Journeyman Electrician's Motors and Motor Controls Flashcards
7 cards from real Journeyman Electrician’s Exam practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 7 Journeyman Electrician's Motors and Motor Controls flashcards as text
A reduced-voltage soft starter is primarily used to:
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.
According to NEC 430.83, a motor controller must have a voltage rating not less than:
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.
What does the term 'locked-rotor current' (LRC) describe?
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.
Which type of motor protection device responds to both overtemperature and overcurrent conditions by monitoring the motor winding temperature directly?
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.
A variable frequency drive (VFD) controls motor speed by varying:
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.
Under NEC 430.52, when the calculated protection device size does not correspond to a standard rating, you must use:
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.
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?
Answer: 62.5A
Branch-circuit conductors must be sized at 125% of FLC: 50A × 1.25 = 62.5A minimum ampacity per NEC 430.22.