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Electrical Engineering
Polyphase Induction Motors
Quiz 3
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Q.1
In squirrel cage induction motors, the rotor slots are usually given slight skew in order to
Reduce windage losses
Reduce eddy currents
Reduce accumulation of dirt and dust
Reduce magnetic hum
Q.2
If an induction motor with certain ratio of rotor to stator slots, runs at
1
/
7
of the normal speed, the phenomenon will be termed as
Humming
Hunting
Crawling
Cogging
Q.3
The 'cogging' of an induction motor can be avoided by
Proper ventilation
Using DOL starter
Autotransformer starter
Having number of rotor slots more or less than the number of stator slots (not equal)
Q.4
In three-phase induction motors sometimes copper bars are placed deep in the rotor to
Improve starting torque
Reduce copper losses
Improve efficiency
Improve power factor
Q.5
The speed characteristics of an induction motor closely resemble the speed-load characteristics of which of the following machines
D.C. series motor
D.C. shunt motor
Universal motor
None of the above
Q.6
For driving high inertia loads best type of induction motor suggested is
Slip ring type
Squirrel-cage type
Any of the above
None of the above
Q.7
An induction motor is
Self-starting with zero torque
Self-starting with high torque
Self-starting with low torque
Non-self starting
Q.8
Lubricant used for ball bearing is usually
Graphite
Grease
Mineral oil
Molasses
Q.9
For which motor the speed can be controlled from rotor side?
Squirrel-cage induction motor
Slip-ring induction motor
Both (A) and (B)
None of the above
Q.10
Step-less speed control of induction motor is possible by which of the following methods?
e.m.f. injection in rotor eueuit
Changing the number of poles
Cascade operation
None of the above
Q.11
The starting torque of a 3 phase squirrel cage induction motor is
Twice the full load torque
1.5 times the full load torque
Equal to full load torque
Any of the above
Q.12
A pump induction motor is switched on to a supply 30% lower than its rated voltage. The pump runs. What will eventually happen? It will
Stall after sometime
Stall immediately
Continue to run at lower speed without damage
Get heated and subsequently get damaged
Q.13
The circle diagram for an induction motor cannot be used to determine
Efficiency
Power factor
Frequency
Output
Q.14
A 500 kW, 3-phase, 440 volts, 50 Hz, A.C. induction motor has a speed of 960 r.p.m. on full load. The machine has 6-poles. The slip of the machine will be
0.01
0.02
0.03
0.04
Q.15
It is advisable to avoid line starting of induction motor and use starter because
It will run in reverse direction
It will pick up very high speed and may go out of step
Motor takes five to seven times its full load current
Starting torque is very high
Q.16
A double squirrel-cage induction motor has
Two rotors moving in opposite direction
Two parallel windings in stator
Two parallel windings in rotor
Two series windings in stator
Q.17
The term 'cogging' is associated with
Three-phase transformers
Compound generators
D.C. series motors
Induction motors
Q.18
Which is of the following data is required to draw the circle diagram for an induction motor?
Block rotor test only
No load test only
Block rotor test and no-load test
Block rotor test, no-load test and stator resistance test
Q.19
The low power factor of induction motor is due to
Rotor leakage reactance
Stator reactance
The reactive lagging magnetizing current necessary to generate the magnetic flux
All of the above
Q.20
The injected e.m.f. in the rotor of induction motor must have
Zero frequency
The same frequency as the slip frequency
The same phase as the rotor e.m.f.
High value for the satisfactory speed control
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