INDUCTION AND SYNCHRONOUS MACHINES
UNIT – II-Characteristics and Testing Methods of Induction Motors
Torque equation – expressions for maximum torque and starting torque – torque slip characteristic –
double cage and deep bar rotors – crawling and cogging – speed control of induction motor with V/f
control method – no load and blocked rotor tests – circle diagram for predetermination of performance –
induction generator operation (Qualitative treatment only)
JNTUK Previous Questions
1. a) Compare the relative merits and demerits of cage rotor and wound rotor induction motors of the
same power rating
[7M]
b) A 6-pole, 50Hz, 3-f induction motor at what value of slip does the maximum torque obtain? and a
motor has a full load slip of 0.04. The maximum torque is twice the full load torque.
[8M]
2. a) Explain the speed control of induction motor with V/f control method.
[6M]
b) A 3-φ, 15.38 kW, 415V, 50Hz star connected Induction motor yielded the following data on testing.
No load test : 415V 7.4A, cos φ0 = 0.25
Blocked rotor test: 220V 48A cos φS = 0.42
From the circle diagram find i) line current, ii) Power factor and iii) Efficiency at rated load?
3. a) Explain the phenomenon of cogging and crawling in the 3-phase induction motor.
(7M)
b) Explain the method of speed control of 3-phase induction motor by varying the supply frequency.
4. a) Explain the no-load and blocked rotor tests and also procedure to predetermine the efficiency using
circle diagram of a three phase induction motor.
(7M)
b) A three phase induction motor has a starting torque of 100% and a maximum torque of 200% of the
full-load torque. Determine :
i) slip at which maximum torque occurs
ii) full-load torque and iii) rotor current at starting in per unit of full-load rotor current.
5. Draw the circle diagram from no-load and short circuit test on a 3-phase, 14.92 kW, 440V, 6-pole
induction motor from the following results:
No-load : 400V , 11A, PF=0.2
Short-circuit test : 100V, 25A, PF=0.4
Rotor Cu losses at stand still is half the total Cu losses from the circle diagram, find
(i) Line current, slip, efficiency and pf at full load
(ii) Maximum torque.
6. a) Explain in detail about torque slip and torque speed characteristics.
(7M)
b) How the equivalent circuit parameters are obtained for an induction motor.
(7M)
7. a) Explain the procedure of drawing the circle diagram of an induction motor. What information can be
drawn from the circle diagram.
(7M)
b) What is the purpose of using deep bar cage rotors? Explain the construction and working principle
of a deep-bar cage motor.
8. a) Explain the phenomenon of crawling and cogging in a three phase induction motor?
(7M)
b) Derive the condition for maximum torque for an induction motor?
(7M)
9. A 400V, 40 H.P, 50 Hz, 4-pole delta connected induction motor give the following test data:
No-load test : 400V, 20A, 1200W
Blocked rotor test : 100V, 45A, 2800W
Draw the circle diagram and determine
(i) full load line current and power factor (ii) maximum output power (iii) full-load rotor speed
Assume stator and rotor Cu losses to be equal at stand still.
(14M)
10. a) Explain the torque slip characteristic of 3-phase induction motor.
(7M)
b) What is induction generator? Discuss the principle of operation of induction generator?
(7M)
11. a) Draw the torque-slip characteristics of a 3-phase induction motor. Explain them briefly? (7M)
b) A 3-phase, 400 V, 50 Hz induction motor has a rotor resistance of 0.1Ω and standstill reactance of
0.9Ω per phase. The full load slip is 4%, Calculate (i) Full load torque as a percentage of maximum
torque and the value of extra resistance to be added in the rotor circuit to have 80% of maximum
torque at start. (7M)
SURESH MIKKILI, Associate Professor, EEE Department
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INDUCTION AND SYNCHRONOUS MACHINES
12. a) Explain the phenomena of cogging and crawling in three phase induction motor
(7M)
b) The short circuit current of SCIM on normal voltage is 3 times the full load current and the full
load slip ids 4%. Determine the percentage tapping required to an autotransformer starter to start
the motor against 1/3rd full load torque. Neglect magnetizing current.
(7M)
13. a) Explain how the circle diagram for a poly-phase induction motor can be drawn from its test data.
b) For a three phase induction motor, the rotor ohmic loss at maximum torque is
16 times that at full load torque. The slip at full load torque is 0.03.If the stator resistance and
rotational losses are neglected, then calculate
(i) The slip at maximum torque
(ii) The max. torque in terms of full load torque (iii) The starting torque in terms of full load torque
SURESH MIKKILI, Associate Professor, EEE Department
Page 2