Induction Machine -IAS previous year questions.....
1. Consider the following statements regarding the equivalent circuit parameters of an induction motor :
1. Leakage reactance is dependent on supply current
2. Magnetising reactance is dependent on the air-gap flux
3. Core loss is dependent on the input voltage and frequency.
4. Rotor resistance is dependent on the speed Of these statements [IAS-1993]
a. 2 & 4 correct
b. 1,3 & 4 correct
c. 1,2,3 & 4 correct
d. 1,2 & 4 correct
2. A 3-phase induction machine operates on 3-phase fixed frequency ac mains at a per unit slip of 1.5 Consider the following statements regarding the operating condition of the machine.
1. It draws electric power from the mains
2. It draws mechanical power through the shaft
3. It delivers electrical power to the mains
4. It delivers mechanical power through the shaft . Of these statements : [IAS-1995]
a. 1 and 2 correct
b. 1 and 4 correct
c. 2 and 3 correct
d. 3 and 4 correct
3. A 3-phase induction motor draws 1000 kVA at a pf of 0.8 lag. A synchronous condenser is connected in parallel to draw an additional 750 kVA ot 0.6 pf lead" The pf of the total load supplied by the mains is [IAS-1996]
a. unity b. 0.707 lead
b. 0.707 lag d. Zero
4. Consider the following statements about the operation of a three-phase induction motor running at full load : If one of the supply line fuse blows off, the motor will
1. stall
2. continue to run with increased slip,
3. continue to run with normal supply current
4. continue to run with excessive supply current
Of these statements [IAS-1997]
a. 1 alone correct
b. 3 alone correct
c. 2 and 3 correct
d. 2 and 4 correct
5. Consider the following statements: As a 3-phase induction motor is loaded from no load to rated load,
1.there is an improvement in the power factor.
2. the torque increases almost in proportion to slip.
3. the air-gap flux falls sharply. Of these statements [IAS-1998]
a. 1, 2 and 3 correct
b. 1 and 3 correct
c. 1 and 2 correct
d. 2 and 3 correct
6. A 3-phase induction motor has a full-load slip of 3 per cent at normal voltage. Which one of the following will be the value of the slip of the motor if it develops the same torque theoretically while operating at 110 per cent of its normal voltage? [IAS-1999]
a. 2.48 % b. 0.248%
c. 0.483% d. 4.83%
7. Two separate induction motors having 6 poles and 4 poles are cascaded 3-phase, 400 V, 60Hz is the supply to the motors. The synchronous speeds (in rpm) of cascaded set will be [IAS-1999]
a. 720, 1200, 1800 and 2400
b. 600, 1000, 1500 and 3000
c. 720, 1200, 1800 and 3600
d. 600, 1200, 1800 and 2400
8. In a 3-phase induction motor,the rotor impedance angle at standstill is e. The load angle will be [IAS-2001]
a. Pi + thita
b. Pi - thita
c. Pi/2 + thita
d. Pi/2
9. The power input to an induction motor is 50 kW when it is running at 4% slip. If the stator resistance and core losses are neglected, the rotor copper loss is equal to [IAS-2002]
a. 38Kw b. 20 Kw c. 4Kw d. 2Kw
10. Under variable frequency operation of 3-phase induction motor air gap flux is kept constant by adjusting the motor voltage.For constant air gap flux machine develops the same torque when the [IAS-2003]
a. Slip is kept constant
b. Slip speed is kept constant
c. power flow across the air gap is kept constant
d. Power input to stator is kept constant
11. 1) Match List -I (Motor) with List -II (Application) and select the correct answer using the code given below the Lists :
List-I (Motor)
A. Hysteresis motor B. Shaded-pole motor C. Reluctance motor
D. Universal motor
List-II (Application)
1. Photocopying machine
2. Phonograph turntable
3. Electric Shaver
4. Record player [IAS-2006]
CODES. A B C D
a. 4 3 2 1
b. 2 1 4 3
c. 4 1 2 3
d. 2 3 4 1
12. In an induction motor, what will happen if air-gap is increased ?
[IAS-2008]
a. Speed will reduces
b. Efficiency will improves
c. Power factor will lowered
d break-down torque will reduces
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