# Electrical Machines Questions and Answers – Synchronous Machine Stability

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This set of Electrical Machines Multiple Choice Questions & Answers (MCQs) focuses on “Synchronous Machine Stability”.

1. Steady state stability limit is defined as maximum power flow possible through a particular point without loss of stability when the ___________
b) power is increased suddenly
d) power is reduces suddenly

Explanation: Steady state analysis involves study of stability at gradual changes in the machine.

2. If the load angle is increased till 135°, the consequence will be ___________
a) loss of synchronism
b) no alternation of synchronism
c) increase in stability limit
d) reduction in stability limit

Explanation: Because load angle is more than 90°.

3. The steady state limit can be improved by ___________

i. Increasing excitation
ii. reducing reactance
iii. using series capacitors
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a) i, ii, iii
b) i, ii
c) iii only
d) ii, iii

Explanation: Steady state limit = EV/X.

4. Series capacitors will improve the ___________
a) SSSL
b) Transient stability
c) Hunting
d) Stiffness

Explanation: Steady state limit = EV/X.

5. Transient stability limit is less important than SSSL.
a) True
b) False

Explanation: Transient stability is more of concern due to the reason that the machine may loose synchronism due to sudden disturbances.

6. (i) Transient stability is lower than SSSL.
(ii) Transient disturbances are more common.
a) i and ii both are true
b) i false, ii true
c) i true, ii false
d) Both are false

Explanation: Both are correct statements.

7. The supply voltage to an induction motor is decreased by 10%. By what percentage, approximately, will the maximum torque decrease?
a) 5%
b) 10%
c) 20%
d) 40%

Explanation: Torque is proportional to V2.
Torque will change by twice percentage of the change in voltage i.e. 20%.

8. In a salient pole synchronous machine the air gap is least under the middle of the pole shoe and increases outwards.
a) True
b) False

Explanation: Air gap for a salient pole structure is minimum at mid of a given pole.

9. If it is planned to bring an incoming alternator to the operating alternator in parallel, then which of the criteria can be ignored?
a) Prime mover speed
b) Voltage magnitude
c) Frequency
d) Phase sequence of the alternator

Explanation: To operate under synchronising conditions, there must be same voltage, frequency and phase sequence for the machines in the parallel.

10. Which one of the following is the base reason to place field on rotor in the alternator?
a) Small power in field circuit
b) Insulation of high voltage is made easy on stator than on rotor
c) Huge power in stator
d) Large current in the stator

Explanation: Providing insulation becomes easier as the stator will be stationary.

Sanfoundry Global Education & Learning Series – Electrical Machines.

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