Motors and Controls Flashcards

A three-phase induction motor runs slower than its synchronous speed. What is the difference between synchronous speed and actual rotor speed called?

Answer: Slip

Slip is the difference between synchronous speed and actual rotor speed, expressed as a percentage of synchronous speed.

Which motor protection device opens a circuit when it detects excessive heat caused by motor overloading?

Answer: Thermal overload relay

A thermal overload relay monitors motor current heat buildup and trips to protect the motor windings from damage due to overload.

What happens to the speed of a DC shunt motor if the field current is reduced while the load stays the same?

Answer: Speed increases

Reducing field current weakens the magnetic field, which reduces back-EMF and causes the motor to speed up to restore balance.

A NEMA 4X enclosure for a motor controller is rated for which environment?

Answer: Watertight and corrosion-resistant

NEMA 4X enclosures are designed to be watertight, dust-tight, and corrosion-resistant, suitable for outdoor and washdown environments.

Which starting method for a large AC induction motor connects the motor at reduced voltage through a transformer during startup?

Answer: Autotransformer starting

Autotransformer starting uses a tapped autotransformer to apply reduced voltage during startup, limiting inrush current.

In a motor control circuit, what component physically switches the power contacts to start or stop the motor?

Answer: Contactor

A contactor is an electromagnetically operated switching device that opens and closes the main power contacts to the motor.

A motor nameplate shows a service factor of 1.15. What does this mean?

Answer: The motor can operate at 115% of rated load for short periods

A service factor of 1.15 means the motor can handle 115% of its rated horsepower without damage under normal operating conditions.