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DC 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: 2 computed question types on this topic.

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Questions

1. A 4-pole, lap-wound DC generator has 500 armature conductors and a flux of 0.02 Wb per pole. What EMF does it generate at 1200 rpm?

  1. 200 V
  2. 400 V
  3. 100 V
  4. 800 V
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Answer: A. 200 V

E = PφZN/(60A). For lap winding A = P = 4, so E = 4 × 0.02 × 500 × 1200/(60 × 4) = 200 V.

2. A 230 V DC shunt motor has an armature resistance of 0.25 Ω and draws an armature current of 40 A. What is the back EMF?

  1. 240 V
  2. 220 V
  3. 230 V
  4. 200 V
Show the answer and solution

Answer: B. 220 V

Eb = V − Ia Ra = 230 − 40 × 0.25 = 220 V.

3. The same 230 V shunt motor (Ra = 0.25 Ω) runs at 1200 rpm with an armature current of 40 A. If the load increases so that the armature current becomes 60 A with constant flux, what is the new speed?

  1. 1228 rpm
  2. 800 rpm
  3. 1173 rpm
  4. 1200 rpm
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Answer: C. 1173 rpm

Eb1 = 230 − 40(0.25) = 220 V, Eb2 = 230 − 60(0.25) = 215 V. With constant flux N ∝ Eb, so N2 = 1200 × 215/220 = 1173 rpm.

4. Which DC motor must never be started or run without a mechanical load connected?

  1. Shunt motor
  2. Separately excited motor
  3. Cumulative compound motor
  4. Series motor
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Answer: D. Series motor

A series motor's flux falls with load current, so at no load the speed rises to dangerously high values (runaway).

5. A DC shunt generator delivers 100 A at 250 V terminal voltage. The shunt field resistance is 125 Ω and the armature resistance is 0.1 Ω. What is the generated EMF?

  1. 260.2 V
  2. 260.0 V
  3. 250.2 V
  4. 239.8 V
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Answer: A. 260.2 V

Field current = 250/125 = 2 A, so Ia = 100 + 2 = 102 A. Eg = V + Ia Ra = 250 + 102 × 0.1 = 260.2 V.

6. What is the main function of the commutator in a DC generator?

  1. To provide excitation current for the field poles
  2. To convert the alternating EMF induced in the armature coils into direct voltage at the brushes
  3. To reduce armature reaction
  4. To limit the starting current
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Answer: B. To convert the alternating EMF induced in the armature coils into direct voltage at the brushes

The EMF induced in each armature coil alternates; the commutator reverses the coil connections at the right instant so the brushes deliver unidirectional voltage.

7. A DC motor has a back EMF of 220 V and an armature current of 40 A while running at 1200 rpm. What electromagnetic torque does it develop?

  1. 73.2 N·m
  2. 7.33 N·m
  3. 70.0 N·m
  4. 1.17 N·m
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Answer: C. 70.0 N·m

Developed power = Eb Ia = 220 × 40 = 8800 W. ω = 2π × 1200/60 = 125.66 rad/s. T = 8800/125.66 = 70.0 N·m.

8. How is the speed of a DC shunt motor raised above its base (rated) speed?

  1. By adding resistance in series with the armature
  2. By reducing the armature supply voltage
  3. By strengthening the field current
  4. By adding resistance in the shunt field circuit to weaken the field
Show the answer and solution

Answer: D. By adding resistance in the shunt field circuit to weaken the field

Speed is proportional to Eb/φ. Weakening the field (field rheostat) reduces φ and raises speed above base speed; armature resistance or lower voltage only gives speeds below base.

Worked examples with computed numbers

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

9. A 440 V DC motor with 0.1 Ω armature resistance draws 20 A. What is its back EMF?

  1. 2 V
  2. 438 V
  3. 440 V
  4. 442 V
Show the answer and solution

Answer: B. 438 V

Motor: Eb = V − IaRa = 440 − 20 × 0.1 = 438 V.

10. A separately excited DC generator delivers 50 A at 110 V terminal voltage; its armature resistance is 0.1 Ω. What EMF does it generate?

  1. 5 V
  2. 110 V
  3. 115 V
  4. 105 V
Show the answer and solution

Answer: C. 115 V

Generator: E = Vt + IaRa = 110 + 50 × 0.1 = 115 V.

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 · Induction motors · Synchronous machines · Power systems and transmission · Protection: fuses, breakers, relays · Illumination · Power plant engineering · PEC: branch circuits, ampacity and protection

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