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Motors, starters and control circuits: board exam questions with solutions

On the RME exam under Technical 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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Build it in the lab

Thermal flasher and relay latch (start/stop): the same circuits on a live bench, with meters you can move.

Questions

1. Compared with direct-on-line (DOL) starting, a star-delta starter reduces the starting line current to what fraction?

  1. One-third
  2. One-half
  3. 1/√3
  4. Two-thirds
Show the answer and solution

Answer: A. One-third

In star each winding gets 1/√3 of line voltage, so phase current falls by 1/√3. In delta the line current is √3 × phase current, so the net line-current ratio is (1/√3)² = 1/3. Starting torque is also cut to 1/3.

2. A motor with a full-load current of 20 A draws 6 times full-load current when started direct-on-line. What starting line current would a star-delta starter draw?

  1. 120 A
  2. 40 A
  3. 69 A
  4. 20 A
Show the answer and solution

Answer: B. 40 A

DOL starting current = 6 × 20 = 120 A. Star-delta gives 1/3 of this: 120/3 = 40 A.

3. An autotransformer starter is set on its 65% tap. What is the starting torque as a percentage of the direct-on-line starting torque?

  1. 65%
  2. 35%
  3. 42.25%
  4. 80.6%
Show the answer and solution

Answer: C. 42.25%

Motor voltage is 65% of rated, and torque ∝ V², so T = 0.65² = 0.4225 = 42.25%. The line current is also reduced to 42.25% of the DOL value.

4. In a standard three-wire start/stop control circuit, which contact keeps the contactor energized after the START button is released?

  1. A normally closed auxiliary contact wired in series with the STOP button
  2. The overload relay's normally open contact
  3. A normally closed contact in parallel with the STOP button
  4. A normally open auxiliary (holding/seal-in) contact of the contactor wired in parallel with the START button
Show the answer and solution

Answer: D. A normally open auxiliary (holding/seal-in) contact of the contactor wired in parallel with the START button

The NO auxiliary contact closes when the coil energizes and bypasses the momentary START button, sealing the circuit until STOP or the overload contact opens it.

5. Why do forward-reverse motor starters use interlocking between the forward and reverse contactors?

  1. To prevent both contactors closing at once, which would short-circuit two supply phases
  2. To reduce the motor's starting current
  3. To let the motor run in both directions at once for braking
  4. To protect the motor from single-phasing
Show the answer and solution

Answer: A. To prevent both contactors closing at once, which would short-circuit two supply phases

The reverse contactor swaps two phases. If both closed together, those two phases would be shorted, so mechanical and/or electrical (NC auxiliary contact) interlocks are used.

6. In a typical magnetic motor starter with an upstream fuse or circuit breaker, which device is intended to protect the motor against a sustained running overload?

  1. The branch-circuit short-circuit fuse or breaker
  2. The overload relay
  3. The STOP push button
  4. The contactor's holding contact
Show the answer and solution

Answer: B. The overload relay

The overload relay (thermal or electronic) trips on sustained overcurrent above its setting. The fuse or breaker provides short-circuit and ground-fault protection and is sized too high to protect against running overload.

7. A 400 V delta-rated motor is started in star by a star-delta starter on a 400 V supply. What voltage appears across each winding during the star (start) connection?

  1. 400 V
  2. 133 V
  3. 231 V
  4. 693 V
Show the answer and solution

Answer: C. 231 V

In star each winding receives the line voltage divided by √3: 400/1.732 = 231 V.

8. In a star-delta starter the overload relay is connected in the motor phase-winding circuit (carrying winding current, not line current). The motor's full-load line current is 30 A. To what current should the overload relay be set?

  1. 30.0 A
  2. 52.0 A
  3. 10.0 A
  4. 17.3 A
Show the answer and solution

Answer: D. 17.3 A

In delta (run) connection the winding current is the line current divided by √3: 30/1.732 = 17.3 A. An OL relay in the winding circuit is set to this value.

Worked examples with computed numbers

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

9. A motor draws 90 A from the line when started direct-on-line in delta. About what line current does a star-delta starter draw at start?

  1. 90 A
  2. 52 A
  3. 156 A
  4. 30 A
Show the answer and solution

Answer: D. 30 A

In star each winding sees 1/√3 of the voltage, and the line current falls to one third of the delta value: 90/3 = 30 A.

Other topics in the same subjects

DC circuits · AC circuits · Three-phase systems · Tools and test equipment · Transformers · Induction motors · Illumination · Protection: fuses, breakers, relays · RA 7920 and ethics (electrical)

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.