Operating Principles of Three-Phase Induction Motors

Question 1

The flux created by any winding will be at a maximum when the current through that winding is at a _____________

Answer 1

maximum

Question 2

When the current through the winding reverses, the flux direction created by that winding _________

Answer 2

reverses

Question 3

What effect does increasing the frequency of the supply have on the speed of the rotating magnetic field?

Answer 3

Frequency increases, speed of rotating field also increases

Question 4

What effected does increasing the number of poles have on the speed of the rotating magnetic field?

Answer 4

Decreases

Question 5

Define synchronous speed

Answer 5

Speed at which the field rotates

Question 6

Since there is no electrical connection between the stator and the rotor, how is current made to flow in the rotor of a squirrel-cage induction motor?

Answer 6

Electromagnetic induction

Question 7

Define slip speed

Answer 7

Difference between synchronous speed and rotor speed

Question 8

Define percent slip

Answer 8

Percentage of synchronous speed

Question 9

Define speed regulation

Answer 9

Difference between no load speed and full load speed of the rotor

Question 10

How is the direction of rotation reversed in a squirrel-cage induction motor?

Answer 10

Reversing direction of rotating magnetic field. Done by interchanging any two of the stator line connections to the source

Question 11

Define full load torque

Answer 11

Torque necessary to produce rated horsepower at full-load speed

Question 12

Define breakdown torque

Answer 12

Maximum torque motor can produce before slowing down and stalling

Question 13

Define locked rotor torque

Answer 13

Torque produced at the moment power is applied to the stator

Question 14

Define starting torque

Answer 14

Maximum torque motor will develop at rest for all angular positions of rotor with rated voltage applied at rated frequency

Question 15

At what rotor speed will the stator current be the highest?

Answer 15

When rotor is at stand still (100% slip)

Question 16

What type of loads is a NEMA design class C motor suitable for?

Answer 16

High inertia starts. Has locked rotor torque equal to or slightly larger than its breakdown torque

Question 17

What types of loads is a NEMA design class B motor suitable for?

Answer 17

Low inertia starts. General purpose motor with starting torque typically lower than breakdown torque

Question 18

What type of loads is a NEMA design class D motor suitable for?

Answer 18

Punch presses, shears, cranes, hoists, elevators and oil well pumping jacks. Very high breakdown torque

Question 19

AC induction motors are normally designed to develop full-load torque at about __ to __ slip

Answer 19

2-5%

Question 20

Breakdown torque occurs at approximately __ slip

Answer 20

25%

Question 21

Breakdown torque has a magnitude of between ___% and ___% of the full load torque

Answer 21

175%, 300%

Question 22

On standard squirrel cage induction motors, the starting torque has a magnitude of between ___ and ___ of full-load torque, depending on speed and power ratings of motor

Answer 22

70%, 275%

Question 23

What part of a linear-induction motor moves?

Answer 23

Stator

Question 24

In what units is the synchronous speed of a linear-induction motor measured?

Answer 24

metres per second