Operating Characteristics of Wound-Rotor Induction Motors

Question 1

Calculate the rotor speed of an eight-pole, 60Hz wound-rotor motor operating at 30% slip.

Answer 1

Slip speed = f x 60/p x 30%
Rotor speed = f x 60/p x (100% - 30%)
= (60Hz x 60/4) x 70%
= 630 rpm

Question 2

A wound-rotor motor is designed mainly to be a variable-_________ motor.

Answer 2

torque

Question 3

With respect to locked rotor reactance, what value of rotor resistance will give maximum starting torque?

Answer 3

The resistance should be approximately equal to the locked rotor reactance

Question 4

Torque is directly proportional to rotor current. Why is it, then, that increasing the resistance of the rotor circuit increases the torque output of the motor?

Answer 4

Increasing the resistance of the rotor circuit improves its power factor and brings the rotor current more in phase with the stator flux

Question 5

With the secondary terminals M1, M2, and M3 short-circuited, will the starting torque of the wound-rotor motor be high or low?

Answer 5

low

Question 6

Why is the efficiency of the wound-rotor motor poor when resistance is left in the rotor circuit?

Answer 6

The resistors dissipate energy in the form of heat

Question 7

To decrease the speed of a wound rotor motor, the resistance is:

a. added to the rotor circuit, or
b. taken out of the rotor circuit

Answer 7

a. Resistance is added to the rotor circuit to decrease the rotor speed

Question 8

Why is it not practical to control the speed of a wound-rotor motor below 50% slip?

Answer 8

The speed of the motor becomes too unstable beyond 50% slip

Question 9

What does the term “duty cycle” mean when applied to the secondary resistors?

Answer 9

The amount of time the resistors are expected to carry current with respect to the amount of time they are not

Question 10

How is the direction of rotation reversed on a wound-rotor motor?

Answer 10

By interchanging any two of the line leads to the stator

Question 11

State the three advantages a wound-rotor motor has over a squirrel-cage induction motor.

Answer 11

• High starting torque with low current
• Smooth acceleration under heavy load
• No abnormal heating during starting
• Good running characteristics after the starting resistance is removed
• Adjustable speed

Question 12

State three disadvantages a wound-rotor motor has over a squirrel-cage induction motor.

Answer 12

• Large physical size
• Initial and maintaining costs are higher
• More complicated control
• Poor speed regulation and effeciency when the secondary resistance is left in the circuit