Journeyman Electrician’s Exam Journeyman Electrician's Motors and Motor Controls 4 — Questions and Answers
Question 1: In a two-speed motor (consequent pole), changing from low speed to high speed requires:
- Increasing the supply voltage
- Reconnecting the stator windings to change the number of effective poles (Correct answer)
- Adding resistance to the rotor circuit
- Replacing the capacitor run winding
Correct answer: Reconnecting the stator windings to change the number of effective poles
Consequent pole motors change speed by reconnecting stator windings to alter the effective number of magnetic poles, which changes synchronous speed.
Question 2: What is the primary function of the overload relay in a magnetic motor starter?
- To protect the motor against short circuits
- To protect the motor against sustained overcurrent that causes overheating (Correct answer)
- To prevent voltage surges from reaching the motor
- To limit starting current during acceleration
Correct answer: To protect the motor against sustained overcurrent that causes overheating
Overload relays protect the motor from sustained overcurrent (overloads) that would cause excessive heat buildup and winding damage over time.
Question 3: According to NEC 430.32(A), a continuous-duty motor with a service factor of 1.15 must have overload protection set to trip at no more than what percentage of nameplate FLA?
- 115%
- 125% (Correct answer)
- 140%
- 150%
Correct answer: 125%
NEC 430.32(A)(1) requires overload protection to trip at not more than 125% of FLA for motors with a service factor of 1.15 or higher.
Question 4: A 'wye-delta' reduced-voltage starter transitions the motor from wye to delta configuration. What is the purpose of this transition?
- To reduce starting current to approximately 1/3 of across-the-line current during start (Correct answer)
- To increase torque during acceleration
- To allow the motor to run in both directions
- To provide dynamic braking when stopping
Correct answer: To reduce starting current to approximately 1/3 of across-the-line current during start
During wye starting, each winding receives line voltage ÷ √3, reducing starting current to about 1/3 of across-the-line (delta) starting current.
Question 5: Under NEC Article 430, the term 'controller' refers to:
- Only magnetic contactors
- Any switch or device used to start and stop a motor (Correct answer)
- Only devices rated above 2 HP
- The overload relay only
Correct answer: Any switch or device used to start and stop a motor
NEC 430.81 defines a controller as any switch or device that is normally used to start and stop a motor by making and breaking the motor circuit current.
Question 6: A motor's NEMA enclosure type '4X' indicates the enclosure is:
- Drip-proof and suitable for indoor use only
- Watertight, dust-tight, and corrosion-resistant (Correct answer)
- Explosion-proof for Class I hazardous locations
- Totally enclosed fan-cooled (TEFC) only
Correct answer: Watertight, dust-tight, and corrosion-resistant
NEMA Type 4X enclosures are watertight, dust-tight, and constructed of corrosion-resistant material for demanding indoor or outdoor applications.
Question 7: A jogging (inching) circuit differs from a standard start/stop circuit because:
- Jogging uses a higher voltage contactor
- The holding contact (seal-in) is bypassed so the motor runs only while the jog button is held (Correct answer)
- Jogging requires two separate motors
- The overload relay is bypassed during jogging
Correct answer: The holding contact (seal-in) is bypassed so the motor runs only while the jog button is held
In a jog circuit, the memory (seal-in) contact is not energized, so releasing the jog button immediately de-energizes the contactor and stops the motor.
Journeyman Electrician’s Exam Journeyman Electrician's Motors and Motor Controls 4
In a two-speed motor (consequent pole), changing from low speed to high speed requires: