Quiz: Electrical Engineering 4 June 2020

Quiz: Electrical Engineering
Exam: UPPSC-AE
Topic: Single phase induction motor

Each question carries 1 mark.
Negative marking: 1/4 mark
Time: 10 Minute

Q1. Speed higher than synchronous speed can be held by
(a) Line voltage control
(b) rotor slip power control
(c) rotor resistance control
(d) frequency control

Q2. In a slip power recovery scheme for 3 phase induction motor, if slip power is
(a) returned to supply, constant power drive is obtained
(b) added to the main shaft, constant power drive is obtained
(c) subtracted from the main shaft, constant torque drive is obtained
(d) obtained from the supply, constant torque drive is obtained

Q3. During plugging of an induction motor
(a) phase sequence is reverted
(b) a dc source is connected to stator
(c) one phase is open circuited
(d) power is fed back to mains

Q4. In dynamic breaking of 3 phase induction motors
(a) the supply terminals of any two stator phases are interchanged
(b) any two stator terminals are earthed
(c) the stator terminals are switched over to a dc source from the ac supply
(d) a dc voltage is injected in the rotor circuit

Q5. The stator of 2/4 pole changing cage motor is initially wound for 2 poles. The reconnection of the stator winding to 4-pole through a changeover switch, while the motor is running would result in
(a) constant torque drive
(b) constant hp drive
(c) plugging to stand still
(d) regenerative breaking to half the original speed.

Q6. A squirrel cage induction motor runs slow, probably cause could be
(a) low voltage
(b) short stator coil
(c) one phase open
(d) Any of the above

Q7. Three phase induction motor should preferably be mounted on
(a) wooden structure
(b) PVC platform
(c) solid concreate foundation
(d) none of the above

Q8. The factors to be considered in the selection of motors is/are
(a) voltage rating of motor
(b) kW output rating of motor
(c) speed and frame enclosure size
(d) all of the above

Q9. Induction generators deliver power at
(a) leading power factor only
(b) lagging power factor only
(c) leading as well as lagging power factor
(d) unity power factor only

Q10. What is the rotor copper loss of a 3 phase 550 volts, 50 Hz, 6 pole induction motor developing 4.1 kW at the shaft with mechanical loss of 750 W T 970 rpm?
(a) 275 W
(b) 150 W
(c) 100 W
(d) 250 W

Solutions
S1. Ans. (b)
Sol. rotor slip power control are best suited for mentioned situation.

S2. Ans. (b)
Sol. added to the main shaft, constant power drive is obtained

S3. Ans. (a)
Sol. phase sequence is reverted for desired result quickly.

S4. Ans. (c)
Sol. the stator terminals are switched over to a dc source from the ac supply

S5. Ans. (d)
Sol. regenerative breaking to half the original speed.

S6. Ans. (d)
Sol. All the condition result in slow running of motor. Over load and open stator coil may be additional reason for slow running.

S7. Ans. (d)
Sol. solid concreate foundation

S8. Ans. (d)
Sol. All the mentioned factors should be considered.

S9. Ans. (a)
Sol. Induction generators deliver power at leading power factor only.

S10. Ans. (b)
Sol. Synchronous speed,
Ns = 120f/P
=(120×50)/6 = 1000 rpm
=(N_s-N)/N_s = (1000-970)/1000
=0.03
Mechanical Power developed,
Pmech = Shaft power + mechanical loss
=4100+750 W
= 4850 W
Rotor copper loss,
= s/(1-s) ×4850 W
= 150 W

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