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

A VFD rectifies AC to DC, then inverts it back to variable-frequency AC; since synchronous speed is directly proportional to frequency, varying frequency varies motor speed.

Question 2

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

Accepted Answer

Synchronous motor

Answer

Synchronous motors use DC-excited rotor poles that lock in step with the rotating stator field, running at precisely synchronous speed regardless of load (up to the pull-out torque limit).

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

A normally closed contact is closed when the coil is de-energized; energizing the coil causes the armature to pull in, which mechanically opens the NC contact.

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

Plugging reconnects the motor with reversed phase sequence while still rotating, creating a braking torque that rapidly stops the motor; a zero-speed switch must disconnect power before reversal occurs.

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

A single-phase supply produces a pulsating (not rotating) magnetic field; the auxiliary winding and capacitor create a second phase, shifted approximately 90°, producing the rotating field needed to develop starting torque.

Ramsay Test Practice Test

Ramsay Test Practice Test

Ramsay Test Motors and Controls 3