Journeyman Electrician’s Exam Journeyman Electrician's Motors and Motor Controls 5

Question 1

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?

Accepted Answer

2.78%

Answer

Slip = (Synchronous speed − Rotor speed) ÷ Synchronous speed = (1800 − 1750) ÷ 1800 = 50 ÷ 1800 ≈ 2.78%.

Question 2

What is the purpose of a 'control transformer' in a motor control circuit?

Accepted Answer

To reduce line voltage to a safer control voltage (typically 120V) for the control circuit

Answer

A control transformer steps down line voltage (e.g., 480V) to a safer level (typically 120V) for operating pilot devices, relays, and coils.

Question 3

Under NEC 430.102(B), a motor disconnecting means must be located within sight of the motor and the driven machinery, OR:

Accepted Answer

The disconnect may be out of sight if it is lockable in the open position

Answer

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.

Question 4

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?

Accepted Answer

Electrical interlocking

Answer

Electrical interlocking uses NC auxiliary contacts from each contactor in the opposing contactor's coil circuit to prevent simultaneous energization (common in reversing starters).

Question 5

A motor with an NEMA Design D characteristic would be most appropriate for:

Accepted Answer

High-inertia loads like punch presses requiring very high starting torque

Answer

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.

Journeyman Electrician’s Exam Practice Test

Journeyman Electrician’s Exam Practice Test

Journeyman Electrician’s Exam Journeyman Electrician's Motors and Motor Controls 5