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This set of Electric Drives Multiple Choice Questions & Answers (MCQs) focuses on “Dynamics of Motor-Load Combination”.

1. The axis along which no emf is produced in the armature conductors is called as ____________
a) Geometrical Neutral Axis (GNA)
b) Magnetic Neutral Axis (MNA)
c) Axis of rotation
d) Axis of revolution

Explanation: The coil undergoing commutation must lie along the magnetic neutral axis so that no emf is induced and there is no sparking at the time of commutation.

2. The generated e.m.f from 25-pole armature having 200 conductors driven at 10 rev/sec having flux per pole as 20 mWb, with two parallel paths is ___________
a) 400 V
b) 500 V
c) 200 V
d) 300 V

Explanation: The generated 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 = .02×25×200×600÷60×2= 500 V.

3. The unit of the flux is Weber.
a) True
b) False

Explanation: Flux is the total amount of magnetic field lines passing through a given area. Φ is a dot product of magnetic flux density and area. The unit of the flux is Weber (Wb).

4. Which of the following motor can be referred as a universal motor?
a) DC shunt motor
b) DC compound motor
c) Permanent magnet motor
d) DC series motor

Explanation: DC series motor can operate on DC and AC. It is a universal motor. Universal motors are those motors that can operate on both DC and AC. DC shunt motor can only operate on DC because of pulsating torque in AC.

5. The phase difference between voltage and current in the inductor.
a) 45°
b) 90°
c) 80°
d) 55°

Explanation: In the case of an inductor, the voltage leads the current by 90° or the current lags the voltage by 90o. The phase difference between voltage and current is 90°.

6. The phase difference between voltage and current in the resistor.
a) 85°
b) 90°
c) 0°
d) 5°

Explanation: In the case of a resistor, the voltage and current are in the same phase. The phase difference between voltage and current is 0°. The voltage drop in the resistor is given as V=IR.

7. The phase difference between voltage and current in the capacitor.
a) 90°
b) 80°
c) 95°
d) 91°

Explanation: In the case of a capacitor, the voltage lags the current by 90° or the current leads the voltage by 90o. The phase difference between voltage and current is 90°.

8. The slope of the I-V curve is 30°. Calculate the value of resistance. Assume the relationship between I and V is a straight line.
a) 1.732 Ω
b) 2.235 Ω
c) 1.625 Ω
d) 1.524 Ω

Explanation: The slope of the I-V curve is reciprocal of resistance. The slope given is 30° so R=1÷tan(30°)=1.732 Ω. The slope of the V-I curve is resistance.

9. What is a mark to space ratio?
a) Ton÷Toff
b) Ton÷(Ton- Toff)
c) Ton÷2×(Ton*Toff)
d) Ton÷2×Toff

Explanation: Mark to space is Ton÷Toff. It is the ratio of time for which the system is active and the time for which is inactive. It has no unit.

Sanfoundry Global Education & Learning Series – Electric Drives. 