IEC Motors and Motor Controls 3

Question 1

In a star-delta (wye-delta) motor starting method, the motor initially starts in wye and then switches to delta. What is the starting current relative to across-the-line starting?

Accepted Answer

Reduced to 1/3 of full-voltage starting current

Answer

Wye-delta starting reduces the starting current to 1/3 of full-voltage (across-the-line) starting current.

Question 2

A two-speed motor uses a consequent pole winding. Switching from low speed to high speed changes the:

Accepted Answer

Number of poles

Answer

Consequent pole motors change speed by reconnecting the winding to create a different number of magnetic poles, since motor speed is inversely proportional to the number of poles.

Question 3

What is the function of a control transformer in a motor control circuit?

Accepted Answer

To step down line voltage to a safer control voltage (typically 120V) for the control circuit

Answer

Control transformers step down the higher line voltage to a safer, standardized control voltage (typically 120V) used by the pilot devices and coils.

Question 4

Which motor protection device opens the circuit when the motor windings reach an excessive temperature?

Accepted Answer

Thermal protector (thermistor or thermostat embedded in windings)

Answer

Thermal protectors or PTC thermistors embedded in motor windings directly sense winding temperature and open the circuit when unsafe temperatures are reached.

Question 5

What does NEMA design letter 'B' indicate about a motor's torque and starting current characteristics?

Accepted Answer

Normal starting torque, normal starting current, suitable for most applications

Answer

NEMA Design B motors have normal starting torque, normal starting current, and low slip, making them the most commonly used general-purpose motor design.

IEC - Independent Electrical Contractors Practice Test

IEC - Independent Electrical Contractors Practice Test

IEC Motors and Motor Controls 3