Motor Controls Flashcards
7 cards from real EETC practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 7 Motor Controls flashcards as text
What is regenerative braking in an AC drive system?
Answer: Returning braking energy back to the power supply instead of dissipating it as heat
Regenerative braking returns the motor's kinetic energy back to the AC supply line, improving energy efficiency compared to resistive braking.
Which control component is used to detect that a motor has reached a specific speed before advancing to the next starting step?
Answer: Centrifugal speed switch or back-EMF relay
A centrifugal speed switch or back-EMF relay detects when the motor has accelerated to a set speed, triggering transition to the next starting step.
What is the purpose of a phase sequence relay in a motor control circuit?
Answer: To detect incorrect phase rotation and prevent motor damage or reverse operation
A phase sequence relay monitors the order of three-phase voltages and trips if the sequence is incorrect, preventing a motor from running in the wrong direction.
In DC injection braking, what stops an AC induction motor?
Answer: DC current injected into the stator creates a stationary magnetic field that brakes the rotor
DC injection braking applies a DC voltage to the AC stator windings, creating a stationary magnetic field that exerts a braking torque on the spinning rotor.
A motor nameplate shows a service factor (SF) of 1.15. What does this indicate?
Answer: The motor can operate at 115% of rated horsepower under specified conditions without damage
A service factor of 1.15 means the motor can safely handle 15% overload beyond its nameplate horsepower rating under proper conditions.
What is the function of a soft starter compared to an across-the-line starter?
Answer: A soft starter gradually ramps up voltage to reduce mechanical shock and inrush current during starting
A soft starter uses SCRs to gradually increase voltage from zero to full value, reducing mechanical shock, inrush current, and torque pulsations.
When testing a motor contactor coil with an ohmmeter, a reading of infinity (OL) indicates:
Answer: The coil winding is open (broken)
An infinite resistance reading on a coil indicates an open winding, meaning the coil is broken and cannot produce a magnetic field to close the contactor.