Operation Of Three Phase Induction Motors

Question 1

What is slip

Answer 1

It is the ratio of slip speed to synchronous speed, expressed as a decimal or a percent

Most induction motors have a small speed variation of 2-5% of synchronous speed

Question 2

Describe rotor resistance in a SCIM

Answer 2

It stays constant

Question 3

Describe rotor resistance in a WRIM

Answer 3

Can be adjusted using external resistors connected through brushes and slip rings.
This enables the operator to reduce starting current while increasing rotor torque

Question 4

What is blocked rotor

Answer 4

The instant when the stator is first energized but before the rotor begins to turn

Question 5

Describe rotor voltage

Answer 5

1.Proportional to slip
2. Rotor voltage equals the slip times the blocked rotor voltage
3. Induced voltage is at its maximum at the instant of start and decreases linearly as the rotor accelerates

Question 6

Describe rotor frequency

Answer 6

1. Proportional to slip
2. Rotor frequency equals the blocked rotor frequency times the slip
3. At its maximum value at the instant of start and decreases linearly as the motor accelerates

Question 7

Describe rotor reactance

Answer 7

1. Proportional to slip
2. It is effectively inductive reactance because the rotors capacitive reactance is negligible.
3. Equal to slip times blocked rotor reactance
4. At its maximum at the instant of start and decreases Iinearly as the rotor accelerates

Question 8

Describe rotor impedance

Answer 8

1. At any time it is the phasor sum of rotor resistance and rotor reactance
2. At its maximum at the instant of start and decreases as the rotor accelerates

Question 9

What are the 2 components of induced rotor currents

Answer 9

1. Active rotor current (in phase with with induced voltage)
2. Reactive rotor current (90 degrees out of phase with induced voltage)

Question 10

Describe active rotor current

Answer 10

1. It is the rotor current times the power factor
2. It creates the copper losses in a fully loaded motor
3. It produces torque

Question 11

Describe reactive rotor current

Answer 11

1. Establishes the magnetic field that the motor needs to operate
2. Does not produce torque

Question 12

Describe Rotor power factor

Answer 12

1. It is at its minimum at the instant of start, and increases as the rotor accelerates
2. Decreases with slip

Question 13

What is breakdown torque

Answer 13

1. Achieved when the active rotor current equals the reactive rotor current
2. Rotor angle is 45 degrees
3. Power factor is 0.707

Question 14

Is Rotor slip speed and slip inversely or directly proportional to rotor speed?

Answer 14

Inversely

Question 15

Describe a Design “A” motor

Answer 15

2. Starting current of 600% FLC
3. Starting torque of 150% FLT
4. Breakdown torque 200% FLT
5. Suitable for most loads but not high inertia loads, or other hard to start loads

Question 16

Describe a Design “B” rotor

Answer 16

2. Starting current is 500% FLC
3. Starting torque is 150% FLT
4. Breakdown torque is 200% FLT
5. Suitable for most loads but not high inertia or other hard to start loads

Question 17

Describe a design “C” rotor

Answer 17

1. Double squirrel cage design
2. Starting current is 500% FLC
3. Starting torque is 225% FLT
4. Produce breakdown torque at instant of start
5. Suitable for most loads but not high inertia or hard to start loads (unless they are started infrequently)

Question 18

Describe a Design “D” rotor

Answer 18

2. Starting torque is 400% of FLC
3. Starting current is 275% FLT
4. Breakdown torque is at the instant of start
5. Most suitable for high inertia loads and frequently started, hard to start loads

Question 19

Why is the WRIM the ideal induction motor

Answer 19

Combines the best starting characteristics ( high resistance and low inductance) with the best running characteristics (low rotor resistance and low rotor inductance)

Question 20

Disadvantages of a WRIM

Answer 20

1. Expensive
2. Larger frame
3. Extra auxiliary equipment required
4. Extra maintenance