Quiz: Electrical Engineering 19 May 2021

Quiz: Electrical Engineering
Exam: DFCCIL
Topic: Synchronous motor

Each question carries 1 mark.
Negative marking: 1/4 mark
Time: 10 Minute

Q1. The rotor of which motor does not have winding on it?
(a) Repulsion motor
(b) Universal motor
(c) Reluctance motor
(d) Hysteresis motor

Q2. The ratio of starting torque to running torque in a synchronous motor is:
(a) 0
(b) 1
(c) 2
(d) ꝏ

Q3. When the field circuit of an unloaded salient pole synchronous motor gets suddenly open circuited, then………
(a) the motor stops
(b) it runs at a slower speed
(c) it continues to run at the same speed
(d) it runs at a very high speed

Q4. Hunting occurs in a/an………………motor.
(a) Induction
(b) Synchronous
(c) Dc shunt
(d) Dc series

Q5. If the field of the synchronous motor is under-excited, the power factor will be:
(a) Lagging
(b) Leading
(c) Zero
(d) Unity

Q6. If the field of the synchronous motor is over-excited, the power factor will be:
(a) Lagging
(b) Leading
(c) Zero
(d) Unity

Q7. Damper winding is used in a synchronous machine to?
(a) eliminate hunting
(b) improve efficiency
(c) reduce temperature rise
(d) improve power factor

Q8. In synchronous motor damper winding is placed at:
(a) stator
(b) rotor
(c) both stator and rotor
(d) yoke

Q9. An over excited synchronous motor is used for…………
(a) variable speed loads
(b) low torque loads
(c) power factor corrections
(d) high torque loads

Q10. In the salient pole of 3-phase synchronous machine the rotor will have………….
(a) Larger diameter, larger axial length
(b) Smaller diameter, smaller axial length
(c) Larger diameter, shorter axial length
(d) Small diameter, larger axial length

SOLUTIONS
S1. Ans.(d)
Sol. A Hysteresis Motor is a synchronous motor with a uniform air gap and without DC excitation.
The rotor of the motor is made up of smooth chrome steel cylinder and it has no winding.

S2. Ans.(a)
Sol. when supply is given to stator starts rotating and stator poles too rotate at synchronous speed. these poles interchange their position due to alternating nature of current. the rotor cannot pick up the quick reversal of stator. Due to this clockwise and counter clockwise directions of torque will develop making average torque zero.

S4. Ans.(b)
Sol. Hunting is a phenomenon occurs in a synchronous machine, involving the oscillations of the rotor about its final equilibrium position.

S5. Ans.(a)
Sol. In case of synchronous motor:
• Under excitation: lagging pf
• Over excitation: leading pf

S6. Ans.(b)
Sol. In case of synchronous motor:
• Under excitation: lagging pf
• Over excitation: leading pf

S7. Ans.(a)
Sol. Damper winding eliminates hunting which is caused by pulsating frequency or by changing load in synchronous motor.

S8. Ans.(b)
Sol. in synchronous motor damper winding is placed at rotor pole faces and it is used to eliminate hunting and also provides starting torque.

S9. Ans.(c)
Sol. over-excited synchronous motor running at no-load, called “Synchronous condenser”.
It is used for power factor correction.

S10. Ans.(c)
Sol. 3-phase synchronous machine rotor construction:
• Salient pole rotor: larger diameter, shorter axial length
• Cylindrical pole rotor: smaller diameter, larger axial length

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