# Electric Drives Questions and Answers – DC Motors Characteristics – Basic Relations

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This set of Electric Drives Multiple Choice Questions & Answers (MCQs) focuses on “DC Motors Characteristics – Basic Relations”.

1. Swinburne’s test can be conducted on ___________
a) Series motor
b) Shunt motor
c) Compound motor
d) Shunt and compound motor

Explanation: The test is practically applicable for machines which have flux constant like the shunt and compound machines as this is a no-load test and 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

Explanation: The generated e.m.f can be calculated using the formula Eb = Φ×Z×N×P÷60×A, Φ represent 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

Explanation: The active power in the electrical circuits is a useful power. It determines the power factor of the system. It is expressed in terms of Watt. P=VIcosΦ.

4. Calculate the mass of the ball having a moment of inertia 4.5 kgm2 and radius of 14 cm.
a) 229.59 kg
b) 228.56 kg
c) 228.54 kg
d) 227.52 kg

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

Explanation: Field control method is generally used for obtaining the speeds greater than the base speed. It is also known as 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

Explanation: Intensity is defined as the amount of power incident on a particular area. It is mathematically expressed as I = Power incident (Watt)÷Area(m2).

7. Calculate the value of the frequency if the signal completes half of the cycle in 70 sec. Assume signal is periodic.
a) 0.00714 Hz
b) 0.00456 Hz
c) 0.00845 Hz
d) 0.00145 Hz

Explanation: The frequency is defined as the number of oscillations per second. It is reciprocal of 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

Explanation: 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

Explanation: 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 South pole.

Sanfoundry Global Education & Learning Series – Electric Drives. 