Speed Control Using Kramer Method

1 DEPARTMENT OF ELECTRICAL ENGINEERING,
DAVIET, JALANDHAR
SPEED
CONTROL
USING
KRAMER
METHOD

DAVIET, JALANDHAR
CONTENTS
Title Page No.
Abstract 2
Acknowledgement 3
List of Figures 4
List of Tables 5
Chapter 1: INTRODUCTION 7
1.1 Induction Motor 7
1.2 Working Principle of Induction motor 8
1.3 Synchronous Motor 9
1 .4 Why Induction Motor Never Runs At Synchronous Speed? 10
1.5 Types of Induction Motor 10
1.6 Working of Three Phase Induction Motor 11
1.7 Why There Is Need To Control Speed Of Induction Motor? 15
1.8 Method to Control Speed of Induction Motor 15
Chapter 2: PROJECT IDEA 22
2.1 Kramer Method 22
Chapter 3: PROJECT DESCRIPTION 24
3.1 The Various Equipment’s Used Kramer Method 24
CHAPTER 4: RESULTS 28
CHAPTER 5: CONCLUSION 29
REFERENCES 30

DAVIET, JALANDHAR
CHAPTER 1
INTRODUCTION
1.1 INDUCTION MOTOR
An induction or asynchronous motor is an AC electric motor in which the electric current in the
rotor needed to produce torque is obtained by electromagnetic induction from the magnetic field of
the stator winding. Induction motors are widely used as industrial drives because they are rugged,
reliable and economical. Single-phase induction motors are used extensively for smaller loads, such
as household appliances like fans. Although traditionally used in fixed-speed service, induction
motors are increasingly being used with variable-frequency drives (VFDs) in variable-speed service.
VFDs offer especially important energy savings opportunities for existing and prospective induction
motors in variable-torque centrifugal fan, pump and compressor load applications
Figure 1.1: Induction Motor
1.2 WORKING PRINCIPAL OF INDUCTION MOTOR

DAVIET, JALANDHAR
When the stator winding is fed with an AC supply. Alternating flux is produced around the stator winding
due to AC supply. This alternating flux revolves with synchronous speed. The revolving flux is called as
"Rotating Magnetic Field" (RMF).The relative speed between stator RMF and rotor conductors causes an
induced emf in the rotor conductors, according to the Faraday's law of electromagnetic induction.
The rotor conductors are short circuited, and hence rotor current is produced due to induced emf.
That is why such motors are called as induction motors. Now, induced current in rotor will also
produce alternating flux around it. This rotor flux lags behind the stator flux. The direction of
induced rotor current, according to Lenz's law, is such that it will tend to oppose the cause of its
production.
As the cause of production of rotor current is the relative velocity between rotating stator flux and
the rotor, the rotor will try to catch up with the stator RMF. Thus the rotor rotates in the same
direction as that of stator flux to minimize the relative velocity. However, the rotor never succeeds
in catching up the synchronous speed. This is the basic working principle of induction motor of
either type, single phase of 3 phase.
Figure 1.2 Rotating magnetic field in a 3-phase induction motor

DAVIET, JALANDHAR
1.3 SYNCHRONOUS MOTOR
A synchronous electric motor is an AC motor in which, at steady state, the rotation of the shaft is
synchronized with the frequency of the supply current; the rotation period is exactly equal to an
integral number of AC cycles. Synchronous motors contain multiphase AC electromagnets on the
stator of the motor that create a magnetic field which rotates in time with the oscillations of the line
current. The rotor with permanent magnets or electromagnets turns in step with the stator field at
the same rate and as a result, provides the second synchronized rotating magnet field of any AC
motor. A synchronous motor is only considered doubly fed if it is supplied with independently
excited multiphase AC electromagnets on both the rotor and stator.
The synchronous motor and induction motor are the most widely used types of AC motor. The
difference between the two types is that the synchronous motor rotates at a rate locked to the line
frequency. The synchronous motor does not rely on current induction to produce the rotor's magnetic
field. By contrast, the induction motor requires "slip", the rotor must rotate slightly slower than the
AC current alternations, to induce current in the rotor winding. Small synchronous motors are used
in timing applications such as in synchronous clocks, timers in appliances, tape recorders and
precision servomechanisms in which the motor must operate at a precise speed; speed accuracy is
that of the power line frequency, which is carefully controlled in large interconnected grid systems.
The synchronous speed on induction motor is based on the supply frequency and the number of
poles in the motor windings and can be expressed as:
Ns = 120.f / P
Where
Ns = synchronous speed
F = Frequency
P = Number of Poles
1.4 WHY INDUCTION MOTOR NEVER RUNS AT SYNCHRONOUS SPEED?

DAVIET, JALANDHAR
The working principle of induction motor is electromagnetic induction. If induction motor runs at
synchronous speed then there would be no relative speed between stator field and rotor. Then there
will be no induced emf, no current and no torque will be produced. Hence induction motor always
runs at a speed less than synchronous speed
1.5 TYPES OF MOTOR
1.5.1 Single Phase Induction Motor
a) Split phase induction motor
b) Capacitor start induction motor
c) Capacitor start capacitor run induction motor
d) Shaded pole induction motor
1.5.2 Three Phase Induction Motor
a) Squirrel cage induction motor
b) Slip ring induction motor
1.6 THREE PHASE INDUCTION MOTOR

DAVIET, JALANDHAR
The three phase induction motor is the most widely used electrical motor. Almost 80% of the
mechanical power used by industries is provided by three phase induction motors because of its
simple and rugged construction, low cost, good operating characteristics, absence of commutator
and good speed regulation. In three phase induction motor the power is transferred from stator to
rotor winding through induction. The induction motor is also called a synchronous motor as it runs
at a speed other than the synchronous speed.
1.6.1Stator:
Stator of three phase induction motor is made up of numbers of slots to construct a 3 phase winding
circuit which is connected to 3 phase AC source. The three phase winding are arranged in such a
manner in the slots that they produce a rotating magnetic field after 3Ph. AC supply is given to them.
Figure 1.3 Three Phase Induction Motor
1.6.2 Rotor

DAVIET, JALANDHAR
Rotor of three phase induction motor consists of cylindrical laminated core with parallel slots that
can carry conductors. Conductors are heavy copper or aluminum bars which fits in each slots & they
are short circuited by the end rings. The slots are not exactly made parallel to the axis of the shaft
but are slotted a little skewed because this arrangement reduces magnetic humming noise & can
avoid stalling of motor.
1.6.3 Squirrel-cage rotor is the rotating part of the common "squirrel cage" induction motor. It
consists of a cylinder of steel laminations, with aluminum or copper conductors embedded in its
surface. In operation, the non-rotating "stator" winding is connected to an alternating current power
source; the alternating current in the stator produces a rotating magnetic field. The rotor winding
has current induced in it by the stator field, and produces its own magnetic field. The interaction of
the two sources of magnetic field produce torque on the rotor.
By adjusting the shape of the bars in the rotor, the speed-torque characteristics of the motor can be
changed, to minimize starting current or to maximize low-speed torque
The motor rotor shape is a cylinder mounted on a shaft. Internally it contains longitudinal conductive
bars (usually made of aluminium or copper) set into grooves and connected at both ends by shorting
rings forming a cage-like shape. The name is derived from the similarity between this rings-and-
bars winding and a squirrel cage .
The solid core of the rotor is built with stacks of electrical steel laminations. Figure 3 shows one of
many laminations used. The rotor has a smaller number of slots than the stator and must be a non-
integer multiple of stator slots so as to prevent magnetic interlocking of rotor and stator teeth at the
starting instant. The rotor bars may be made either of copper or aluminium. A very common structure
uses die cast aluminium poured into the rotor after the laminations are stacked. Some larger motors
have aluminium or copper bars which are welded or brazed to end-rings. Since the voltage
developed in the squirrel cage winding is very low, no intentional insulation layer is present between
the bars and the rotor steel.

DAVIET, JALANDHAR
Rotor of an Induction Motor skewed due to following reason…
1. With the bar skewed,the amount of the bar cutting the field line grows continuously and the
next bar starts cutting the field lines as the first finishes.Due to this,we get Uniform Torque.
2. To run quietly by reducing the magnetic hum , reduce rotor locking tendency. Rotor locking
tendency occurs when rotor teeth remain directly under stator teeth thus they might be
magnetically attracted.
3. Primarily to prevent the cogging phenomenon. It is a phenomenon in which, if the rotor
conductors are straight, there are chances of magnetic locking or strong coupling between
rotor & stator.
4. Increase effective Magnetic Coupling between Stator and Rotor Fluxes
1.6.4 Phase wound-rotor motor is a type of induction motor where the rotor windings are connected
through slip rings to external resistances. Adjusting the resistance allows control of the speed/torque
characteristic of the motor. Wound-rotor motors can be started with low inrush current, by inserting
high resistance into the rotor circuit; as the motor accelerates, the resistance can be decreased.
Compared to a squirrel-cage rotor, the rotor of the slip ring motor has more winding turns; the
induced voltage is then higher, and the current lower, than for a squirrel-cage rotor. During the start-
up a typical rotor has 3 poles connected to the slip ring. Each pole is wired in series with a variable
power resistor.
Figure 1.4 Difference between Slip Ring and Squirrel Cage Induction Motor
Table 1.1 - Difference between Slip Ring and Squirrel Cage Induction Motor

DAVIET, JALANDHAR
Slip ring or phase wound Induction motor Squirrel cage induction motor
Construction is complicated due to presence of slip
ring and brushes
Construction is very simple
The rotor winding is similar to the stator winding
The rotor consists of rotor bars which are
permanently shorted with the help of end rings
We can easily add rotor resistance by using slip
ring and brushes
Since the rotor bars are permanently shorted, its
not possible to add external resistance
Due to presence of external resistance high starting
torque can be obtained
Staring torque is low and cannot be improved
Slip ring and brushes are present Slip ring and brushes are absent
Frequent maintenance is required due to presence
of brushes
Less maintenance is required
The construction is complicated and the presence
of brushes and slip ring makes the motor more
costly
The construction is simple and robust and it is
cheap as compared to slip ring induction motor
This motor is rarely used only 10 % industry uses
slip ring induction motor
Due to its simple construction and low cost. The
squirrel cage induction motor is widely used
Rotor copper losses are high and hence less
efficiency
Less rotor copper losses and hence high
efficiency
Speed control by rotor resistance method is
possible
Speed control by rotor resistance method is not
possible
Slip ring induction motor are used where high
starting torque is required i.e. in hoists, cranes,
elevator etc.
Squirrel cage induction motor is used in lathes,
drilling machine, fan, blower printing machines
etc.
1.7 WHY THERE IS NEED TO CONTROL SPEED OF INDUCTION MOTOR ?
Three phase induction motor is a constant speed motor. So it is difficult to control its speed. But by
using different methods we can control its speed. Because speed of Induction motor is inversely

DAVIET, JALANDHAR
proportional to torque. And at the starting, motor runs at maximum slip. Torque is directly
proportional to slip. If supply voltage reduced then Induction motor draws more current to magnetise
rotor and operate under normal condition. But because of excessive current flowing through motor
windings and motor get overheat up. Hence motor get damaged, Because of this reason, we have to
control the speed of three phase induction motor.
In AC drives, or traction drives , sometimes you need to accelerate , sometimes you have to slow
down , sometimes you have to stop and apply brakes, to carry out this whole range of operation ,
speed control during motoring and braking of Induction motor is highly necessary.
1.8 SPEED CONTROL OF INDUCTION MOTOR
The Speed of Induction Motor is changed from Both Stator and Rotor Side
The speed control of three phase induction motor from stator side are further classified as :
1. V / f control or frequency control.
2. Changing the number of stator poles.
3. Controlling supply voltage.
4. Adding rheostat in the stator circuit.
The speed controls of three phase induction motor from rotor side are further classified as:
1. Adding external resistance on rotor side.
2. Cascade control method.
3. Injecting slip frequency emf into rotor side.
4. By using Kramer method
1.8.1Speed Control from Stator Side
1.8.1(a) V / f control or frequency control - Whenever three phase supply is given to three phase
induction motor rotating magnetic field is produced which rotates at synchronous speed given by

DAVIET, JALANDHAR
In three phase induction motor emf is induced by induction similar to that of transformer which is
given by
Where, K is the winding constant, T is the number of turns per phase and f is frequency. Now if we
change frequency synchronous speed changes but with decrease in frequency flux will increase and
this change in value of flux causes saturation of rotor and stator cores which will further cause
increase in no load current of the motor . So, its important to maintain flux , φ constant and it is only
possible if we change voltage. i.e if we decrease frequency flux increases but at the same time if we
decrease voltage flux will also decease causing no change in flux and hence it remains constant. So,
here we are keeping the ratio of V/f as constant. Hence its name is V/ f method. For controlling the
speed of three phase induction motor by V/f method we have to supply variable voltage and
frequency which is easily obtained by using converter and inverter set.
1.8.1(b) Controlling supply voltage - The torque produced by running three phase induction motor
is given by

DAVIET, JALANDHAR
In low slip region (Sx)2 is very very small as compared to R2. So, it can be neglected. So torque
becomes
Since rotor resistance, R2 is constant so the equation of torque further reduces to
We know that rotor induced emf E2 ∝ V. So, T ∝ sV2.
From the equation above it is clear that if we decrease supply voltage torque will also decrease. But
for supplying the same load, the torque must remain the same and it is only possible if we increase
the slip and if the slip increases the motor will run at reduced speed. This method of speed control
is rarely used because small change in speed requires large reduction in voltage, and hence the
current drawn by motor increases, which cause over heating of induction motor.
1.8.1(c) Adding rheostat in the stator circuit - In this method of speed control of three phase
induction motor rheostat is added in the stator circuit due to this voltage gets dropped .In case of
three phase induction motor torque produced is given by T ∝ sV2
2. If we decrease supply voltage
torque will also decrease. But for supplying the same load, the torque must remains the same and it
is only possible if we increase the slip and if the slip increase motor will run reduced speed.

DAVIET, JALANDHAR
1.8.1(d) Pole Changing Method is one of the main methods of the speed control of an induction
motor. This method of controlling the speed by pole changing is used mainly for cage motor only
because the cage rotor automatically develops a number of poles, which is equal to the poles of the
stator winding.
1.8.1(e) Pole amplitude modulation is a flexible method of pole changing which can be used in
applications where speed ratios other than 2:1 are required. The motors designed for speed changing
based on the poled amplitude modulation scheme are known as PAM motors.
1.8.2 SpeedControl from Rotor Side
1.8.2(a) Adding external resistance on rotor side - In this method of speed control of three phase
induction motor external resistance are added on rotor side. The equation of torque for three phase
induction motor is
The three phase induction motor operates in low slip region. In low slip region term (sX)2 becomes
very small as compared to R2. So, it can be neglected. And also E2 is constant. So the equation of
torque after simplification becomes

DAVIET, JALANDHAR
Now if we increase rotor resistance, R2 torque decreases but to supply the same load torque must
remain constant. So, we increase slip, which will further results in decrease in rotor speed. Thus by
adding additional resistance in rotor circuit we can decrease the speed of three phase induction
motor. The main advantage of this method is that with addition of external resistance starting torque
increases but this method of speed control of three phase induction motor also suffers from some
disadvantages:
1. The speed above the normal value is not possible.
2. Large speed change requires large value of resistance and if such large value of resistance is added
in the circuit it will cause large copper loss and hence reduction in efficiency.
3. Presence of resistance causes more losses.
4. This method cannot be used for squirrel cage induction motor.
1.8.2(b) Cascade control method - In this method of speed control of three phase induction motor,
the two three phase induction motor are connected on common shaft and hence called cascaded
motor. One motor is the called the main motor and another motor is called the auxiliary motor. The
three phase supply is given to the stator of the main motor while the auxiliary motor is derived at a
slip frequency from the slip ring of main motor.
Let
NS1 = be the synchronous speed of main motor.
NS2 = be the synchronous speed of auxiliary motor.
P1 = be the number of poles of the main motor.

DAVIET, JALANDHAR
P2 = be the number of poles of the auxiliary motor.
F = is the supply frequency.
Now, slip of motor
A, S1 = (Ns1 - N) / Ns1
Frequency of the rotor induced emf in motor A, f1 = S1f
Now, auxiliary motor is supplied with the rotor induced emf
Therefore, Ns2 = (120f1) / p2 = (120 s1f ) /P2
Now putting the value of S1 = (Ns1 – N) / Ns1
Figure 1.5 Cascade Method
Now at no load , the speed of auxiliary rotor is almost same as its synchronous speed i.e N = NS2
In this method of speed control of three phase induction motor, four different speeds can be obtained
1. When only main induction motor work, having speed corresponds to =

DAVIET, JALANDHAR
2. When only auxiliary induction motor work, having speed corresponds to = .
3. When cumulative cascading is done, then the complete set runs at a speed of = .
3. When differential cascading is done, then the complete set runs at a speed of = .
CHAPTER 2
PROJECT IDEA
2.1 KRAMER METHOD
The three-phase wound-rotor induction motor exhibits the advantage over other induction motors in
that its speed can be adjusted over a considerable range. On the other hand, it suffers the

DAVIET, JALANDHAR
disadvantage in that its speed regulation is poor. That is, a variation in the loading tends to produce
a large change in speed. Additionally, the speed-control rheostats in the rotor circuit sometimes
waste considerable power. There is a relatively simple way to obtain up to a four-to-one speed range
with good speed regulation, and without high-power dissipation. The arrangement shown in fig is
known as the Kramer drive. A dc shunt-field motor shares the same shaft as the wound-rotor
induction motor. Electronic circuitry is limited to rectification in the basic setup
Figure 1.6 Kramer Method Circuit
The speed-control system. The combination of ac and dc motors coupled to a common shaft provides
an efficient adjustable-speed range with good speed regulation. Speed changes from varying the
load on the output shaft are much smaller than would result from either motor alone.
In this unique scheme, the ac slip-power from the rotor of the induction machine is rectified and
delivered to the armature of the dc motor. Speed control of this combination is obtained by adjusting
the field current to the dc motor. The net effect is that the shaft speed is considerably stabilized
against load changes. Note that both machines contribute torque to the common shaft.

DAVIET, JALANDHAR
There are other ways of viewing this duo. It bears some resemblance to the con catenation of
induction motors described elsewhere. Also, the dc shunt motor performs somewhat as a magnetic
amplifier in that the control of low power in its field circuit serves to control high power in its
armature circuit. In any event, it’s noteworthy that there are no rheostats in heavy-current portions
of the circuit. The maximum speed of this technique is that of the induction motor. A commonly
attained speed range for 60 Hz is 450 to 1700 RPM. The minimum speed is a consequence of the
relationship between voltage provided by the rotor of the induction motor and that needed by the
armature of the dc motor. This relationship tends to be contradictory to extending the speed-control
range much beyond the alluded four to one.
Figure1.7 control of commutator Machine The Kramer speed-control system
CHAPTER 3
PROJECT DESCRIPTION
3.1 THE VARIOUS EQUIPMENT’S USED KRAMER METHOD:-
3.1.1 Digital Voltmeter (0-750v):- A voltmeter is an instrument used for measuring electrical
potential difference between two points in an electric circuit. Digital voltmeters give a numerical
display of voltage by use of an analog to digital converter. Voltmeters are made in a wide range of
styles. Instruments permanently mounted in a panel are used to monitor generators.

DAVIET, JALANDHAR
Figure 1.8 Digital voltmeter
3.1.2 Analog Ammeter (0-5mA) :- An ammeter (from Ampere Meter) is a measuring instrument
used to measure the current in a circuit. Electric currents are measured in amperes (A), hence the
name. Instruments used to measure smaller currents, in the mili ampere or microampere range, are
designated as millimeters or micro ammeters. Early ammeters were laboratory instruments which
relied on the Earth's magnetic field for operation.
Figure 1.9 Digital Ammeter
3.1.3 Connecting Wires :- A wire is a single, usually cylindrical, flexible strand or rod of metal.
Wires are used to bear mechanical loads or electricity and telecommunications signals. Wire is
commonly formed by drawing the metal through a hole in a die or draw plate. Wire gauges come in
various standard sizes, as expressed in terms of a gauge number. The term wire is also used more
loosely to refer to a bundle of such strands, as in "multi stranded wire", which is more correctly
termed a wire rope in mechanics, or a cable in electricity.

DAVIET, JALANDHAR
Figure 2.0 Connecting Leads
3.1.4 Terminals : A banana connector (commonly banana plug for the male, banana socket or
banana jack for the female) is a single-wire (one conductor) electrical connector used for joining
wires to equipment. The plugs are frequently used to terminate patch cords for electronic test
equipment, while sheathed banana plugs are common on multi-meter probe leads
Figure 2.1 Terminals
3.1.5 Bakelite Sheet is an early plastic. It is a thermosetting phenol formaldehyde resin, formed
from a condensation reaction of phenol with formaldehyde. It was developed by the Belgian-
American chemist Leo Baekeland in Yonkers, New York, in 1907.
Making Bakelite was a multi-stage process. It began with the heating of phenol and formaldehyde
in the presence of a catalyst such as hydrochloric acid, zinc chloride, or the base ammonia. This
created a liquid condensation product, referred to as Bakelite A, which was soluble in alcohol,

DAVIET, JALANDHAR
acetone, or additional phenol. Heated further, the product became partially soluble and could still
be softened by heat. Sustained heating resulted in an "insoluble hard gum". However, the high
temperatures required to create this tended to cause violent foaming of the mixture, which resulted
in the cooled material being porous and breakable
Properties
1. Bakelite has a number of important properties. It can be molded very quickly, allowing identical
units to be mass-produced. Moldings are smooth, retain their shape and are resistant to heat,
scratches, and destructive solvents. It is also resistant to electricity, and prized for its low
conductivity. It is not flexible.
2. Phenolic resin products may swell slightly under conditions of extreme humidity or perpetual
dampness. When rubbed or burnt, Bakelite has a distinctive, acrid, sickly-sweet or fishy odor.
Figure 2.2 Bakelite Sheet
3.1.6 Tachometer (revolution-counter, tach, rev-counter, RPM gauge) is an instrument
measuring the rotation speed of a shaft or disk, as in a motor or other machine. The device usually
displays the revolutions per minute (RPM) on a calibrated analogue dial, but digital displays are
increasingly common
Tachometers or revolution counters on cars, aircraft, and other vehicles show the rate of rotation of
the engine's crankshaft, and typically have markings indicating a safe range of rotation speeds. This

DAVIET, JALANDHAR
can assist the driver in selecting appropriate throttle and gear settings for the driving conditions.
Prolonged use at high speeds may cause inadequate lubrication, overheating (exceeding capability
of the cooling system), exceeding speed capability of sub-parts of the engine (for example spring
retracted valves) thus causing excessive wear or permanent damage or failure of engines.
Figure 2.3 Tachometer
CHAPTER 4
RESULTS
Observations:-
S.NO DC VOLTAGE

DAVIET, JALANDHAR
AC
VOLTAGE
FIELD
CURRENT
SPEED OF
MOTOR
SPEED OF
CONVERTOR
1.
2.
3.
4.
5.
0
415
415
415
415
0
110
105
100
95
0.05
0.15
0.20
0.25
0.30
1410
1017
817
700
690
0
740
1330
1460
1580
Table 1.2 – Readings of Kramer Method
CHAPTER - 5
5.1 CONCLUSION

DAVIET, JALANDHAR
We had designed control panel board which controls the speed of induction motor in order to
improve the efficiency and to improve the power factor of the machine. This method is differ from
other as there is injection of rotary convertor takes place which was not in others. This is also
different from the other which presented in our college as our design it reduces the complexity of
the circuit connection and A.C parts are present on one side and dc on other hand. This panel also
reduces the complexity of making the connection during performing the experiment hence, it can
operated by any on beginner by its own .this is the method for the speed control of induction motor
A.C as well D.C. In this we use rotary convertor instead of using slip ring.
REFRENCES
[1] www.wikipedia.org

DAVIET, JALANDHAR
[2] www.electrical4u.com
[3] www.google images.com
[4] www.electricaleasy.com
[5] www.google.co.in
[6] www.quora.com
[7] www.ieee.com
[8] www.electricalengineeringschools.com
[9] www.learn.adafruit.com
[10] www.what-when-how.com
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Speed Control Using Kramer Method

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