Motors and Motor Controls Flashcards
7 cards from real Licensed Electrical Contractor practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 7 Motors and Motor Controls flashcards as text
A variable frequency drive (VFD) controls motor speed primarily by varying:
Answer: Output frequency and voltage simultaneously
VFDs vary both output frequency and voltage (maintaining a constant V/Hz ratio) to control induction motor speed while preserving magnetic flux.
What is the NEC maximum distance allowed between a motor and its disconnecting means when the disconnect is not in sight of the motor?
Answer: No distance limit if lockable
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.
Plugging a 3-phase induction motor means:
Answer: Reversing two supply phases while the motor is running to brake it rapidly
Plugging reverses the phase sequence while the motor is running, creating a strong opposing torque that rapidly decelerates and stops the motor.
A motor's full-load speed is 1750 RPM on a 60 Hz system. Its slip is approximately:
Answer: 2.78%
Synchronous speed for a 4-pole, 60 Hz motor is 1800 RPM; slip = (1800−1750)/1800 × 100 = 2.78%.
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:
Answer: The largest motor HP in the group
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.
In a motor control circuit, an overload relay's normally closed (NC) contact is wired:
Answer: In series with the contactor coil to interrupt the control circuit on trip
The OL relay NC contact is in series with the motor starter coil; when the relay trips, this contact opens, de-energizing the coil and stopping the motor.
What motor starting method provides the highest starting torque per ampere of line current drawn?
Answer: Autotransformer reduced-voltage starting
Autotransformer starters provide the best torque-per-amp ratio because they apply reduced voltage via transformer action, which is more efficient than wye-delta or part-winding methods.