IEC Motors and Motor Controls 5

Question 1

What is the synchronous speed of a 4-pole, 60 Hz induction motor?

Accepted Answer

1800 RPM

Answer

Synchronous speed = 120 × frequency ÷ poles = 120 × 60 ÷ 4 = 1800 RPM.

Question 2

When a motor is rewound with a higher number of turns per coil using thinner wire, the motor will have:

Accepted Answer

Higher voltage rating and lower current draw

Answer

More turns increase the winding impedance, raising the voltage required and reducing the current drawn for equivalent power output.

Question 3

A motor's power factor is 0.75 lagging. To correct the power factor to near unity, you would install:

Accepted Answer

Capacitors in parallel with the motor

Answer

Capacitors supply reactive power locally, offsetting the lagging reactive power demand of inductive motors and improving the overall power factor.

Question 4

Which type of DC motor provides constant speed regardless of load and is NOT suitable for starting under heavy loads?

Accepted Answer

Shunt DC motor

Answer

Shunt DC motors have field windings connected in parallel with the armature, providing relatively constant speed but low starting torque.

Question 5

What is the purpose of a motor's 'reduced voltage solid-state starter' (soft starter)?

Accepted Answer

To gradually ramp up the voltage to the motor during starting to reduce inrush current and mechanical stress

Answer

Soft starters use SCRs to gradually increase the voltage applied to the motor, reducing starting current and mechanical shock to the driven equipment.

IEC - Independent Electrical Contractors Practice Test

IEC - Independent Electrical Contractors Practice Test

IEC Motors and Motor Controls 5