# Electric Drives Questions and Answers – DC Motors – Three Phase Synchronous Motors

«
»

This set of Electric Drives Multiple Choice Questions & Answers (MCQs) focuses on “DC Motors – Three Phase Synchronous Motors”.

1. The direct axis is taken along ________
a) Inter-polar axis
b) Rotor pole axis
c) In between inter polar and rotor axis
d) Parallel to interpolar axis

Explanation: The direct axis is oriented along the rotor pole axis and the quadrature axis is 90° electrical to rotor pole axis. The direct axis is not oriented along the inter-polar axis.

2. A 4-pole, 3-phase, 60 Hz induction motor is operating at a speed of 1500 rpm. The frequency of the rotor current of the motor in Hz is __________
a) 5
b) 4
c) 2
d) 7

Explanation: Given a number of poles = 4. Supply frequency is 60 Hz. Rotor speed is 1500 rpm. Ns = 120×f÷P = 120×60÷4 = 1800 rpm. S=Ns-Nr÷Ns = 1800-1500÷1800 = .166. F2=sf=.166×60=4 Hz.

3. Calculate the phase angle of the sinusoidal waveform z(t)=8cos(45t+2π÷15).
a) 2π÷39
b) 2π÷15
c) π÷4
d) 2π÷44

Explanation: Sinusoidal waveform is generally expressed in the form of V=Vmsin(ωt+α) where Vm represents peak value, ω represents angular frequency, α represents a phase difference.

4. The leakage flux paths are ________ on the angular position of the rotor.
a) Dependent
b) Proportional
c) Independent
d) Dependent and independent

Explanation: Leakage flux and leakage reactance are constant irrespective of rotor angular position. Armature reaction though is dependent on the angular position of the rotor in salient pole machine but in cylindrical rotor machine, both the quantities are independent of rotor position.

5. The hunting phenomenon in a synchronous motor is also referred to as _________
a) Surging
b) Phase swinging
c) Cogging
d) Surging and phase swinging

Explanation: During hunting, power oscillates so power surges are observed and hence it can be called as surging. Also, the rotor phase angle oscillates and it is called as phase swinging.

6. Which of the following are used in preventing the hunting phenomenon in synchronous generators?
a) Damper bars
b) Short pitch chords
c) Distributed winding
d) Damper bars and short pitch chords

Explanation: Damper bars try to maintain synchronism between the rotating magnetic field and the rotor so they help in preventing hunting. It produces surges in the machine.

7. In a synchronous machine, the phase sequence can be reversed by reversing the _________
a) Rotor direction
b) Field polarities
c) Armature terminal
d) Rotor direction and armature terminal

Explanation: In synchronous generator, the phase sequence is governed by the direction of rotation of the rotor and in a synchronous motor, the phase sequence governs the direction of rotation of the rotor.

8. The slope of the V-I curve is 7°. Calculate the value of resistance. Assume the relationship between voltage and current is a straight line.
a) .122 Ω
b) .360 Ω
c) .377 Ω
d) .578 Ω

Explanation: The slope of the V-I curve is resistance. The slope given is 7° so R=tan(7°)=.122 Ω. The slope of the I-V curve is reciprocal of resistance.

9. In an induction motor, when the number of stator slots is not equal to an integral number of rotor slots _________
a) There may be a discontinuity in torque slip characteristics
b) A high starting torque will be available
c) The machine performs better
d) The machine may fail to start

Explanation: When the number of stator slots is not an integral multiple of a number of rotor slots the machine will not fail to start. It does not cause the cogging phenomenon.

10. A 3-phase induction motor runs at almost 1500 rpm at no load and 900 rpm at full load when supplied with power from a 50 Hz, 3-phase supply. What is the corresponding speed of the rotor field with respect to the rotor?
a) 300 revolution per minute
b) 400 revolution per minute
c) 600 revolution per minute
d) 500 revolution per minute

Explanation: Supply frequency=50 Hz. No-load speed of motor = 1500 rpm. The full load speed of motor=900 rpm. Since the no-load speed of the motor is almost 1500 rpm, hence synchronous speed near to 1500 rpm. Speed of rotor field=1500 rpm. Speed of rotor field with respect to rotor = 1500-900 = 600 rpm.

11. For a practical synchronous motor, the pull-out torque will occur when the torque angle is nearly equal to ________
a) 0°
b) 30°
c) 45°
d) 75°

Explanation: In a practical synchronous motor, the armature resistance cannot be neglected and hence the pull-out occurs at delta=beta which is the impedance angle and is practically 75°.

12. Calculate the active power in an 8 Ω resistor with 8 A current flowing through it.
a) 512 W
b) 514 W
c) 512 W
d) 518 W

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 90°. P = I2R = 8×8×8 = 512 W.

Sanfoundry Global Education & Learning Series – Electric Drives.

To practice all areas of Electric Drives, here is complete set of 1000+ Multiple Choice Questions and Answers. 