EETC Motor Controls 4

Question 1

What is the purpose of a Variable Frequency Drive (VFD) in motor control?

Accepted Answer

To control motor speed by varying output frequency and voltage

Answer

A VFD controls AC motor speed and torque by varying the frequency and voltage of the power supplied to the motor.

Question 2

Which NEMA enclosure type is required in locations where ignitable concentrations of flammable gases or vapors may exist?

Accepted Answer

NEMA 7

Answer

NEMA 7 enclosures are rated for Class I, Division 1 hazardous locations where flammable gases or vapors may be present.

Question 3

In a jogging (inching) circuit, what prevents the seal-in contact from latching the contactor ON?

Accepted Answer

A jog/run selector switch that breaks the holding circuit path during jogging

Answer

The jog selector or jog button arrangement breaks the path to the seal-in contact, so the contactor energizes only while the jog button is held.

Question 4

What is the effect on motor starting torque when using an autotransformer starter set to the 65% voltage tap?

Accepted Answer

Torque is reduced to approximately 42% of full-voltage starting torque

Answer

Starting torque varies as the square of the voltage, so at 65% voltage, torque is 0.65² ≈ 42% of full-voltage starting torque.

Question 5

A motor control center (MCC) bucket typically contains which combination of components?

Accepted Answer

A circuit breaker or fuses, contactor, overload relay, and control wiring

Answer

An MCC bucket is a self-contained unit housing the disconnect, protection (fuses/breaker), contactor, overload relay, and associated control wiring.

(EETC) Electrical Technician Certification Practice Test