Journeyman Electrician’s Exam Journeyman Electrician's Motors and Motor Controls 3 — Questions and Answers
Question 1: A reduced-voltage soft starter is primarily used to:
- Increase motor speed above nameplate rating
- Reduce mechanical stress and inrush current during motor starting (Correct answer)
- Provide regenerative braking
- Convert single-phase power to three-phase
Correct answer: Reduce mechanical stress and inrush current during motor starting
Soft starters gradually ramp up voltage to limit starting current and reduce mechanical shock on the driven load during startup.
Question 2: According to NEC 430.83, a motor controller must have a voltage rating not less than:
- The motor's full-load current rating
- The maximum circuit voltage (Correct answer)
- The motor's locked-rotor current rating
- The branch-circuit fuse ampere rating
Correct answer: The maximum circuit voltage
NEC 430.83 requires controllers to have a voltage rating not less than the maximum circuit voltage where they are applied.
Question 3: What does the term 'locked-rotor current' (LRC) describe?
- Current drawn during normal full-speed operation
- Current drawn at the instant of starting when the rotor is stationary (Correct answer)
- Current drawn during dynamic braking
- Minimum current to keep the motor running
Correct answer: Current drawn at the instant of starting when the rotor is stationary
Locked-rotor current is the high inrush current drawn when voltage is applied and the rotor has not yet begun to rotate.
Question 4: Which type of motor protection device responds to both overtemperature and overcurrent conditions by monitoring the motor winding temperature directly?
- Bimetallic overload relay
- Thermistor (PTC) embedded in winding (Correct answer)
- Dual-element time-delay fuse
- Magnetic-only circuit breaker
Correct answer: Thermistor (PTC) embedded in winding
A PTC thermistor embedded in the motor winding changes resistance dramatically at a set temperature, triggering protection based on actual winding temperature.
Question 5: A variable frequency drive (VFD) controls motor speed by varying:
- Supply voltage only, keeping frequency constant
- Both the output frequency and voltage applied to the motor (Correct answer)
- Motor field current only
- The number of stator poles
Correct answer: Both the output frequency and voltage applied to the motor
VFDs vary both frequency and voltage (maintaining a constant V/Hz ratio) to control motor speed while preventing magnetic saturation.
Question 6: Under NEC 430.52, when the calculated protection device size does not correspond to a standard rating, you must use:
- The next lower standard rating
- The next higher standard rating (Correct answer)
- Exactly the calculated value — no rounding allowed
- Double the calculated value
Correct answer: The next higher standard rating
NEC 430.52(C)(1) Exception 1 permits using the next higher standard device rating when the calculated value does not correspond to a standard size.
Question 7: A 10 HP, 230V, single-phase motor has a full-load current of 50A per NEC Table 430.248. What minimum ampacity must the branch-circuit conductors have?
- 50A
- 57.5A
- 62.5A (Correct answer)
- 75A
Correct answer: 62.5A
Branch-circuit conductors must be sized at 125% of FLC: 50A × 1.25 = 62.5A minimum ampacity per NEC 430.22.
Journeyman Electrician’s Exam Journeyman Electrician's Motors and Motor Controls 3
A reduced-voltage soft starter is primarily used to: