EETC Motor Controls 3 — Questions and Answers
Question 1: A star-delta (wye-delta) starter is used primarily to:
- Reverse motor direction
- Reduce starting current by initially connecting windings in wye (Correct answer)
- Provide dynamic braking
- Increase motor torque during acceleration
Correct answer: Reduce starting current by initially connecting windings in wye
In star-delta starting, windings are first connected in wye (reducing voltage by 1/√3), then switched to delta for full-speed operation.
Question 2: What is 'plugging' in motor control?
- Inserting a plug-type connector into a motor terminal block
- Applying reverse phase voltage to quickly stop or reverse a running motor (Correct answer)
- Connecting power factor correction capacitors
- Bypassing the overload relay for testing
Correct answer: Applying reverse phase voltage to quickly stop or reverse a running motor
Plugging applies reverse-phase power to a running motor to produce rapid braking, creating high braking torque.
Question 3: In a motor control ladder diagram, what does an X inside a circle symbol typically represent?
- A contactor coil
- A pilot light (Correct answer)
- A normally open contact
- A fuse
Correct answer: A pilot light
A circle with an X (or letter) inside typically represents a pilot light or indicator lamp in ladder logic diagrams.
Question 4: What is the primary purpose of a time-delay relay (TDR) in a reduced-voltage starter?
- To limit overload current
- To allow the motor to accelerate before switching to full voltage (Correct answer)
- To detect phase loss conditions
- To reverse motor direction automatically
Correct answer: To allow the motor to accelerate before switching to full voltage
The TDR provides a timed delay so the motor can accelerate to near full speed before the circuit transitions from reduced to full voltage.
Question 5: Which enclosure type is rated for outdoor use and protection against rain, sleet, and snow according to NEMA standards?
- NEMA 1
- NEMA 4 (Correct answer)
- NEMA 7
- NEMA 12
Correct answer: NEMA 4
NEMA 4 enclosures are weatherproof and protect against rain, sleet, snow, windblown dust, and hose-directed water.
Question 6: In a two-wire control circuit, what is a key characteristic that differentiates it from a three-wire circuit?
- It uses a maintained-contact device and will restart automatically after a power loss (Correct answer)
- It requires a separate start and stop button
- It uses three conductors for the control circuit
- It cannot be used with contactors larger than NEMA size 2
Correct answer: It uses a maintained-contact device and will restart automatically after a power loss
Two-wire circuits use maintained-contact devices (like float switches) and restart automatically when power is restored, unlike three-wire circuits with low-voltage protection.
Question 7: What is dynamic braking as applied to a DC motor?
- Applying mechanical brake pads to the motor shaft
- Disconnecting the armature and connecting it to a resistor to dissipate kinetic energy (Correct answer)
- Reversing field polarity to slow the motor
- Using regenerative energy back to the supply
Correct answer: Disconnecting the armature and connecting it to a resistor to dissipate kinetic energy
Dynamic braking disconnects the armature from supply and connects it across a resistor; the motor acts as a generator, converting kinetic energy to heat.
EETC Motor Controls 3
A star-delta (wye-delta) starter is used primarily to: