Licensed Electrical Contractor Motors and Motor Controls 4

Question 1

A variable frequency drive (VFD) controls motor speed primarily by varying:

Accepted Answer

Output frequency and voltage simultaneously

Answer

VFDs vary both output frequency and voltage (maintaining a constant V/Hz ratio) to control induction motor speed while preserving magnetic flux.

Question 2

What is the NEC maximum distance allowed between a motor and its disconnecting means when the disconnect is not in sight of the motor?

Accepted Answer

No distance limit if lockable

Answer

NEC 430.102(B) allows a motor disconnect that is not in sight if it is capable of being locked in the open position, with no stated distance limit.

Question 3

Plugging a 3-phase induction motor means:

Accepted Answer

Reversing two supply phases while the motor is running to brake it rapidly

Answer

Plugging reverses the phase sequence while the motor is running, creating a strong opposing torque that rapidly decelerates and stops the motor.

Question 4

A motor's full-load speed is 1750 RPM on a 60 Hz system. Its slip is approximately:

Accepted Answer

2.78%

Answer

Synchronous speed for a 4-pole, 60 Hz motor is 1800 RPM; slip = (1800−1750)/1800 × 100 = 2.78%.

Question 5

Per NEC 430.87, if a single motor controller is used for a group of motors, it must have a horsepower rating not less than:

Accepted Answer

The largest motor HP in the group

Answer

NEC 430.87 requires the controller HP rating to be at least equal to the horsepower of the largest motor in the group it controls.

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