Electrical machines MCQs : This section focuses on the "Electrical machines". These Multiple Choice Questions (MCQs) should be practiced to improve the Electrical machines skills required for various interviews (campus interview, walk-in interview, company interview), placement, entrance exam and other competitive examinations.
Question 1
The parameter of an equivalent curcuit of a 3-phase induction motor affected by reducing the rms value of the supply voltage at the rated frequency is-
A. rotor resistance
B. rotor leakage reactance.
C. magnetizing reactance
D. stator resistance
Question 2
Which class has the lowest and the highest temperature?
A. Class Y, Class C
B. Class Y, Class H
C. Class H, Class C
D. Class B, Class H
Question 3
How many number of insulation classes are present with respect to electrical equipment?
A. 5
B. 6
C. 7
D. 8
Question 4
Silicone rubber is an example of Class H.
A. True
B. False
Question 5
A 3-phase induction motor can also be run as ______________
A. synchronous generator
B. induction generator
C. dc shunt motor
D. any of the mentioned
Question 6
Induction generator ____________
A. can not work in isolation
B. can work in isolation
C. should work in parallel with synchronous generators
D. any of the mentioned
Question 7
A self excited induction generator ____________
A. must have static capacitor bank
B. can have static capacitor bank
C. should have static capacitor bank
D. any of the mentioned
Question 8
For an induction motor to work in isolation, it should have ____________
A. lagging reactive power source
B. leading reactive power source
C. leading reactive power sink
D. any of the mentioned
Question 9
The magnetization curve in the induction generator working in isolation has similar characteristic as ____________
A. dc shunt generator
B. dc cumulative field generator
C. synchronous generator
D. all of the mentioned
Question 10
If the residual flux is absent in the rotor iron of the isolated induction generator ____________
A. it will not run
B. it will run as induction motor
C. it will burn the winding
D. it will not affect the characteristic of the generator
Question 11
What is/are the shortcomings of an induction generator in real time system?
A. It needs considerable amount of reactive power
B. Efficiency is poor
C. It can work at leading power factor only
D. All of the mentioned
Question 12
Induction generator is self excited machine.
A. True
B. False
Question 13
Induction generator can only work at leading reactive power.
A. True
B. False
Question 14
Which is the most acceptable application field where an induction generator can be used?
A. wind power system
B. steam power system
C. hydro power system
D. nuclear power station
Question 15
Voltage induce in the induction motor is highest at __________
A. starting
B. standstill
C. rated speed
D. any of the mentioned
Question 16
Starters are required in the induction motor because __________
A. of high starting current
B. they are not self starting
C. torque produced is very low at starting to overcome inertia
D. any of the mentioned
Question 17
Full voltage starting of induction motor may cause __________
A. dip in the voltages
B. high inrush current
C. high losses
D. All of the above
Question 18
External methods of induction motor starting is employed to __________
A. Reduce the voltage
B. Reduce the current
C. Improve efficiency
D. All of the mentioned
Question 19
Series reactor method of starting has main disadvantage of __________
A. lowering the starting power
B. increasing the starting current
C. increasing starting torque
D. All of the mentioned
Question 20
Series reactor method of starting has main disadvantage of lowering the starting power. This can be overcome by __________
A. using series resistance
B. using parallel reactor
C. using parallel resistance
D. using series reactor
Question 21
For the application requiring high starting torque and minimum starting current, the most suitable starting method will be?
A. Auto transformer starting method
B. Reactance starting method
C. DOL method
D. Star-Delta method
Question 22
Auto transformer method of starting has most acceptable method for the starting of the induction motor due to __________
A. maximum torque
B. maximum reduction of inrush current
C. variable tapping
D. All of the above
Question 23
The primary and secondary of a transformer are ________ coupled but _______ connected.
A. magnetically, not electrically
B. electrically, not magnetically
C. magnetically, also magnetically
D. electrically, also electrically
Question 24
We can employ transformers for a power range of _________
A. lower and higer values
B. lower values
C. higher values
D. medium values
Question 25
A transformer has comparitively much higher efficiency than a similar induction machine due to _________
A. small air gaps
B. no moving parts
C. strong coupling
D. All of the above
Question 26
Which is the most widely used material in the core of the transformer?
A. cold rolled grain oriented sheet steel
B. cold rolled grain steel
C. soft iron
D. steel
Question 27
The leakage flux is the flux in side the transformer which?
A. links either of the windings
B. links both of the windings
C. yoke of the core
D. windows of the core
Question 28
It is advised that staggering of the butt joints _________
A. reduces reluctance of the path
B. increases air gap
C. increases mechanical strength
D. All of the above
Question 29
Transformers do not require any type of cooling as it well ventilated.
A. True
B. False
Question 30
A coupling magnetic field inside a rotating machine or static machine like transformers must involve with _________
A. mechanical parts
B. electrical parts
C. both electrical and mechanical
D. either of the electrical or mechanical parts
Question 31
Stepping of the core is implemented in the core to _________
A. reduce conductor material and copper losses
B. reduce core loss
C. to provide mechanical strength
D. to reduce magnetizing current
Question 32
Core type transformers have, LV and HV windings are arranged such that _________
A. Half LV near the core and half HV outside LV on each limb
B. LV one one limb and HV on the other
C. Half LV outside the core and half HV inside LV on each limb
D. LV and HV windings are sandwiched
Question 33
The shell type transformers have, LV and HV windings are arranged such that _________
A. Half LV near the core and half HV outside LV on each limb
B. LV one one limb and HV on the other
C. Half LV outside the core and half HV inside LV on each limb
D. LV and HV windings are sandwiched
Question 34
The PWM control of DC motor varies _________
A. linearly with speed
B. inversely with speed
C. parabolically with speed
D. exponentially with speed
Question 35
Ward-Leonard system of system of speed control is not recommended for _________
A. constant speed operation
B. Wide speed
C. frequent-motor reversed
D. very slow speed
Question 36
At a very low speed, increase in field resistance will _________
A. decrease the speed of motor
B. increase the speed of motor
C. not have signicant effect on speed
D. no effect
Question 37
Small DC motors have best speed control by _________
A. armature voltage control
B. field resistance control
C. any of the methods
D. None of the above
Question 38
To implement armature voltage control, it must be ensured that _________
A. it is used on separately excited machine
B. it is used on shunt machine
C. it is used on series machine
D. any of the mentioned machine
Question 39
Armature voltage control works for speeds _____ base speed and field resistance control works well for speed _____ base speed.
A. below, above
B. above, below
C. above, above
D. below, below
Question 40
The torque limit of speed for a shunt motor _________
A. remains constant till base speed
B. remains constant after base speed
C. varies linearly after base speed
D. varies inversely till base speed
Question 41
Run-away for large DC machine is avoided by using _________
A. a turn or two of cumulative compounding
B. compensating winding
C. stablizer winding
D. All of the above
Question 42
Why does DC motor sometimes run too fast when under-loaded?
A. Due to weak field
B. Due to high line voltage
C. Due to brush-shifted to neutral
D. All of the above