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Synchronous machines: board exam questions with solutions

On the REE exam under Electrical Engineering Professional Subjects. Try each question before you open its solution, then practise with fresh numbers below: 1 computed question type on this topic.

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Questions

1. At what speed must a 10-pole alternator be driven to generate 60 Hz?

  1. 720 rpm
  2. 600 rpm
  3. 1200 rpm
  4. 360 rpm
Show the answer and solution

Answer: A. 720 rpm

N = 120 f/P = 120 × 60/10 = 720 rpm.

2. A hydroelectric generator runs at 300 rpm and supplies a 60 Hz system. How many poles does it have?

  1. 12
  2. 24
  3. 20
  4. 48
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Answer: B. 24

P = 120 f/N = 120 × 60/300 = 24 poles.

3. An over-excited synchronous motor connected to an infinite bus operates at what power factor?

  1. Lagging
  2. Always unity
  3. Leading
  4. Zero lagging only
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Answer: C. Leading

Over-excitation makes the motor deliver reactive power to the system, so it draws leading current and behaves like a capacitor (synchronous condenser).

4. An alternator has a terminal voltage of 400 V at full load. When the load is removed with excitation and speed unchanged, the voltage rises to 480 V. What is the voltage regulation?

  1. 16.7%
  2. 80.0%
  3. 8.0%
  4. 20.0%
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Answer: D. 20.0%

VR = (E − V)/V × 100 = (480 − 400)/400 × 100 = 20.0%.

5. A wye-connected 400 V three-phase alternator delivers 50 A at 0.8 power factor lagging. Its synchronous reactance is 2 Ω per phase and armature resistance is negligible. What is the generated EMF per phase?

  1. 301.7 V
  2. 251.7 V
  3. 188.7 V
  4. 330.9 V
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Answer: A. 301.7 V

V_ph = 400/√3 = 230.94 V. E = V + jXs I = 230.94 + j2 × 50∠−36.87° = 230.94 + 60 + j80 = 301.7 V per phase.

6. Why is a damper (amortisseur) winding fitted to the rotor of a synchronous motor?

  1. To supply DC excitation to the field
  2. To let the motor start as an induction motor and to damp hunting
  3. To raise the motor's synchronous speed
  4. To reduce core losses in the stator
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Answer: B. To let the motor start as an induction motor and to damp hunting

A synchronous motor has no starting torque of its own. The squirrel-cage damper winding gives induction-motor starting torque and opposes speed oscillations (hunting) once synchronized.

7. Which of the following is NOT a required condition before closing the breaker to parallel an incoming alternator with a bus?

  1. Equal terminal voltages
  2. Equal frequencies
  3. Equal kVA ratings of the two machines
  4. Same phase sequence
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Answer: C. Equal kVA ratings of the two machines

Synchronizing requires equal voltage, equal frequency, the same phase sequence and the voltages in phase. Machines of different ratings are routinely paralleled.

8. A plant draws 400 kW at 0.8 power factor lagging. A synchronous motor taking 100 kW at 0.8 power factor leading is added. What is the new overall power factor?

  1. 0.800 lagging
  2. 0.800 leading
  3. 1.000
  4. 0.912 lagging
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Answer: D. 0.912 lagging

Plant Q = 400 tan(36.87°) = 300 kvar lag; motor Q = 100 × 0.75 = 75 kvar lead. Total P = 500 kW, Q = 225 kvar lag. pf = 500/√(500² + 225²) = 0.912 lagging.

Worked examples with computed numbers

These numbers are fixed on this page; in practice they change every time.

9. How many poles does a 60 Hz synchronous generator need to run at 3600 rpm?

  1. 1
  2. 2
  3. 4
  4. 3
Show the answer and solution

Answer: B. 2

n = 120f/P, so P = 120 × 60/3600 = 2 poles.

Other topics in the same subjects

DC circuits · AC circuits · Three-phase systems · Transients (RC, RL, RLC) · Semiconductors and diodes · Power electronics · Instrumentation and measurement · Control systems · Transformers · DC machines · Induction motors · Power systems and transmission · Protection: fuses, breakers, relays · Illumination · Power plant engineering · PEC: branch circuits, ampacity and protection

An independent, free reviewer: not affiliated with the PRC or the Boards. Subject weights and passing rules are from the PRC examination programs and the laws (RA 9292, RA 7920); check the current PRC program before you file. Code questions follow the Philippine Electrical Code 2017 Part 1; check them against your own copy of the Code.