Ramsay Test Motors and Controls 3

Question 1

What does a Variable Frequency Drive (VFD) do to control an AC motor's speed?

Accepted Answer

It varies the frequency and voltage of the AC power supplied to the motor

Answer

It converts AC power to DC, then varies the DC voltage to the motor

Answer

It adjusts the motor's field current to change the speed

Answer

It switches between different numbers of stator poles to change speed

Question 2

Which type of AC motor operates at exactly synchronous speed under any load?

Accepted Answer

Synchronous motor

Answer

Squirrel cage induction motor

Answer

Wound rotor induction motor

Answer

Shaded-pole motor

Question 3

In a control circuit, what happens to a normally closed (NC) auxiliary contact when its associated contactor coil is energized?

Accepted Answer

It opens, breaking the circuit path through that contact

Answer

It closes, completing an additional circuit path

Answer

It remains in its current state until manually reset

Answer

It alternates between open and closed at the supply frequency

Question 4

What is 'plugging' as a method of stopping or reversing an electric motor?

Accepted Answer

Applying reverse-phase voltage to a running motor to rapidly decelerate it to a stop

Answer

Inserting a physical plug into the motor control panel to isolate the circuit

Answer

Connecting a large resistor across the motor terminals to absorb braking energy

Answer

Using a friction clutch to mechanically stop the motor shaft

Question 5

Why does a single-phase induction motor require a starting capacitor or auxiliary winding?

Accepted Answer

To create a phase shift that produces a rotating magnetic field necessary for self-starting

Answer

To filter voltage spikes that would otherwise damage the motor windings at startup

Answer

To reduce the power consumption of the motor during the startup phase

Answer

To maintain constant speed under varying loads after reaching operating speed

Question 6

What is dynamic braking of a DC motor?

Accepted Answer

Disconnecting the armature from supply and connecting a resistor across it so the motor acts as a generator dissipating energy

Answer

Applying full reverse supply voltage to the armature to brake the motor as quickly as possible

Answer

Clamping a mechanical friction brake onto the motor shaft using electromagnetic force

Answer

Reducing field current to zero while maintaining full armature current to create braking torque

Question 7

A motor nameplate lists a NEMA enclosure designation of 'TEFC.' What does TEFC mean?

Accepted Answer

Totally Enclosed Fan Cooled — the motor is sealed and cooled by an external fan on the shaft

Answer

Totally Enclosed Free Circulation — the motor relies on natural convection for cooling

Answer

Temperature Enclosed Fan Controlled — an internal thermostat controls the cooling fan speed

Answer

Thermally Efficient Fan Cooled — the motor uses a high-efficiency internal cooling fan