EETC Motor Controls 2 — Questions and Answers
Question 1: What is the purpose of an overload relay in a motor control circuit?
- To protect against short circuits
- To protect the motor from sustained overcurrent due to mechanical overload (Correct answer)
- To limit inrush current during starting
- To disconnect power during a phase imbalance
Correct answer: To protect the motor from sustained overcurrent due to mechanical overload
Overload relays monitor motor current and trip the circuit if sustained overcurrent occurs, protecting windings from thermal damage.
Question 2: In a three-phase motor starter, what does a 'contactor' do?
- Measures motor speed
- Provides overcurrent protection
- Connects and disconnects the motor from the power supply (Correct answer)
- Regulates motor voltage
Correct answer: Connects and disconnects the motor from the power supply
A contactor is an electrically operated switch that connects or disconnects the motor from its power source via a control signal.
Question 3: Which type of motor starting method applies reduced voltage during starting to limit inrush current?
- Full-voltage (across-the-line) starting
- Autotransformer starting (Correct answer)
- Dynamic braking
- Plugging
Correct answer: Autotransformer starting
Autotransformer starting uses a transformer to reduce the voltage applied to the motor during starting, reducing inrush current.
Question 4: What is the function of a holding contact (seal-in contact) in a motor starter circuit?
- Maintains the contactor energized after the start button is released (Correct answer)
- Prevents the motor from starting under load
- Disconnects power if the motor overheats
- Limits the maximum motor speed
Correct answer: Maintains the contactor energized after the start button is released
The seal-in contact (auxiliary NO contact) in parallel with the start button keeps the coil energized once the start button is released.
Question 5: In a forward/reverse motor control circuit, what prevents both forward and reverse contactors from energizing simultaneously?
- Thermal overload relays
- Electrical and mechanical interlocks (Correct answer)
- Time-delay relays
- Current-limiting reactors
Correct answer: Electrical and mechanical interlocks
Electrical interlocks (NC auxiliary contacts) and mechanical interlocks prevent both contactors from closing simultaneously, avoiding a phase-to-phase short circuit.
Question 6: What does the abbreviation 'NEMA' refer to in motor control equipment ratings?
- National Electrical Manufacturers Association (Correct answer)
- National Emergency Motor Authority
- North American Electrical Motor Association
- National Equipment Maintenance Authority
Correct answer: National Electrical Manufacturers Association
NEMA (National Electrical Manufacturers Association) establishes size and performance standards for motor control equipment in the US.
Question 7: Which wiring diagram shows the physical location and connections of components in a control panel?
- Ladder diagram
- Schematic diagram
- Wiring diagram (Correct answer)
- One-line diagram
Correct answer: Wiring diagram
A wiring diagram shows actual physical placement and wire routing of components, unlike a ladder diagram which shows only logical function.
EETC Motor Controls 2
What is the purpose of an overload relay in a motor control circuit?