Electrical Machines - Induction Machines

Induction Machines - IES previous year questions....

1. The rotor power output of a 3-phase induction motor is 15kWand the corresponding slip is 4%. The rotor copper loss will be [IES-1997]
a. 600w  b. 625w  c. 650w  d. 700w

2. A 3-phase induction motor fed a 3-phase voltage controller is suitable for driving loads whose torque is [IES-1998]
a. Irrespective of the speed
b. Invertially proportional to square of the speed
c. Constantly proportional to the square of speed
d. Invertially proportional to the speed

3. Which one of following statements regarding a capacitor start, capacitor run single-phase induction motor is correct? [IES-1999]
a. One of the capacitors is in the circuit during starting and both are in the circuit during running
b. Both the capacitors are in the circuit during starting and one of them is in the circuit during running
c. Both the capacitors are in the circuit during starting and running
d. One of the capacitors is in the circuit during starting and the other is in the circuit during running.

4. A 3-phase slip-ring induction motor is wound for 4 poles on stator and 6 poles on rotor. When 3-phase balanced voltage source at 50 Hz is applied to the motor, it will run at : [IES-2000]
a. 1500 rpm              b. 1000 rpm
c. 750 rpm                 d. Zero

5. An induction motor when started on load does not accelerate up to full speed but runs at 1/7th of the rated speed. The motor is said to be [IES-2001]
a. Locking                b. Plugging
c. Crawling              d. Coggling

6. Assertion(A): A single -phase induction motor is not self starting. Reason(R): A three-phase induction motor is self starting [IES-2002]
a. Both A and R are true and R is the correct explanation of A
b. Both A and R are true but R is not the correct explanation of A
c.  A is true but R is false
d.  A is false but R is true

7. Match List-I(Machines) with List -II(Tests) and select the correct answer:
List-I
A. Transformer
B. DC Motor
C. Alternator
D. Induction Motor
List-II
1. Slip Test
2. Blocked rotor test
3. Sumpner's Test
4. Swinburne's Test [IES-2003]
a. A-3, B-4, C-1, D-2
b. A-3, B-4, C-2, D-1
c.  A-4, B-3, C-1, D-2
d.  A-4, B-3, C-2, D-1

8. Which one of the following statement is correct?In an induction motor, if the air gap is increased, [IES-2004]
a. Speed reduces
b. Efficiency increases
c. Break down talk reduces
d. Power factor reduces

9. A starting torque of 80Nm is developed in an induction motor by an auto- transformer starter with a tapping of 30%. If the tapping of auto-transformer starter is 60%, then what is the starting torque? [IES-2005]
a. 40 Nm             b. 160 Nm
c. 240 Nm           d. 320 Nm

10. If the load on an induction motor is increased from no load to full load, its slip and the power factor will, respectively  [IES-2006]
a. Decrease, Decrease
b. Decrease, Increase
c. Increase, Decrease
d. Increase, Increase

11. Which one of the following is the correct statement?In a 3-phase induction motor, the resultant flux of a constant nature and is  [IES-2007]
a. equal to (flux)m ,where (flux)m is maximum flux due to an phase
b. 1.5 time maximum value of flux due to any phase
c. Square root of 3/2 time maximum value of flux due to any phase
d. 3 times of (flux)m

12. Why is a centrifugal switch used in a single-phase induction motor [IES-2008]
a. To protect the motor from over loading
b. To improve starting performance of the motor
c. To cut off the starting winding at an appropriate instant
d. To cut in the capacitor during runing of the motor

13.  A three phase 6 pole 50 Hz induction motor is running at 5% slip. What is the speed of the motor? [IES-2009]
a.800 rpm            b. 850 rpm
c.950 rpm            d. 1000 rpm

14. The MMF produced by the rotor currents of a 3-phase induction motor [IES-2010]
a. Rotates at the speed of rotor in the air gap
b. is at stand with respect to stator MMF
c. Rotates at slip speed with respect to stator MMF
d. Rotates at synchronous speed with respect to rotar 

15.  Assertion (A) : A squirrel cage induction motor is preferred to the slip ring induction motor. Reason (R): Higher starting torque is the main consideration. [IES-2011]
a. Both A and R are true and R is the correct explanation of A
b. Both A and R are true but R is not the correct explanation of A
c.  A is true but R is false
d.  A is false but R is true

16. A 15 kW ,400 V, 4-pole ,50 Hz, star -connected 3 - phase induction motor has full-load slip of 4 %. The output torque of the machine at full load is  [IES-2012[
a. 1.66 Nm                b. 95.5 Nm
c. 99.47 Nm              d. 624.73 Nm

17. In a synchronous machine the rotor speed becomes more than the synchronous speed during hunting, the damping bars develop [IES-2013]
a. Synchronous motor torque
b. DC motor torque
c. Induction motor torque
d. Induction generator torque