting and braking cannot be
ignored.
• 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, strapdown mechanism of blooming mill,
several machine tool drives, drives for electric suburban trains and mine hoist are some
examples of this duty.
6. 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.
• The load period and no load period being too short for the respective temperatures to be
attained.
• Pressing, cutting, shearing and drilling machine drives are the examples.
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57. 7. 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.
8. 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.
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