ITMC Motor Controls 5 — Questions and Answers
Question 1: In a jogging (inching) control circuit, why is a jogging relay sometimes used instead of simply using the start button?
- To allow higher voltage during jogging
- To prevent the holding contact from sealing in the circuit during jog operation (Correct answer)
- To bypass the overload relay during jogging
- To reverse the motor during jogging
Correct answer: To prevent the holding contact from sealing in the circuit during jog operation
A jogging relay breaks the seal-in (holding) contact circuit during jog mode so the motor runs only while the jog button is held, without latching into continuous run.
Question 2: What does the term 'FVNR' stand for in motor starting methods?
- Frequency Variable Neutral Resistance
- Full Voltage Non-Reversing (Correct answer)
- Field Voltage Neutral Rectifier
- Fixed Voltage Nominal Rating
Correct answer: Full Voltage Non-Reversing
FVNR stands for Full Voltage Non-Reversing, describing the most common across-the-line motor starter that applies full line voltage to start the motor in one direction only.
Question 3: Which motor braking method injects DC current into the stator windings to stop the motor quickly?
- Regenerative braking
- Plugging
- Dynamic braking
- DC injection braking (Correct answer)
Correct answer: DC injection braking
DC injection braking applies direct current to the stator windings after AC power is removed, creating a stationary magnetic field that produces a braking torque to stop the rotor.
Question 4: A motor contactor's power contacts show pitting and carbon deposits. What is the recommended corrective action?
- Polish contacts with emery cloth to restore conductivity
- Replace the contact tips or the entire contactor per manufacturer recommendations (Correct answer)
- Apply conductive grease to reduce arcing
- Increase the overload relay setting to compensate
Correct answer: Replace the contact tips or the entire contactor per manufacturer recommendations
Pitted and carbonized contacts should be replaced per manufacturer guidelines because filing or sanding contacts removes the silver-alloy facing and shortens contact life.
Question 5: What is the function of a reduced-voltage soft starter compared to a VFD?
- It provides precise variable speed control during running
- It controls acceleration and deceleration during start/stop only, running the motor across-the-line at full speed (Correct answer)
- It converts single-phase power to three-phase
- It provides power factor correction during running
Correct answer: It controls acceleration and deceleration during start/stop only, running the motor across-the-line at full speed
A soft starter controls voltage (and therefore current and torque) only during the starting and stopping ramp; once at full speed, it typically bypasses itself and the motor runs directly across the line.
Question 6: In a motor control ladder diagram, which symbol represents a normally open (NO) pushbutton?
- Two parallel horizontal lines (capacitor symbol)
- Two vertical lines with a diagonal bar through them
- Two vertical lines with no connection (open gap) (Correct answer)
- A circle with an arrow through it
Correct answer: Two vertical lines with no connection (open gap)
A normally open pushbutton is represented by two vertical lines (contact symbol) with an open gap between them, indicating the circuit is open until the button is pressed.
Question 7: An operator reports that a motor controlled by a three-wire circuit starts but immediately stops when the start button is released. Which component is MOST likely faulty?
- Stop pushbutton (stuck open)
- Holding (auxiliary) contact on the contactor (open or missing) (Correct answer)
- Overload relay (tripping instantly)
- Control transformer (low voltage output)
Correct answer: Holding (auxiliary) contact on the contactor (open or missing)
If the motor starts but stops when the start button is released, the holding (auxiliary) contact is not sealing in the circuit, indicating it is open, worn, or wired incorrectly.
ITMC Motor Controls 5
In a jogging (inching) control circuit, why is a jogging relay sometimes used instead of simply using the start button?