1. PRACTICAL MANUAL
Electrical Machine Lab
TO PERFORM LOAD TEST ON THREE PHASE INDUCTION
MOTOR AND DRAW
(1) TORQUE-SPEED CHARACTERISTICS.
(2) POWER-FACTOR-LINE CURRENT CHARACTERISTICS.
2. Experiment No. – 1
Aim: -
To perform load test on a three phase induction motor and draw:
(1) Torque-speed characteristics.
(2) Power-factor-line current characteristics.
APPARATUS REQUIRED: -
S.No Equipments Type Specification/Range/Rating Quantity
1.
2.
3.
4.
5.
6.
Ammeter
Voltmeter
Wattmeter
Tachometer
2-phase variac
Star/Delta
starter
MI
MI
Dynamometer
Digital
Fully variable
Manual Type
0-10A
0-500 V
0-750W
0-2000 i.e.
10A,0-470V
1 No.
1 No.
2 No.
1 No.
1 No.
1 No.
THEORY: -
The load test on induction motor is performed to its complete performance i.e. torque, slip, efficiency,
power factor etc. During this test, the motor is operated at rated voltage and frequency and normally
loaded mechanically by brake and pulley arrangement from the observed data, the performance can be
calculated, following the steps given below:
TORQUE: Mechanical loading is the most common type of method employed in laboratories, the brake
drum is coupled to the shaft of the motor and the load is applied by tightening the belt, provide on brake
drum. The net force exerted at the brake drum in kg is obtained from the readings W1 and W2 of the
spring balance i.e.
Output =Torque x Speed
Thus as the speed of the motor does not vary appreciably with the load torque will increase with
increasing load.
Net Force exerted, W= (W1-W2) kg
Then Load torque, T= F x d/2 kg-m
= F x d/2 x 9.8 N-m
Where d is effective diameter of brake drum in meters.
SPEED: when the induction motor is NO-LOAD speed is slightly below the synchronous speed. The
current due to induced emf in the rotor winding is responsible for production of torque required at NO-
LOAD. As the load is increased the rotor speed is slightly reduced. The emf induced in the rotor causes
the current increases to produce higher torque, until the torque developed is equal to the torque required
by the load on motor.
SLIP: The speed of the rotor, Nr drops slightly as the load on the motor is increased. The synchronous
speed, Ns of the rotating magnetic field is calculated, based on the number of poles, p and the supply
frequency, f i.e.
Synchronous speed, Ns = 120f/p rpm
Then,
Slip = (Ns-Nr)/Ns x 100
3. Normally, the range of slip at full load is from 2 to 5 %.
Output power, Po The output power in watts developed by the motor is given by,
Output power, Po = 2∏ NT/60 watts
Where, N is the speed of the motor in rpm.
Input power, Pi Input power is measured by the two watt meters, properly connected in the circuit i.e.
Input power, Pi = (P1+P2) watts
Where P1 and P2 are the readings of the two watt meters.
POWER FACTOR: Power factor of the induction motor on NO-LOAD is very low because of the high
value of magnetizing current. With the increase in load the power factor increases because the power
component of the current is increased. Low power factor operation is one of the disadvantages of
induction motor. An induction motor draws heavy amount of magnetizing current due to presence of air
gap between the stator and rotor. Thus to reduce the magnetizing current in induction motor the air-gap
is kept small is possible.
Input power factor can also be calculated from the readings of two wattmeter for balanced load. If p is
the power angle, then
Tan Ф = √3 [P1-P2]/[P1+P2]
It may be noted clearly that the power factor of induction motor is very slow at no load, hardly 0.1 to
0.25 lagging. As such, one of the wattmeter will record a negative reading, till the power factor is less
than 0.5, which may be measured by reversing the connection of either the current coil or pressure coil
of the multimeter.
EFFICIENCY:
% efficiency of the motor, = (output power / input power) x100%ὴ
Full load efficiency of 3 phase induction motor lies in the range of 72% (for small motors) to 82% (for
very large motors).
4. CIRCUIT DIAGRAM:
OBSERVATION- TABLE:
S.No. Line Voltage
(volts)
Input current
(amp)
P1
(Watt)
P2
(Watt)
W1
(kg)
W2
(kg)
Speed
(rpm)
Torque
(Nm)
CALCULATION: Net force exerted, W kg
Then load torque, T = (W1-W2) x 9.8 N-m
RESULT:
5. VIVA VOICE QUESTIONS
Q.1 Explain the reasons for lower power factor of low speed, 3- phase induction motor as compared to
that of high speed motors.
Ans. Because of larger no. of poles in low speed 3-phase induction motor as compared to high speed
motors, magnetizing current is more due to increase in leakages, which increases with the increase in
number of poles. So the parameter of low speed induction motor is poor in comparison to that of high
speed induction motor.
Q.2 Why the power factor of an induction motor is low at starting?
Ans. The rotor frequency & rotor reactance are high under starting condition and therefore, rotor
currents leg the rotor emf by large angle. This results in low power factor at starting.
Q.3 What are the types of induction motors?
Ans. There are two types of induction motors:
(a) Squirrel-cage induction motor
(i) Single-cage
(ii) Double cage
(b) Wound-rotor induction motor
Q.4 Why do you require starters for the starting of 3 phase induction motor?
Ans. The object of using starters for the starting of three phase induction motor is to limit the starting
current to safe values so that voltage dip in the line voltage is not objectionable to other consumers
connected to the same line.
Q.5 Name the four types of starter used for 3 phase induction motor?
Ans.1 Direct online starter
2. Stator reactor starter
3. Auto transformer starter
4. Star-delta starter.
Q.6. Why the air – gap between stator core and rotor of induction is very small?
Ans. The air –gap is made as small as possible so as to produce required flux with a minimum exciting
current & give leakage reactance as small as possible. This results in improved power factor.
Q.7 which type of induction motor can not run at synchronous speed?
Ans. Slip ring induction motor
Q.8 why an induction motor cannot run at synchronous speed?
Ans. The direction of rotation of the 3- phase induction motor may be reversed by interchanging the
connection to the supply of any leads of the motor.
Q.9 What is the speed of the rotor mmf of 3 phase induction motor with its stator mmf?
Ans. Zero.
Q.10 How does the slip very with the load?
Ans. The greater the load , the greater the slip.
7. Experiment No. 2
AIM: To perform no load and block rotor test on a single phase induction motor and determine its efficiency.
APPARATUS REQUIRED:
S.No. Name Type Range Quantity
1 Voltmeter MI 0-300V 1
2 Ammeter MI 0-10 A 1
3 Wattmeter UPF 5/10A, 150/300/600V 1
4 Variac Single phase 6 A, 0-270 V 1
THEORY: Circuit diagram shows the laboratory setup of no load and block rotor test on capacitor start single
phase induction motor. The motor is provided with the centrifugal switch phase on the rotor shaft and are
connected in series with the starting winding. This switch is closed, when the motor at rest and thus the starting
winding is in the circuit and as such the motor can be started as split phase motor. The switch gets opened and
disconnect the starting winding, when the speed of the motor approaches to the approximately 75% of the rated
speed.
For performing NO-LOAD test, the range of voltmeter should be higher than the rated voltage of the
machine i.e 300 V and the range of the ammeter should be nearly equal to the half of the full load current of the
machine.
For performing on Block Rotor test ,the range of voltmeter should be approximately 40% of the
rated voltage of the machine i.e. 150v and the range of ammeter should be nearly equal to the full load current
of the machine.
CIRCUIT DIAGRAM:
8. PROCEDURE:
1. Connect the circuit as per circuit diagram.
2. Ensure that the motor is unloaded and the variac is set as zero output voltage.
3. Switch on 3-phase ac mains and start the motor at reduced applied voltage. Increase the applied voltage,
till its rated value.
4. Observe the direction of rotation of the motor. In case, it is reverse, change the phase sequence of the
applied voltage.
5. Take the readings of all the meters and the speed under no load running.
6. Increase the load on the motor gradually by turning of the hand wheels, thus tightening the belt. Record
the readings of all the meters and the speed at every setting of the load. Observation may be continued
up to the full load current rating of the motor.
7. Reduce the load on the motor and finally unload it completely.
8. Switch off the supply to stop the motor.
9. Note down effective diameter of the brake drum.
PRECAUTION:
While loading the induction motor by brakes, check whether cooling water is circulated in the
drum. Before starting the motor, loosen the strap and then tighten it gradually when the motor has picked up
speed.
RESULT:
VIVA VOICE QUESTIONS
Q.1 What is the effect of increasing rotor resistance in a single phase induction motor?
9. Ans. Increase in the rotor resistance of a single phase induction motor reduces its breakdown torque, lowers the
efficiency and increases the slip corresponding to the maximum torque.
Q.2 A single phase induction motor is provided with a main winding and an auxiliary winding .Which of the
two winding should be more resistive?
Ans. Auxiliary winding should be more resistive.
Q.3 How is the direction of rotation of a single phase induction motor reversed?
Ans. The direction of rotation of a single phase induction motor is reversed by reversing the leads to the main or
starting winding, but not both.
Q.4 Does the capacitor start induction motor have a high or low starting torque?
Ans. The capacitor start induction motor has the high starting torque.
Q.5 In a two phase capacitor motor which capacitor is of fairly high value and to what order?
Ans. In a two phase capacitor motor the starting capacitor is about 10-15 times as large as the running capacitor.
Q.6 What happens when the auxiliary winding of a capacitor motor is disconnected during running condition?
Ans, The motor will continue running but will develop small torque. Once it stops , it will not start again.
Q.7 How will you resolve a single pulsating field?
Ans. A single pulsating field can be resolved into two components of half it amplitude and rotating in opposite
direction with equal speed.
Q.8 How is eddy current loss reduced?
Ans. By making core of thin laminations.
Q.9 State the faraday’s law.
Ans Faraday’s law states that an emf is induced in a coil when the magnetic flux linking the coil changes with
time. It is expressed as,
e = N(dФ / dt )
where e = induced emf
N = No. of turns in the coil
Q.10 Why we perform no load and blocked rotor test on a single phase induction motor?
Ans. In order to determine the efficiency , losses , power factor and equivalent circuit parameter.
11. Experiment No. 3
AIM: To Perform the V curve and inverted V curve of the three phase synchronous motor.
APPARATUS REQUIRED: -
For dc generator (shunt type) for electrical loading:
1 MC voltmeter 96 x 96 mm flush mounted 0-220 V
2 MC ammeter 96 x 96 mm flush mounted 0-6 A
3 Double pole for cut out (DPIC) 16A
4 Field Rheostat 260 ohm ,1.4 A
5 Lamp load of 200 Watts x10
6 DP switch for load lamp
For synchronous motor:
1 MI voltmeter 96 x 96 mm flush mounted 0-500V
2 MI ammeter 96 x 96 mm flush mounted 0-2.5A
3 Power factor meter, flush mounted, 96 x 96 mm
4 MCB TP
5 Indicating Light
6 Shunt Field Rheostat 260 ohm, 1.4 A
7 DOL Starter
8 Excitation Switch
For Exciter:
1 MC voltmeter 96 x 96 mm flush mounted 0-200V
2 MC ammeter 96 x 96mm flush mounted 0-2.5A
THEORY: -
With constant mechanical load on synchronous motor, the variation of field current change the armature
current drawn by motor and also its operating power factor. As such, the behavior of the synchronous motor is
described below under three different modes of field excitation.
(1) NORMAL EXCITATION: The armature current is minimum at a particular value of field current,
which is called the normal field excitation. The operating power factor of motor is unity at this
excitation and thus the motor is equivalent to a resistive type of load.
(2) UNDER EXCITATION: When the field current is decrease gradually below the normal excitation, the
armature current increase and operating power factor of motor decreases. The power factor under this
condition is lagging. Thus the synchronous motor draws a lagging current, when it is under excited and
equivalent to an inductive load.
(3) OVER EXCITATION: When the field current is increased gradually beyond the normal excitation, the
armature current again increase and operating power factor increases. However, the power factor is
leading current, when it is over excited and is equivalent to a capacitive load.
If the above variation of field current and the corresponding armature current are plotted for a constant
mechanical load, a curve of shape of ‘V’ is obtained as shown in fig. such a characteristics of synchronous
motor is commonly called as V curve of the motor. The characteristic curve plotted between input power
12. factor and field current for a constant mechanical load on the motor are of shape of inverted V and are
known as inverted ‘V’ curve.
For increased constant mechanical load on the motor, ‘V’ curves bodily shift upwards as shown in fig. The
curve joining the minimum current points of various ‘V’ curves plotted for different mechanical loads is
normally called a unity power factor compounding curve.
CIRCUIT DIAGRAM:
14. PROCEDURE:
1. Connect the MG set with starter and measuring instrument as per connection diagram.
2. Switch on the ac supply to be fed to synchronous motor ac input terminal by using triple pole MCB and start
the motor with DOL starter and make sure that dc generator is on NO LOAD and the dc excitation switch is off
position and the rheostat in minimum position.
3. Press the green push button and the full voltage is being applied to the stator of the motor.
Observe the direction of MG set to the marked position, otherwise change the phase sequence to get marked
direction of rotation.
4. At this point armature current Ia, line voltage VL power factor of the motor which will be lagging.
5. Now switch on the dc excitation switch. Under this condition the armature current will tends to a minimum
value and power factor will tends towards unity. This is the position of normal excitation. Note the readings of
armature current, power factor & , excitation current.
6. Now move the excitation rheostat gradually and note down the readings of the meters connected in circuits.
Note carefully the value of excitation current the point at which power factor crosses from unity towards the
leading power factor and now the armature current will increase with excitation current.
Excitation may be increased till the rated value of current of synchronous motor till the behavior of motor is
normal because at large excitation motor will try to fall out of step.
7. Adjust the voltage of the dc generator coupled to the synchronous motor to rated value by varying the field
current using field rheostat.
8.To stop the motor the red push button should be pressed .The excitation rheostat should be brought to
minimum position, dc excitation switch in off position.
9. Repeat steps no. 5 to 8 sequentially for loading the Dc generator to half load, three fourth loads and maintain
it constant through out the experiment. Dc generator is loaded through lamp bank load by switching on DP
switch. Thus V curves at various loading conditions can be drawn and can be verified with the theoretical V
curve graph as shown in fig.
10. The Load on D.C. Generator should be gradually removed.
11. Step no. 9 should be repeated after the half load, and also after the three fourth loads.
PRECAUTIONS:
1. Check the power supply properly.
2. Take the readings carefully.
3. Don’t touch the live wire.
RESULT:
15. VIVA VOICE QUESTIONS
Q.1 What is a synchronous condenser?
Ans. A synchronous condenser is an over excited synchronous motor whose primary function is the to
improve the power factor of an electrical system .It does show by delivering reactive power to the ac
system. A synchronous condenser has no shaft extension, because it is not designed to deliver any
mechanical load.
Q.2 Describe the operating condition at which an alternator will give negative voltage regulation?
Ans. When excitation voltage EF is less than the terminal voltage VT , the alternator voltage regulation
would be negative. This can happen when alternator is delivering a capacitive load.
Q3.Name any two method of starting a synchronous motor?
Ans. 1. Starting of a synchronous motor by means of damper winding (Starting of a synchronous motor
first as slip ring induction motor and then as synchronous motor.)
2. By use of external prime mover.
Q.4 Why will a 3 phase synchronous motor always run at synchronous speed?
Ans.Stator poles and rotor poles of synchronous motor are magnetically locked. This locking is possible
only if relative speed between field poles and synchronously rotating armature poles is zero.
Therefore in order to maintain this magnetic locking between poles of opposite polarity, the rotor must
run at synchronous speed.
Q5. What is meant by synchronizing power?
Ans. Synchronizing power is the power that comes into play when rotor speed departs from synchronous
speed momentarily. Depending upon whether the load angle decreases or increases suddenly to cause
the rotor speed to vary from synchronous speed, synchronizing power flows in order to maintain
synchronize. This power flow is transient in nature.
Q6. How the power factor of a Synchronous motor is changed keeping the Shaft load undistributed?
Ans: By varying the field excitation.
Q7. Does the change in excitation affect the Synchronous motor speed and power factor?
Ans: Change in excitation will affect the power factor (not the speed) of the synchronous motor.
Q8. What is meant by V-curves of Synchronous motor?
Ans: The Curves drawn between armature current and field current for difference constant loads are
known as V-curves.
Q9. What is meant of hunting of Synchronous motor?
Ans: The oscillation of Synchronous motor rotor above its equilibrium position is called the hunting.
Q9. What is the purpose of damper winding?
Ans: The purpose of damper winding is to minimize hunting and make the Synchronous motor self
starting.
16. PRACTICAL MANUAL
Electrical Machine Lab
To determine Xd and Xq of a three phase salient synchronous machine using
slip test and draw the power angle curve.
17. Experiment No.4
AIM: To determine Xd and Xq of a three phase salient synchronous machine using slip test and draw the power
angle curve.
APPARATUS USED:
S.No. Name Type Range Quantity
1 Ammeter MI 0-5/10A 1
2 Ammeter MC 0-5/10A 1
3 Ammeter MC 0-1/2A 1
4 Voltmeter MI 0-300/600V 1
5 Voltmeter MI 0-75/150V 1
6 Voltmeter MC 0-150/300V 1
7 3 Phase variac - 440V/8A 1
8 Rheostat Single tube 260 ,1.4AὨ 2
THEORY:
Direct axis synchronous reactance and quadrature axis synchronous reactance are the steady state reactances of
the synchronous machine. These reactances can be measured by performing ,open circuit ,short circuit test and
the slip test on a synchronous machine.
1 Direct axis synchronous reactance, Xd
The Direct axis synchronous reactances of synchronous machine in per unit is equal to the ratio of field current,
Ifsc at rated armature current from the short circuit test to the field current ,Ifo at rated voltage on the air gap line
i.e.
Direct axis synchronous reactance Xd = Ifsc /Ifo per unit
Thus Direct axis synchronous reactance can be found out by performing open circuit and short circuit test on
alternator.
2 Quadrature axis synchronous reactance, Xq by slip test
For the slip test the alternator should be driven at a speed, slightly less than the synchronous speed with its field
circuit open .Three phase balanced reduced voltage of same frequency is applied to armature terminals of the
synchronous machine .Applied voltage is to be adjusted, should at the current drawn by the stator winding is
full load rated current. The wave shapes of stator current and stator voltage clearly indicated that these are
changing between minimum and maximum value .When the crest of the stator mmf wave coincides with the
direct axis of the rotating field, the inducted emf in the open field is zero, the voltage across the stator terminal
will be maximum and the current drawn by the stator winding is minimum as shown clearly in fig. Thus
approximate value of direct axis synchronous reactance, Xds is given by
Xds = Emax /Imin
When the crest of stator mmf wave coincides with the quadrature axis of the rotating field, the induced emf in
the open circuit field is maximum; the voltage across the terminals will be minimum and current drawn by the
stator winding in maximum as shown in fig. Hence, approximate value of quadrature axis synchronous
reactance, Xqs is given by
Xqs = Emin/Imax
For best result, these values are not taken as the final values .The most accurate method for determining the
direct axis synchronous reactance Xd is the one, that has already been described above. The most accurate value
18. of quadrature axis synchronous reactance Xq can now be found out using the above information i.e. Xds, Xqs and
Xd .
Quadrature axis, synchronous reactance Xq =( Xqs/Xds) x Xd
= ( Emin/Imax )(Imin/ Emax) x Xd per unit
Hence the accurate value of Xq can be found out by calculating minimum and maximum values of the above
quantities .Accurate result can be obtained if the oscillograms are taken during experimentation for stator
current, stator voltage and injected voltage across field.
It may be noted clearly that for synchronous machine Xd is greater than Xq i,e. Xd >Xq
CIRCUIT DIAGRAM:
19. IMPORTANT PRECAUTION FOR CONDUCTING SLIP TEST:
1. Slip should be extremely low during experiment. In case of high slip (more than above 5%) following
effects may be observed :
Current induced in the damper winding of an alternator will produce an appreciable error due to induced
voltage in the open circuit field may reach dangerous value.
2. It should be assure that the induced voltage in the field circuit is less than the rating of the voltmeter
connected in the circuit.
OBSERVATION TABLE:
S.No. Open circuit test Short circuit test Slip test
If (A) Vo (V) Imin (A) Imax (A) Vmin (V) Vmax (V)
1
2
3
4
5
PROCEDURE:
1. Connect the circuit as per circuit diagram.
2. Ensure the external resistance in the field circuit of the dc motor acting as a prime mover for alternator is
minimum and the external resistance in the field circuit of alternator is maximum’
3. Switch on the dc supply to dc motor and field of the alternator .
4. Start the dc motor with the help of the starter. The starter arm should be moved slowly, till the speed of
the motor builds up and finally all the resistance steps are cut out and the starter arm is held in ON
position by the magnet of no volt release.
5. Adjust the speed of the motor to rated speed of the alternator by varying the external resistance in the
field circuit of the motor.
6. Record the field current and its open circuit voltage per phase.
7. Increase the field current of the alternator in steps by decreasing the resistance and record the field
current and open circuit of the alternator for various values of the field current.
8. Similarly perform the short circuit parameter by changing the connections of the alternator.
PRECAUTIONS:
1. Check the power supply properly.
2. Take the readings carefully.
3. Don’t touch the live wire.
RESULT:
20. VIVA VOICE QUESTIONS
Q.1 Name two types of alternators depending on the rotor construction?
Ans. Salient pole alternator and cylindrical rotor alternator.
Q.2 What will be the number of poles of a 50 Hz alternator, if it runs at its greatest speed?
Ans. Two
Q.3Why are the poles and pole shoes laminated?
Ans. To reduce eddy current loss.
Q.4 What is meant by armature reaction of a synchronous machine?
Ans. The effect of armature flux on main field flux is known as armature reaction. Armature reaction has
distorting effect on unity power factor, wholly demagnetizing at zero power factor lagging and wholly
magnetizing at zero power factor leading
Q.5 What is the effect of armature reaction for a alternator for zero power factor lagging?
Ans. Wholly demagnetizing.
Q.6.At what voltage is the field of an Alternator usually excited?
Ans: 125- 250 V d.c.
Q.7. By which tests Synchronous reactance of the Synchronous machine is determined?
Ans: By open circuit and short circuit test.
Q.8. Why is the short circuit characteristic of an Alternator is linear?
Ans: The short circuit characteristic is normally a straight line through the origin because the net excitation
is so small that there is no saturation in the magnetic circuit.
Q.9. State the use of slip test on an Alternator.
Ans: The slip test is perform on an Alternator to determine Xd and Xq.
Q.10. Why an alternator with low value of SCR has lower limit stability?
Ans: An Alternator with low value of SCR has high value of Xd (Xd == 1/SCR) and, therefore, has lower
stability limit because maximum power output of a machine is inversely proportional to Xd.
21. PRACTICAL MANUAL
Electrical Machine Lab
TO STUDY SYNCHRONIZATION OF AN ALTERNATOR
WITH THE INFINITE BUS BY USING:
1. DARK LAMP METHOD
2. TWO BRIGHT AND ONE DARK LAMP METHOD
22. Experiment No. 5
AIM: To study synchronization of an alternator with the infinite bus by using:
1. Dark lamp method
2. Two bright and one dark lamp method
THEORY:
It is an established practice these days to connect a number of synchronous generator in parallel to supply a
common load. In power station, instead of having one large capacity generator, a number of smaller units are
installed and their output terminals connected in parallel. Moreover, for a number of reasons, large number
stations in a country are interconnected through transmission and distribution lines. All the synchronous
generator of the system, therefore, works in parallel and form a very large synchronous machine. Similarly, all
the electrical loads of the consumers are connected in parallel and form a very large variable load.
A supply system with a large number of synchronous generators connected in parallel is referred to as
infinite bus bar. Any additional machine, whether to work as a generator or as a motor is connected in parallel
with the system. The characteristics of an infinite bus-bar system are constant terminals voltage, constant bus-
bar frequency and very small synchronous impedance (since a large number of generators are in parallel). There
are number of advantage of connecting alternators in parallel to such an infinite bus-bar system.
ADVANTAGE OF PARALLEL OPERATION OF SYNCHRONOUS GENERATORS:
The following are the advantage of connecting a large number of synchronous generators in parallel to
supply a common load:
(a) Repair and maintenance of individual generating units can be done keeping the continuity supply by
properly scheduling maintenance of generators one after the other. If only one large generator is
installed, supply is to be cut off for maintenance work.
(b) For operating an alternating on maximum efficiency it is to be run near to its full load capacity. It is
uneconomic to operate large alternators on low loads. If several small units are used, units can be
added or put off depending upon the load requirement and thus the units can be operated at near to
their capacity.
(c) Additional sets can be connected in parallel to meet the increasing demand, thereby reducing the initial
capital cost of buying larger units in anticipation if increasing demands.
(d) There is physical and economics limit to the possible capacity of the alternator that can be built. The
demand of the single power station may be as high as 1200 MVA. It may not be feasible to build a
single alternator of such a high rating due to physical and economics considerations.
PARALLEL CONNECTION OR SYNCHRONISING OF ALTERNATOR
Before a synchronous generator can be put to share the load, it should be properly connected in parallel with
the common bus-bar. Interconnection of the terminals of a generator with the terminals of another or a bus-bar,
to which a large number of synchronous generators are already connected, is called synchronizing.
CONDITION FOR PARALLEL CONNECTION OR SYNCHRONISATION:
For satisfactory parallel connection of alternator, the following three conditions must be fulfilled:
(a) The generated voltage of the incoming alternator to be connected in parallel with a bus-bar should be
equal to the bus-bar voltage.
23. (b) Frequency of the generated voltage of the incoming alternator should be equal to the bus-bar
frequency.
(c) Phase sequence of the voltage of the incoming alternator should be the same as that of the bus-bar.
Fig: ‘A’ Three lamps method of checking three phase sequence of an alternator
Generated voltage of the incoming alternator can be adjusted the field excitation. Frequency of the
incoming alternator can be controlled and made equal to bus-bar frequency by controlling the speed of the
prime mover driving the alternator. Phase sequence of the alternator and the bus-bar can be checked by a phase
sequence indicator. Alternatively, three lamps as shown in fig.A can be used for checking of phase sequence.
Three lamps L1, L2 and L3 are to be connected as shown in the fig. with the synchronous generator driven at
rated speed if all the lamps glow together and become dark together then the phase sequence of the incoming
alternator in the same as that of the bus-bar.
Once the three conditions mentioned above are satisfied, the incoming alternator can be switched on to the
bus-bar, provided the instant when the voltage of the incoming generator and the bus-bar are in expect phase is
known. For this purpose the two commonly used method are described as follows.
METHOD OF SYNCHRONISATION:
Synchronisations are parallel connection of alternator can be achieved by any of the following two methods:
(a) By three lamps(one dark, two bright method)
(b) By using a synchroscope
SYNCHRONISING BY THREE LAMPS METHOD:
In the method of synchronising an alternator, three lamps are connected as show in fig.A two lamps is
cross connected with the bus-bar. In this method the brightness of lamps will vary in sequence. Particular
sequence will indicate if the incoming alternator is running two fast and two slow. Perfect synchronising will
occur when lamp L1 is dark while lamps L2 and L3 are equally bright.
24. When the speed and voltage have been adjusted, the switch of the incoming synchronous machine can be
closed only when lamp L1 is dark while lamps L2 and L3 are equally bright. if the frequency of incoming
alternator is higher than the bus-bar frequency , the phasors R1-Y1-B1 representing the bus-bar voltage. At the
instant when R1 is in phase with R2, lamp L1 will be dark and the other two lamps will be equally bright. After
one third of the cycle, B2 will be in phase with Y2. Since the lamp L2 is connected across B2 and Y2, it will be
dark. After another one third of the cycle, lamp L3 will be dark. Thus if the frequency of incoming alternator is
higher, the lamps will become dark in sequence L1-L2-L3.
Similarly, if the frequency of the incoming alternator is lower, the lamps will become dark in the sequence
L1-L2-L3. The speed of alternator will, therefore have to be slowly adjusted so that the lamp L1 is dark and lamps
L2 and L3 are bright. At this instant, the switch can be closed. The incoming machine thus gets connected in
parallel with the bus-bar.
In this three lamp method, in addition to knowing the exact instant of closing of synchronising switch, it is
also known whether the incoming alternator frequency is less or more than the bus-bar frequency.
SYNCHRONISING BY USING A SYNCHOSCOPE:
A Synchronous determine the instant of synchronism more accurately than the three lamp method. A
synchroscope consists of a rotor(moving coil) and a stator(fixed coil), one of which is connected to the
incoming alternator and the other to the bus-bar as shown in fig. A pointer connected to the rotor will rotate if
25. there is a difference in frequencies of the incoming alternator and bus-bar anticlockwise rotation of the rotor
pointer indicate that the frequency of the incoming alternator is slower, whereas clockwise rotation of the
pointer indicates that the frequency is higher than the bus-bar frequency. The speed of the prime mover driving
the alternator will, therefore, have to be adjusted such that, when the frequencies are equal the pointer is
stationary. The alternator can be switched on the bus-bar by closing the switch, S at this instant.
PROCEDURE:
(a) Always switch ON the MCB for DC motor keeping in view that A.C. Generator is not loaded.
(b) Ensure that the MCB for DC Exciter as should be in off position.
(c) Ensure that the T.P.D.T. switch is in middle position.
(d) Ensure that the MCBs for Synchroscope should be in OFF position.
(e) Ensure that field Rheostat is connected across F.R. terminal.
(f) Never leave Synchroscope and Phase Sequence Indicator permanently connected in the circuit i.e.
MCB meant for Synchroscope be should turned OFF immediately once the synchronizing procedure is
over.
(g) After starting the MG set following the steps for synchronizing.
1. Connection is made as per the circuit diagram shows in fig.D.
2. Ensure that the synchronising switch is open, external resistance in the field circuit of the motor is
zero and external resistance in the field circuit of alternator is maximum.
3. Switch on the DC supply to the DC motor -1 and DC motor-2 and start it using the starter.
26. 4. Adjust the speed of both the DC motor to rated speed of alternators, by varying the rheostat in its
field circuit of respective motors.
Switch on the DC supply to the field of alternator by switching on the MCBs and moving the Variac knob in
clockwise position so that the generated voltage of both the alternators is equal. Check the phase sequence
of both the alternators should be same.
VIVA VOICE QUESTIONS
Q. 1 What parameter of load influences the armature reaction of an alternator?
Ans. Power factor of load.
Q.2 Why are Alternator put in parallel?
Ans: For high efficiency of operation, reliability, convenience and economy in maintenance and repair and
possibility of addition to plant capacity with the growth in load on the power system.
Q.3. What is meant by synchronizing of an Alternator?
Ans. The process of connecting an Alternator in parallel with another Alternator or with the common bus-bar
is called the synchronizing.
Q.4. What are the conditions for parallel operation of an Alternator?
Ans: Terminal voltage, frequency and phase sequence of the incoming machine must be the same as that of the
bus-bars (other alternator already operating in parallel.
Q.5.What are the two methods by which two alternators are put in parallel.
Ans: Synchronizing lamp and synchroscope method.
Q.6. What is meant by infinite bus-bars?
Ans: Infinite bus-bars represent a system of large capacity whose frequency and the phase and magnitude of the
voltage are not affected ever if there is a variation of excitation power of a Synchronous machine connected
to it.
Q.7. At what power angle a Synchronous Generator will develop maximum power?
Ans: Synchronous Generator will develop maximum power when power angle δ = internal angle, θ
Where θ= tan-1
Xs/Re
Q.8. How hunting is reduced in an Alternator?
Ans: The hunting in an Alternator is reduced by employing heavy fly wheels, putting dash-pots on the engine
generator and to use squirrel-cage windings on the surface of the rotor.
Q.9. Compare the salient-pole rotor and cylindrical-pole rotor Synchronous motor on stability point of view.
Ans: Salient-pole rotor machine is more stable that cylindrical-pole rotor machine.
Q.10. Name three important characteristics of a 3-phase synchronous motor not found in a 3-phase induction
motor.
Ans:
27. 1. A 3-phase Synchronous motor has no inherent starting torque, whereas a 3-phase induction motor
possesses inherent starting torque.
2. A 3-phase Synchronous motor always runs at constant speed, equal to synchronous speed, whereas
speed of a 3-phase induction motor varies as it is loaded.
3. A 3-phase Synchronous motor can operate at lagging, unity and leading power factor, whereas a 3-phase
induction motor always operated at lagging power factor.
PRACTICAL MANUAL
Electrical Machine Lab
TO DETERMINE VOLTAGE REGULATION OF 3- PHASE
ALTERNATOR BY SYNCHRONOUS IMPEDANCE METHOD
28. Experiment No. -6
AIM: To determine voltage regulation of 3- phase alternator by synchronous impedance method.
APPARATUS REQUIRED:
Sl. No. Items Type Specifications Quantity
1 Voltmeter M.C. Type 0-300V ( for D.C. Motor) 1
2 Voltmeter M.I. Type 0-300V/500V (for A.C. Generator 1
3 Voltmeter M.C. Type 0-150V/300V ( for D.C. Exciter) 1
4 Ammeter M.C. Type 0-10A/20A ( for D.C. Motor) 1
5 Ammeter M.C. Type 0-1A/2A ( for D.C. Motor Field) 1
6 D.C. Motor Shunt Wound 5 HP,230 V, 18 A, 1500 rpm 1
7 Rheostat Tabular Type 260 Ω, 1.4 A 1
8 Potential
Divider
Variable Type 1
9 Alternator Silent Pole Type 2 KVA, 415 V, 2.8 A,50 Hz,
3 phase, 1500 rpm
1
10 Tachometer Digital 0-2000 rpm 1
11 Connecting
Wires
PVC Insulated As per
requirement
THEORY:
Short Circuit Test:
It is obtained by short circuiting the armature (stator) through a low resistance or ammeter. The excitation is
adjusted to obtain 1.25 times the full load current. During this test the speed is kept constant which need not be
synchronous.
Voltage Regulation: The regulation of alternator is defined as the change in output voltage from no load to full
load.
% Voltage Regulation = (E0±V)/V
Where E0 is the magnitude of voltage induced at no load.
Where V is the magnitude of the voltage at full load.
29. Note: Use “+” sign for lagging power factor load and “-“sign for leading power factor load.
PROCEDURE:
• Open Circuit Test:
• Make the connections as per the circuit diagram.
• Set its field potential divider to zero output position.
• Set the field resistance of motor to minimum value.
Alternator
Short Circuit Test
Short the Stator
Adjust excitation to give 1.25 times
the full load current
Increase the excitation to change Isc
30. • Switch ON the D.C. supply and start the motor with the help of the Starter.
• Adjust the speed of the motor to the Synchronous speed of Alternator with the help of the field rheostat
and note the meter readings.
• Switch ON supply to Alternator field and increase excitation, in steps and note the corresponding meter
readings. Take reading upto 10% more than the rated voltage.
• Short Circuit Test:
• Make the connections as per the circuit diagram.
• Switch ON the supply and start the motor with the help of the Starter. Keep its field rheostat in
minimum resistance position and potential divider to zero output.
• Adjust the speed of the motor to Synchronous speed with the help of field rheostat.
• Switch ON the supply to Alternator field. Note the ammeter readings for different values of
excitation. Take the reading upto 1.25 times of rated armature current.
• Measurement of Armature Resistance:
• Measure the value of Armature resistance of all the three phases separately by ammeter and
voltmeter method or by ohmmeter accurately.
CIRCUIT DIAGRAM:
31. OBSERVATION TABLE:
Synchronous speed = ______ rpm.
• For open circuit test:
Sl. No. Field Current (I2) in Amp Induced voltage (V1) in volt
1
2
3
4
5
6
• For short circuit test:
Sl. No. Field Current (I2) in Amp Armature short circuit
current (I1) in amp
1
2
32. 3
4
5
6
• For measurement of armature resistance:
• Voltage across armature winding(V2)= _______ volt
• Current through armature winding (I3)= _______ Amp
• Armature resistance per phase (Ra)= = _____ = ______ ohms
GRAPH:
• Plot the graph between open circuit voltage and short circuit current against field current.
From this graph calculate the value of Synchronous impedance for rated value of excitation.
• The nature of graph will be as shown in following figure:
CALCULATIONS:
The Synchronous impedance Zs = = _____ ohms
The synchronous reactance (Xs) = = ________= ________ ohms
Calculate % Voltage regulation:
• At 0.8 leading power factor:
• At 0.8 lagging power factor:
• At Unity power factor:
RESULT AND CONCLUSION: The regulation for the alternator is found to be:______ at 0.8 p.f
leading.______ at 0.8 p.f. lagging._______ at unity p.f. load.
33. VIVA VOICE QUESTIONS
Q.1 Name the method giving the pessimistic value of the regulation of an alternator.
Ans. Synchronous impedance or emf method.
Q.2 By which tests synchronous reactance of synchronous machine is determine?
Ans. By open circuit and short circuit test.
Q.3 Why is the short circuit characteristic of an alternator linear?
Ans. The short circuit characteristics are normally a straight line through the origin because the net excitation is
too small that there is no saturation in the magnetic circuit.
Q.4 What is an airgap line?
Ans. Tangent to OCC is called the airgap line.
Q.5 What is an exciter?
Ans. An exciter is a small dc generator to supply dc power to the field magnet system or rotor of the alternator.
Q.6. What do you mean by voltage regulation?
34. Ans: It is defined as the change in terminal voltage, expressed as a percentage (or P.u.) of the rated voltage,
when the load at a given power factor is removed, with speed & field current remaining unchanged,
therefore,
Voltage Regulation= (Ef-Vt)/Vt in P.u.
= (Ef-Vt)/Vt X 100 in percentage
Here, Ef is no load voltage excitation voltage and Vt is full load terminal voltage at the same speed and
field excitation.
Q.7. What do you mean by the short circuit characteristics of an Alternator?
Ans: The short circuit characteristics of an Alternator are a curve (usually a straight line passing through origin)
plotted between short circuit current and field current when the Alternator is being driven at its rated speed.
Q.8. What are the different methods for finding out the voltage regulation in Alternators?
Ans:
1. Synchronous Impedance (or emf) method.
2. Ampere- turn (or mmf) method.
3. Potier- triangle (or zero power factor) method.
Q.9. What is the use of Potier-triangle?
Ans: Potier-triangle is used in determination of voltage regulation of an Alternator.
Q.10. What are the losses takes place in an Alternator?
Ans:
1. Electrical Losses including armature winding loss, brush contact loss and field loss.
2. Core loss.
3. Friction and windage loss.
4. Stray power loss.
PRACTICAL MANUAL
Electrical Machine Lab
TO PERFORM NO LOAD TEST AND BLOCK ROTOR TEST
35. ON A THREE PHASE INDUCTION MOTOR AND DETERMINE
PARAMETER OF EQUIVALENT CIRCUIT
Experiment No. -7
AIM: To perform no load test and block rotor test on a three phase induction motor and determine parameter of
equivalent circuit.
APPARATUS REQUIRED:
Sl. No. Items Type Specifications Quantity
1 Ammeter MI 0-5/10 A 1
2 Voltmeter MI 0-300/600V 1
3 Wattmeter UPF 0-5/10A, 0-300/600V 2
4 Variac 3 phase 0-10A/ 0-400/470V 1
THEORY:
36. In this experiment it is intended to study the effect of variation of applied voltage on the speed, power input,
power factor, stator current of an induction motor running on no-load. The effect of change of applied voltage
on the above mentioned quantities are explained as follows:
• Effect on Speed: Speed remains practically constant until very low voltage are reached. Unless heavily
loaded, the speed of an induction motor is affected very little by fluctuations of voltage.
• Effect on Stator Current: As applied voltage is increased, stator current rises gradually on account of
the increase in magnetizing current required to produce the stator flux. The component of field stator
current which provides field ampere-turns balancing field rotor ampere-turns will steadily diminish as
the rotor current decrease with the increase in rotor speed. The increase in field magnetizing component
is however, more than sufficient to balance this decrease. At normal voltage the rotor current requires
only a small portion of field stator currents to balance them. The highest saturation a much stronger
magnetizing current to maintain the air gap flux.
CIRCUIT DIAGRAM:
EQUIVALENT CIRCUIT:
37. Fig:1
Fig:2
OBSERVATION TABLE:
Sl. No No Load Test Block Rotor Test
V0 (Volt) I0(Amp) P0(Watt) Vsc(Volt) Isc(Amp) Psc(Watt)
1
2
3
4
CALCULATIONS:
PROCEDURE:
38. No Load Test
• Connect the circuit as per the circuit diagram.
• Ensure motor is unloaded and variac is set at zero.
• Switch on the 3 phase a.c. supply and gradually increase the voltage through variac till its rated value.
Thus the motor is running at the rated speed on the No Load Condition.
• Record the readings of all the meters connected in the circuit and tabulate observation.
• Switch OFF the supply.
Block Rotor Test
• Connect the circuit as per the circuit diagram.
• Ensure motor is unloaded and variac is set at zero. Block the rotor either by tightening the belt or by
hand.
• Switch on the 3 phase a.c. supply and gradually increase the voltage through variac till its full load
current value. Thus the motor is under Block Rotor Test condition.
• Record the readings of all the meters connected in the circuit and tabulate observation.
• Switch OFF the supply.
RESULT:
VIVA VOICE QUESTIONS
39. Q.1. Why is the rotor rheostat starter unsuited for a cage motor?
Ans. Because there is no provision for inserting resistance in the rotor circuit of a cage motor.
Q.2. What is the cheapest method of starting a 3-phase induction starter?
Ans. Direct online starter.
Q.3. Why induction motors are called “a Synchronous”?
Ans: Because the rotor does not turn in Synchronism with the rotating field developed by the stator currents.
Q.4. What is the condition for maximum torque at starting in a 3-phase induction motor?
Ans: Torque developed in an induction motor at start will be maximum when rotor resistance equal to rotor
standstill reactance i.e. R2 = X2.
Q.5. Is the maximum torque of a 3-phase induction motor depend on the rotor resistance?
Ans: No, it is independent of rotor resistance.
Q.6. Why are the iron losses in the rotor of a 3-phase induction motor normally neglected?
Ans: Because of very low frequency of emf induced in rotor.
Q.7. Why the power factor of an induction motor slow at starting?
Ans: The rotor frequency and rotor reactance are high under starting condition and therefore rotor currents lag
the rotor emf by a large angle. This results in low power factor at starting.
Q.8. In what ratio line current and starting torque are reduced with star-delta starting.
Ans: Line current and starting torque is reduced to 1/3rd
with star-delta starting.
Q.9. Give three reasons for the preference of fractional-pitch stator winding in 3-phase induction motor.
Ans:
1. To reduce the mmf harmonics produced by the stator windings.
2. To reduce the amount of copper required for the stator windings.
3. To make the torque-slip characteristics a smooth curve so as to avoid the crawling.
Q.10. What is slip of an induction motor?
Ans: The difference between the speed of the stator field and the actual speed of the rotor expressed as a
fraction of synchronous speed is known as slip of an induction motor.
41. Experiment no. 8
AIM: To study speed control of the 3-phase induction motor by keeping V/F control.
APPARATUS USED:
S.No, NAME TYPE RANGE QUANTITIES
1 3 -phase Induction motor Squirrel cage 5 hp/3 Phase /
415V/7.25A
1
2 Autotransformer 3-ф variac 0-470 V 1
3 Voltmeter MI 0-500 V 1
4 Ammeter MI 0-10 A 1
THEORY:
According to torque equation
T = 3/WS x I2
2
x r2 / s
Since, T α V1
2
SIMILARLY,
R2 = V1 / √[(r1 + r2/s)2
+ (x1 + x2ʹ)2
]
I2 = I and I1 α V1
This reducing voltage has the effect of decreasing torque.
There will be voltage below which the motor will not run .If the load is sufficient, the voltage cannot be reduced
below a certain value of continuous running. The change in slip at no load is given by (S1
ʹ
- S1)
While the load is given by (S1 - S2 ) change in speed can be easily calculated .
To study the same effect under load the motor has to below loaded & the loaded convenient method of loading
is to have a coupled dc generator to keep the approximately constant.The dc generator voltage and current are
kept constant with this constant output of a dc generator and note the speed .
CIRCUIT DIAGRAM:
42. PROCEDURE:
1. Arrange the circuit as per circuit diagram.
2. To check the 3 phase variac is in zero position. Then increases it gradually.
3. Note down the reading of voltage and speed of the motor.
4. Draw the graph accordingly.
5. Finally switch off the power supply.
OBSERVATION TABLE:
Sl.No. Voltage(Volts) Speed(rpm)
PRECAUTIONS:
1. Check the power supply properly.
2. Take the readings carefully.
3. Don’t touch the main supply.
RESULT:
43. VIVA VOICE QUESTIONS
Q.1. If frequency variation is used to control the speed of an induction motor , what else must be changed?
Ans: The voltage applied to the stator must be varied in direct proportion to the frequency so as to maintain flux
level constant?
Q.2. On what factor does the speed of an induction motor depends?
Ans. The speed of an induction motor depends on supply frequency, slip and number of poles on stator.
Q.3. What measure can be taken for minimizing the effect of crawling?
Ans. Effect of crawling can be minimized by chording, skewing and integral slot winding.
Q.4. Why that is the V/F ratio is kept constant while controlling the speed of a 3-phase induction motor by
varying the supply frequency?
Ans: In order to maintain flux level constant.
Q.5. What is the relationship of developed torque of a 3-phase induction motor with the supply voltage?
Ans: Torque α s V2
Where s= slip
And V = supply voltage.
Q.6. When the applied rated voltage per phase is reduced to one-half, what will be the starting torque of a
squirrel-cage induction motor in terms of its starting torque with full voltage?
Ans: One-fourth of starting torque with full rated voltage.
Q.7. What are the losses in a 3-phase induction motor?
Ans: Stator copper loss, Stator iron loss, Rotor copper loss, friction and windage losses and stray load loss.
Q.8. How many terminals does u expect to find on the terminal box of a squirrel cage induction motor to be
started by star-delta starter?
Ans: 6
Q.9. What is the condition for producing maximum torque in a 3-phase induction motor?
Ans: for producing maximum torque in an induction motor, the condition is R2=smax X2.
Q.10. While controlling induction motor speed, how super Synchronous speed is achieved?
Ans: It can be achieved by injecting a slip frequency voltage in phase with emf induced in the rotor.
45. Experiment no. 8
AIM: To study speed control of the 3-phase induction motor by varying supply voltage.
APPARATUS USED:
S.No, NAME TYPE RANGE QUANTITIES
1 3 -phase Induction motor Squirrel cage 5 hp/3 Phase /
415V/7.25A
1
2 Autotransformer 3-ф variac 0-470 V 1
3 Voltmeter MI 0-500 V 1
4 Ammeter MI 0-10 A 1
THEORY:
According to torque equation
T = 3/WS x I2
2
x r2 / s
Since, T α V1
2
SIMILARLY,
R2 = V1 / √[(r1 + r2/s)2
+ (x1 + x2ʹ)2
]
I2 = I and I1 α V1
This reducing voltage has the effect of decreasing torque.
There will be voltage below which the motor will not run .If the load is sufficient, the voltage cannot be reduced
below a certain value of continuous running. The change in slip at no load is given by (S1
’
- S1)
While the load is given by (S1 - S2 ) change in speed can be easily calculated .
To study the same effect under load the motor has to below load & the loaded convenient method of loading is
to have a coupled dc generator to keep the approximately constant. The dc generator voltage and current are
kept constant with this constant output of a dc generator and note the speed.
CIRCUIT DIAGRAM:
46. PROCEDURE:
1) Arrange the circuit as per circuit diagram.
2) To check the 3 phase variac is in zero position. Then increases it gradually.
3) Note down the reading of voltage and speed of the motor.
4) Draw the graph accordingly.
5) Finally switch off the power supply.
OBSERVATION TABLE:
Sl.No. Voltage(Volts) Speed(rpm)
GRAPH:
PRECAUTIONS:
1. Check the power supply properly.
2. Take the readings carefully.
3. Don’t touch the main supply.
RESULT:
47. VIVA VOICE QUESTIONS
Q.1. What type of protection is provided in the starter meant for 3-phase induction motor?
Ans: Over load and under voltage protection.
Q.2. What is the serious objection to the practice of employing reduced voltage for the starting of a squirrel-
cage induction motor?
Ans. Large reduction in starting torque because the starting torque varies as the square of voltage applied to the
stator.
Q.3. If frequency variation is used to control the speed of an induction motor, what else must be changed?
Ans. The voltage applied to the stator must varied indirectly proportional to the frequency so as to maintain flux
level constant.
Q.4. What will be the effect on the torque developed by an Induction motor when the applied voltage is reduced
to half, supply frequency unchanged?
Ans: Torque developed by an Induction motor with half rated voltage, supply frequency remaining unchanged,
will be reduced to 1/4th
because T α V2
(i.e. square of the supply voltage).
Q.5. Why the rotor of a slip- ring Induction motor should be wound for the same number of poles as its stator?
Ans: Because the slip-ring Induction motor having the same number of poles on the rotor as that of the stator
will be in a position to run at slip speed.
Q.6. Why an Induction motor can not run at Synchronous speed?
Ans: Because at Synchronous speed the Induction motor can not developed any torque to move the rotor from
its stationary position.
Q.7. A 3-phase, 50 Hz induction motor runs at 960 rpm on NO load and 940 rpm at Full. Find Full load slip?
Ans: 6%
Q.8. What is the speed of stator mmf of 3-phase Induction motor with respect to its rotor mmf?
Ans: 0
Q.9. Speed of a 3-phase Induction motor is varied by varying the supply frequency while (V/F) is kept constant.
How will be maximum torque vary?
Ans: The magnitude of maximum torque does not change but the speed at which it occurs changes in the same
ratio as the supply frequency.
Q.10. Why NO load current of an Induction motor is much higher than that of an equivalent transformer?
Ans: Owing to the air-gap in an Induction motor the magnetizing current is large for the Induction motor as
compare to that for a transformer. The frequency and windage losses are also present in the Induction motor. So
NO load current of an Induction motor is much higher than that of an equivalent transformer.
