Motor Controls Flashcards

What is the purpose of an overload relay in a motor control circuit?

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.

In a three-phase motor starter, what does a 'contactor' do?

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.

Which type of motor starting method applies reduced voltage during starting to limit inrush current?

Answer: Autotransformer starting

Autotransformer starting uses a transformer to reduce the voltage applied to the motor during starting, reducing inrush current.

What is the function of a holding contact (seal-in contact) in a motor starter circuit?

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.

In a forward/reverse motor control circuit, what prevents both forward and reverse contactors from energizing simultaneously?

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.

What does the abbreviation 'NEMA' refer to in motor control equipment ratings?

Answer: National Electrical Manufacturers Association

NEMA (National Electrical Manufacturers Association) establishes size and performance standards for motor control equipment in the US.

Which wiring diagram shows the physical location and connections of components in a control panel?

Answer: Wiring diagram

A wiring diagram shows actual physical placement and wire routing of components, unlike a ladder diagram which shows only logical function.