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 three-phase induction motor running at 1,750 RPM on a 60 Hz supply has a synchronous speed of 1,800 RPM. What is the motor's slip?
Answer: 2.78%
Slip = (Synchronous speed − Rotor speed) ÷ Synchronous speed = (1800 − 1750) ÷ 1800 = 50 ÷ 1800 ≈ 2.78%.
What is the purpose of a 'control transformer' in a motor control circuit?
Answer: To reduce line voltage to a safer control voltage (typically 120V) for the control circuit
A control transformer steps down line voltage (e.g., 480V) to a safer level (typically 120V) for operating pilot devices, relays, and coils.
Under NEC 430.102(B), a motor disconnecting means must be located within sight of the motor and the driven machinery, OR:
Answer: The disconnect may be out of sight if it is lockable in the open position
NEC 430.102(B) Exception allows the disconnect to be out of sight from the motor if it is capable of being locked in the open (off) position.
Which motor protection scheme uses a set of normally-closed auxiliary contacts from one contactor wired in series with the coil of the other contactor to prevent both from energizing simultaneously?
Answer: Electrical interlocking
Electrical interlocking uses NC auxiliary contacts from each contactor in the opposing contactor's coil circuit to prevent simultaneous energization (common in reversing starters).
A motor with an NEMA Design D characteristic would be most appropriate for:
Answer: High-inertia loads like punch presses requiring very high starting torque
NEMA Design D motors provide very high starting torque (275%+ of FLC) with high slip, making them ideal for high-inertia, impact, or peak-load applications.
According to NEC 430.6(A), which current value must be used for determining conductor ampacity and device ratings for AC motors — the nameplate current or the NEC table current?
Answer: Use the NEC table value, not the nameplate current
NEC 430.6(A) requires using the current values from NEC Tables 430.247–430.250 for conductor and device sizing, not the motor nameplate FLA.
A motor running in an ambient temperature of 50°C when its insulation class is rated for 40°C ambient must have its ampacity-corrected conductors selected based on:
Answer: The standard 75°C conductor ampacity table with a temperature correction factor applied
Conductors must be derated using the temperature correction factors in NEC Table 310.15(B)(1) when ambient temperature exceeds the conductor's rated ambient.