# Electric Drives Questions and Answers – Induction Motors – Controlling Speed Using Rotor Resistance

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This set of Electric Drives Multiple Choice Questions & Answers (MCQs) focuses on “Induction Motors – Controlling Speed Using Rotor Resistance”.

1. What is the formula for the maximum active power in the cylindrical rotor synchronous machine? (Eb represents armature emf, Vt represents terminal voltage, δ represents rotor angle, X represents reactance)
a) Eb×Vt×sinδ÷X
b) Eb×Vt2÷X
c) Eb2×Vt×sinδ÷X
d) Eb×Vt÷X

Explanation: The real power in the cylindrical rotor machine is Eb×Vt×sinδ÷X. It is inversely proportional to the reactance. The stability of the machine is decided by the maximum power transfer capability. It is maximum for delta angle=90°.

2. Cylindrical pole machines are more stable than salient rotor machines.
a) True
b) False

Explanation: Cylindrical pole machines are less stable than salient rotor machines because of the less short circuit ratio and less real power transfer capability. The air gap length in salient pole machines is more as compare to cylindrical rotor machines.

3. The unit of oscillating power is VAR.
a) True
b) False

Explanation: The oscillating power is the waste power in case of electric circuits. It is the energy trapped that keeps on oscillating. It is expressed in Volt Ampere reactive.
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4. Calculate the power factor if the power angle is 36.86°.
a) .8
b) .4
c) .6
d) 1

Explanation: Power factor is the ratio of the real power to the apparent power. It measures the useful power contained in the total power.Φ=cos-1(36.86)=.8.

5. Calculate the reactive power in a 2.2 Ω resistor.
a) 1.2 VAR
b) 2.6 VAR
c) 0 VAR
d) 1.9 VAR

Explanation: The resistor is a linear element. It only absorbs real power and dissipates it in the form of heat. The voltage and current are in the same phase in case of the resistor so the angle between V & I is 0°. Q=VIsin0° = 0 VAR.
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6. What is the unit of the distance?
a) m
b) m/s
c) atm/m
d) Volt

Explanation: Distance is the total length traced by the body. It is a scalar quantity. It is expressed in the meter. It is not a tensor quantity. It has no direction.

7. Calculate the reactive power in a 7.2545 Ω resistor.
a) 1.221 VAR
b) 2.611 VAR
c) 0 VAR
d) 1.997 VAR

Explanation: The resistor is a linear element. It only absorbs real power and dissipates it in the form of heat. The voltage and current are in the same phase in case of the resistor so the angle between V & I is 0°. Q=VIsin0° = 0 VAR.

8. The slope of the V-I curve is 180o. Calculate the value of resistance. The graph is parallel to the x-axis.
a) 1.47 Ω
b) 1.81 Ω
c) 0 Ω
d) 4.12 Ω

Explanation: The slope of the V-I curve is resistance. The slope given is 180° so R=tan(180°)=0 Ω. The slope of the V-I curve is resistance. It behaves as a short circuit.

9. Calculate the value of the duty cycle if the system is on for 1 sec and off for 0 sec.
a) 1
b) .4
c) .1
d) .6

Explanation: Duty cycle is Ton÷Ttotal. It is the ratio of time for which the system is active and the time taken by the signal to complete one cycle. D = Ton÷Ttotal=5÷5=1.

10. The phase difference between voltage and current in the choke coil.
a) 90°
b) 40°
c) 50°
d) 68°

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

11. Armature reaction is demagnetizing in nature due to a purely capacitive load in the synchronous generator.
a) True
b) False

Explanation: Due to a purely capacitive load, armature current is in opposite phase with the field magneto-motive force. Armature magneto-motive force produced due to this current will be in opposite phase with the field flux. It will try to reduce the net magnetic field.

12. Armature reaction is magnetizing in nature due to a purely resistive load in the synchronous motor.
a) True
b) False

Explanation: Due to a purely resistive load, armature current is in quadrature with the field magneto-motive force. Armature magneto-motive force produced due to this current will be in quadrature with the field flux. It will try to increase the net magnetic field.

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