Synchronous Motor MCQ

141. A 440 V, 3-phase, 10 pole and 50 Hz synchronous motor delivering a torque of 50/π Nm, delivers a power of

1. 50 W
2. 500 W
3. 1000 W
4. 2000 W

142. What is the effect of the field failure of salient pole synchronous motor connected with infinite bus?

1. Reduces motor torque and speed
2. Not change motor torque and speed
3. Stop the motor
4. Reduces motor torque but motor will continue to run at synchronous speed

143. What is the angle between the induced voltage and supply voltage of a synchronous motor under running condition?

1. Zero
2. Greater than zero but 90°
3. Between 90° and 180°
4. > 180°

144. When does a synchronous motor operate with leading power factor current?

1. While it is under-excited
2. While it is critically excited
3. While it is over-excited
4. While it is heavily loaded

145. Which one of the following statement is not correct in respect of synchronous machines?

1. In salient pole machines, the direct-axis synchronous reactance is greater than the quadrature axis synchronous reactance.
2. The damper bars help the motor to self-start.
3. Short circuit ratio is the ratio of field current required to produce the rated voltage on open circuit to the rated armature current.
4. The V-curve of a synchronous motor represents the variation in the armature current with field excitation at a given output power.

146. A synchronous motor is operated from a bus voltage of 1.0 p.u. and is drawing 1.0 p.u. zero power factor leading current. Its synchronous reactance is 0.5 p.u. What is the excitation emf of the motor?

1. 1.5
2. 2.0
3. 0.5
4. 1.0

147. Consider the following

1. Supply voltage
2. Excitation current
3. Maximum value of load angle

The maximum power developed by a synchronous motor is a function of which of the above?

1. i and iii
2. i and ii
3. i, ii and iii
4. ii and iii

148. If the field of a synchronous motor is underexcited, the power factor will be

1. Lagging
2. Leading
3. Unity
4. More than unity

149. When the excitation of normally operating unloaded salient-pole synchronous motor suddenly gets disconnected, it continues to run as a

1. Schrage motor
2. Spherical motor
3. Switched-reluctance motor
4. Variable-reluctance motor

150. The power factor of a synchronous motor

1. improves with increase in excitation and may even become leading at high excitations
2. decreases with increase in excitation
3. is independent of its excitation
4. increases with loading for a given excitation