Electric Drive Characteristics of DC and AC Motor MCQ [Free PDF]

1. Swinburne’s test can be conducted on ___________

A. Series motor
B. Shunt motor
C. Compound motor
D. Shunt and compound motor

Answer: D

Swinburne’s test is practically applicable for machines that have flux constant like the shunt and compound machines as this is a no-load test and the DC series motor should not be run at no-load because of high speed. 


2. The generated e.m.f from 20-pole armature having 800 conductors driven at 30 rev/sec having flux per pole as 60 mWb, with 16 parallel paths is ___________

A. 1900 V
B. 1840 V
C. 1700 V
D. 1800 V

Answer: D

The generated e.m.f can be calculated using the formula

Eb = Φ×Z×N×P÷60×A


Φ represents flux per pole
Z represents the total number of conductors
P represents the number of poles
A represents the number of parallel paths
N represents speed in rpm

Eb = .06×20×1800×800÷60×16 = 1800 V. 


3. The unit of active power is Watt.

A. True
B. False

Answer: A

The active power in the electrical circuits is useful to power. It determines the power factor of the system. It is expressed in terms of Watt.



4. Calculate the mass of the ball having a moment of inertia of 4.5 kgm2 and a radius of 14 cm.

A. 229.59 kg
B. 228.56 kg
C. 228.54 kg
D. 227.52 kg

Answer: A

The moment of inertia of the ball can be calculated using the formula I=∑miri2.

The moment of inertia of the ball and radius is given.

M=(4.5)÷(.14)2 = 229.59 kg.

It depends upon the orientation of the rotational axis. 


5. The field control method is suitable for constant torque drives.

A. True
B. False

Answer: B

The field control method is generally used for obtaining speeds greater than the base speed. It is also known as the flux weakening method. It is suitable for constant power drives. 


6. What is the unit of the intensity?

A. Watt/m2
B. Watt/m
C. Watt/m4
D. Watt/m3

Answer: A

Intensity is defined as the amount of power incident in a particular area. It is mathematically expressed as

I = Power incident (Watt)÷Area(m2). 


7. Calculate the value of the frequency of the signal that completes half of the cycle in 70 sec. Assume the signal is periodic.

A. 0.00714 Hz
B. 0.00456 Hz
C. 0.00845 Hz
D. 0.00145 Hz

Answer: A

The frequency is defined as the number of oscillations per second. It is reciprocal to the time period. It is expressed in Hz. The given signal completes half of the cycle in 70 seconds then it will complete a full cycle in 140 seconds.

F = 1÷T=1÷140=.00714 Hz. 


8. The slope of the V-I curve is 26°. Calculate the value of resistance. Assume the relationship between voltage and current is a straight line.

A. .487 Ω
B. .482 Ω
C. .483 Ω
D. .448 Ω

Answer: A

The slope of the V-I curve is resistance. The slope given is 26° so

R=tan(26°) = .487 Ω.

The slope of the V-I curve is resistance. 


9. For large DC machines, the yoke is usually made of which material?

A. Cast steel
B. Cast iron
C. Iron
D. Cast steel or cast iron

Answer: A

Yoke in DC machines is made up of cast steel. Yoke provides structural support and mechanical strength to the machine. It helps in carrying the flux from the North pole to the South pole. 


10. Calculate the terminal voltage of the Permanent Magnet DC motor having a resistance of 2 Ω and a full load current of 5 A with 20 V back e.m.f.

A. 30 V
B. 25 V
C. 20 V
D. 31 V

Answer: A

A permanent magnet DC motor is a special type of motor in which flux remains constant. The terminal voltage can be calculated using the relation

Vt = Eb+IaRa = 20+5×2 = 30 V. 

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