AC Motors

Question 1

How do electrical motors work?

Answer 1

Electric motors rely on the interaction between the magnetic field in the stator or rotor. This interaction results in the rotation of

Question 2

Name 4 types of motor?

Answer 2

1- series universal
2- induction motors
3- three phase wound rotor motor
4- synchronous

Question 3

Name 4 advantages of a series universal motor?

Answer 3

-High running speeds
-More power than AC motors of similar size due to high speed and good power factor
- high starting torque
-cheap to produce

Question 4

Name a disadvantage associated with series universal motors?

Answer 4

Creates a lot of heat where carbon brushes arc to/with commuter

Question 5

How does a three phase induction motor work?

Answer 5

When a three phase supply is connected to separate windings equally distributing around the stator creating a magnetic flux. Rotating magnetic flux caused by windings causes shaft to rotate

Question 6

Compared with single phase motors, three phase induction motors are….

Answer 6

Cheaper
Smaller dimensions
More efficient
Self starting

Question 7

What are the advantages of a three phase induction motor?

Answer 7

-Single construction and cheap to make
- cage construction of the rotor is mechanical so provides strength for its industrial uses
- self starting
-very little maintenance
- high efficiency
- good power factor

Question 8

What are the limitations of three phase induction motors?

Answer 8

-speed variation is difficult
- always operate under lagging power factor
-during light load conditions they operate with poor PF
- Poor starting Torque

Question 9

Applications of theee phase induction motors…

Answer 9

Lifts
Cranes
Lathes
Drills
Industrial pumps
Industrial drives

Question 10

Single phase induction motors include:

Answer 10

Split phase motors
Capacitor-start
Capacitor- start, capacitor-run
Shade pole

Question 11

What is the main characteristic of a single phase induction motor?

Answer 11

Not self starting and often requires a started winding to lag or lead

Question 12

What are the main characteristics of a split phase induction motor?

Answer 12

High starting torque
High starting current

Question 13

What are the applications for a split phase induction motor?

Answer 13

Anything with light staying conditions
Washing machines
Agitators
Tumble driers

Question 14

What are the Main characteristics of a capacitor-start motor?

Answer 14

Good starting torque
Good power factor
Quite running

Question 15

What applications are capacitor start motors used?

Answer 15

Compressors
Refrigeration
Applications with a hard starting load

Question 16

What are the main characteristics of a capacitor-start, capacitor- run induction motor?

Answer 16

Uses capacitors that are constantly energised to start the motor, achieved by switching from start to run capacitor.

Lower full load current
High efficiency

Question 17

What applications are capacitor start capacitor run used?

Answer 17

High pressure water pumps
Auger motor
Vaccume pumps
High torque applications

Question 18

What is Slip?

Answer 18

The difference between actual rotor speed (Nr) and synchronous speed (NS)

Question 19

What is the equation for SLIP

Answer 19

S = NS - Nr / NS

rotor speed (Nr)
Synchronous speed (NS)

Question 20

What is the equation for synchronous speed?

Answer 20

NS = F / P. X 60

NS is synchronous speed
P is number of pole pairs
F is supply frequency

Question 21

What are the advantages of a three phase synchronous motor?

Answer 21

High efficiency
Ability to control power factor
High torque and low start currents
Constant speed over load variation
Low operation and maintenance costs

Question 22

What are the limitations of a three phase synchronous motor?

Answer 22

Can’t be started on load
0 start torque
Single speed
Dc supply
Not self starting
Can’t be used on applications with frequent starts

Question 23

What applications are three phase synchronous motors used?

Answer 23

Clocks
Timers
Crushers
Grinding mill
Belt conveyor
Rolling mills
Compressors