EETC Motor Controls 5 — Questions and Answers
Question 1: What is regenerative braking in an AC drive system?
- Using mechanical brakes to stop the motor
- Returning braking energy back to the power supply instead of dissipating it as heat (Correct answer)
- Applying DC injection to stop the motor
- Using a braking resistor to absorb energy
Correct 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.
Question 2: Which control component is used to detect that a motor has reached a specific speed before advancing to the next starting step?
- Thermal overload relay
- Centrifugal speed switch or back-EMF relay (Correct answer)
- Current-limiting reactor
- Phase sequence relay
Correct 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.
Question 3: What is the purpose of a phase sequence relay in a motor control circuit?
- To limit starting current to a safe level
- To detect incorrect phase rotation and prevent motor damage or reverse operation (Correct answer)
- To balance load across all three phases
- To provide undervoltage protection only
Correct 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.
Question 4: In DC injection braking, what stops an AC induction motor?
- A reversed AC voltage is applied to the stator
- DC current injected into the stator creates a stationary magnetic field that brakes the rotor (Correct answer)
- Mechanical brakes are clamped to the rotor
- The VFD ramps frequency to zero
Correct 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.
Question 5: A motor nameplate shows a service factor (SF) of 1.15. What does this indicate?
- The motor can only be used at 85% of rated load
- The motor can operate at 115% of rated horsepower under specified conditions without damage (Correct answer)
- The motor efficiency is 115%
- The motor insulation class is rated for 115°C
Correct 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.
Question 6: What is the function of a soft starter compared to an across-the-line starter?
- A soft starter provides faster acceleration than across-the-line starting
- A soft starter gradually ramps up voltage to reduce mechanical shock and inrush current during starting (Correct answer)
- A soft starter reverses motor direction automatically
- A soft starter only works with single-phase motors
Correct 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.
Question 7: When testing a motor contactor coil with an ohmmeter, a reading of infinity (OL) indicates:
- The coil has a short circuit
- The coil winding is open (broken) (Correct answer)
- The coil resistance is within normal range
- The contactor contacts are welded closed
Correct 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.
EETC Motor Controls 5
What is regenerative braking in an AC drive system?