IEC Motors and Motor Controls 2 — Questions and Answers
Question 1: A motor nameplate shows a service factor of 1.15. What does this mean?
- The motor can operate continuously at 115% of its rated load (Correct answer)
- The motor efficiency is 115%
- The motor requires 115% of rated voltage to start
- The motor has a 15% safety margin before it will trip
Correct answer: The motor can operate continuously at 115% of its rated load
A service factor of 1.15 means the motor can operate continuously at 115% of its nameplate horsepower without damaging insulation.
Question 2: Which type of motor starter uses reduced voltage to limit inrush current during starting?
- Across-the-line starter
- Autotransformer starter (Correct answer)
- Manual motor starter
- Reversing starter
Correct answer: Autotransformer starter
An autotransformer starter reduces starting voltage via a transformer, which significantly reduces inrush current during motor startup.
Question 3: What is the purpose of a motor's overload relay?
- To protect the motor from voltage surges
- To protect the motor from sustained overcurrent due to overloading (Correct answer)
- To prevent the motor from starting under load
- To protect against phase reversal
Correct answer: To protect the motor from sustained overcurrent due to overloading
Overload relays protect motors from sustained overcurrents caused by mechanical overloads, which generate excess heat and can damage windings.
Question 4: A three-phase motor is wired for wye (Y) connection. Compared to a delta connection, the phase voltage is:
- Equal to the line voltage
- Higher than the line voltage
- Lower than the line voltage by a factor of √3 (Correct answer)
- Lower than the line voltage by a factor of 2
Correct answer: Lower than the line voltage by a factor of √3
In a wye connection, phase voltage equals line voltage divided by √3 (approximately 0.577 × line voltage).
Question 5: Which NEC article primarily governs the installation of motors, motor circuits, and controllers?
- Article 240
- Article 310
- Article 430 (Correct answer)
- Article 450
Correct answer: Article 430
NEC Article 430 covers motors, motor circuits, and controllers including overcurrent protection and disconnecting means requirements.
Question 6: What type of enclosure is rated NEMA 4 and is suitable for outdoor use?
- Provides protection against falling dirt only
- Watertight and provides protection against rain, splashing, and hose-directed water (Correct answer)
- Explosion-proof for hazardous locations
- Drip-proof for indoor use only
Correct answer: Watertight and provides protection against rain, splashing, and hose-directed water
NEMA 4 enclosures are watertight and dusttight, protecting against windblown dust, rain, sleet, and hose-directed water.
Question 7: When sizing branch circuit conductors for a single motor, the NEC requires the conductors to be rated at least:
- 100% of the motor's full-load current
- 115% of the motor's full-load current
- 125% of the motor's full-load current (Correct answer)
- 150% of the motor's full-load current
Correct answer: 125% of the motor's full-load current
Per NEC 430.22, branch circuit conductors supplying a single motor must have an ampacity of at least 125% of the motor's full-load current rating.
IEC Motors and Motor Controls 2
A motor nameplate shows a service factor of 1.15.
What does this mean?