Electrician Motors and Controls 3

Question 1

What is 'locked rotor current' (LRC) in a motor?

Accepted Answer

The high inrush current drawn when the motor shaft cannot rotate at startup

Answer

Locked rotor current is the maximum current a motor draws at standstill (when the shaft is held or before it begins to turn).

Question 2

What NEC requirement applies to the motor disconnecting means?

Accepted Answer

It must be in sight from the motor and controller

Answer

NEC 430.102 requires the motor disconnecting means to be within sight from the motor location and within sight from the controller.

Question 3

What is the primary advantage of a wye (star) motor winding configuration?

Accepted Answer

Reduced starting current and lower starting torque

Answer

In wye-start configurations, voltage per winding is reduced to 58%, lowering starting current to approximately 33% of delta-start current.

Question 4

What does a contactor do in a motor control circuit?

Accepted Answer

Electrically switches the motor power circuit on and off

Answer

A contactor is an electrically operated switch (controlled by a low-voltage coil) used to switch the high-power motor circuit on and off.

Question 5

Per the NEC, overload protection for a motor must trip at what maximum percentage of the motor's full-load current for motors with a 1.15 service factor?

Accepted Answer

125%

Answer

NEC 430.32 requires overload devices to trip at no more than 125% of the motor's nameplate full-load current for motors with a 1.15 service factor.

Electrician Practice Test

Electrician Practice Test

Electrician Motors and Controls 3