INDUCTION AND SYNCHRONOUS MACHINES
UNIT – I: 3-phase Induction Motors
Construction details of squirrel cage and slip ring induction motors – production of rotating magnetic
field – principle of operation – Equivalent circuit –phasor diagram- slip speed-rotor emf and rotor
frequency – rotor current and pf at standstill and during running conditions – rotor power input, rotor
copper loss and mechanical power developed and their interrelationship.
JNTUK Previous Questions
1. a) Discuss the points of similarities between a transformer and an induction machine. Hence, explain
why an induction machine is called a generalized transformer.
[8M]
b) Show that the voltage generated in the rotor circuit of a 3-phase induction motor at any slip S is
equal to S times the voltage generated at standstill.
[7M]
2. a) Explain why the power factor of a 3-phase induction motor is low at no load and also under
overloads.
[6M]
b) A 3-φ, 50Hz, 6- pole, 400V induction motor draws an input power to the rotor is 70kW. The motor
emf is observed to make 120 cycles/minute. Calculate i) the slip, ii) rotor speed, iii) Mechanical
power developed, iv) rotor resistance loss/phase and v) the rotor resistance/phase, if the rotor current
is 65A.
[9M]
3. a) Develop the equivalent circuit of a 3-phase induction motor and explain how the mechanical power
developed is taken care in the equivalent circuit.
(7M)
b) A 4-pole 50 Hz, 3-phase induction motor has a rotor resistance of 0.02 Ω per phase and standstill
reactance of 0.5Ω per phase. Calculate the speed at which the maximum torque is developed.(7M)
4. a) Explain in detail the constructional feature of wound rotor three phase induction motor.
(7M)
b) A 25 hp, 400 V, 50 Hz, 4-pole, star connected induction motor has the following impedances per
phase in ohms referred to the stator side : Rs= 0.641, R´r = 0.322: Xs=1.106, X´r = 0.464 and Xm =
26.30. Rotational losses are assumed constant and are 1.1 kW and the core losses are assumed
negligible. If the slip is 2.2% at rated voltage and frequency , find i) speed ii) stator current
iii) power factor iv) output and input power and v) efficiency of motor.
(7M)
5. a) Explain, why the speed of 3-phase induction motor cannot be equal to synchronous speed? (4M)
b) A 3-phase, 4-pole, 50 Hz, induction motor has a star connected wound rotor. The rotor emf is 50V
between the slip rings at standstill. The rotor resistance and standstill reactance are 0.4Ω and 2.0 Ω
respectively. Calculate
(i) rotor current per phase at starting when slip rings are short circuited
(ii) Rotor current per phase at starting if 50 Ω per phase resistance is connected between slip rings.
(iii) Rotor EMF when the motor us running at full load at 1440 rpm
(iv) Rotor current at full load and
(v) Rotor power factor at full load
(10M)
6. a) Describe the construction of a 3-phase cage type induction motor with neat sketch.
(7M)
b) A cage induction motor when started by means of a star-delta starter takes 190% of full load line
current and develops 40% of full load torque at starting. Determine the starting torque and current in
terms of full load values, if an auto transformer with 80% tapping were employed.
(7M)
7. a) Discuss in detail about the principle of operation of a 3-phase induction motor.
(7M)
b) A 4-pole, 50 Hz, 3-phase induction motor develops a maximum torque of 120 Nm at 1460 r.p.m.
The resistance of the star connected rotor is 0.35Ω/phase. Determine the value of resistance that
must be inserted in series, with each rotor phase to produce a starting torque equal to half the
maximum torque.
(7M)
8. a) List the difference between squirrel cage rotor and slip ring rotor?
(7M)
b) A 10kw, 400V, 3-phase, 4-pole, 50 HZ delta connected induction motor is running at no lode with a
line current of 8A and an input power is 660 kW. At full load, the line current is 18 A and the input
power is 11.20 kW. Stator effective resistance per phase is 1.2 ohm and friction, winding loss is 420
watts. For negligible rotor ohmic losses at no load, calculate (i) stator core loss (ii) slip at full load
(iii) total rotor ohmic losses at full load (iv) full load speed.
(7M)
SURESH MIKKILI, Associate Professor, EEE Department
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INDUCTION AND SYNCHRONOUS MACHINES
9. a) Draw a power flow diagram of a three phase induction motor and explain all the stages?
(4M)
b) A 3-phase, 4-pole, 50 hz, star connected induction motor running on full load develops a useful
torque of 300 N-m the rotor EMF is completing 120 cycles per minute. If torque lost is frictional is
50 N-m, calculate (i) slip, (ii) Net power, (iii) rotor copper loss per phase, (iv) Rotor efficiency
(v) Rotor resistance per phase if rotor current is 60A in running condition. (10M)
10. a) Explain how the rotating magnetic field is created in three phase induction motor.
(7M)
b) A 1000 V, 50 HZ, 3-phase induction motor has star connected stator. The ratio of stator to rotor is
3.6 the standstill impedance of rotor per phase is 0.01+j 0.2 ohm calculate (i) Rotor current at start
(ii) Rotor P.F at start (iii) Rotor current at slip of 3% (iv) external resistance per phase the rotor
circuit to limit starting rotor current to 200A.
(7M)
11. a) Describe the principle of operation of three phase induction motor. Explain why the rotor is forced
to rotate in the direction of rotating magnetic field (7M)
b) A 20 kW,6 pole ,400 V, 50 Hz, 3 Phase induction motor has a full load slip of 0.02.If the torque
lost in mechanical losses is 20 Nm, find the rotor ohmic losses, motor input and efficiency. Stator
losses are 900 watts.
(7M)
12. a) Develop the equivalent circuit of a polyphase induction motor. Explain how this equivalent circuit
is similar to the transformer equivalent circuit?
(7M)
b) A 10 kW, 400 V, 3 Phase , 4 pole , 50 Hz delta connected induction motor is running at no load
with a line current of 8 A an input power of 660 watts. At full load, the line current is 18 A and the
input power is 11.2 kW. Stator effective resistance per phase is 1.2Ω and friction and windage loss
is 420 watts. For negligible rotor ohmic losses at no load, calculate (i) stator core loss (ii) total
ohmic loss of rotor at full load (iii) full load speed (iv) shaft torque and (v) motor efficiency(7M)
13. a) Develop the phasor diagram for a polyphase induction motor. How does it different from the
phasor diagram of a transformer?
(7M)
b) The power supplied to a three phase induction motor is 40 kW and the corresponding stator losses
are 1.5 kW. Calculate net mechanical power developed and rotor cu losses when the slip is 0.04 per
unit. What will be the net power developed if the speed of the above motor is reduced to 40 % of
the synchronous speed by means of external rotor resistors assuming the torque and stator losses to
remain unaltered? Friction and windage losses may be assumed to be 0.8 kW.
(7M)
SURESH MIKKILI, Associate Professor, EEE Department
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