Single Phase AC Motors

Question 1

How is the rotating magnetic field developed in a split phase motor?

Answer 1

Single phase AC supply feeding two parallel connected windings (run & start) resulting in two currents ideally phase displaced by 90°

Question 2

Physical characteristics of the “start” & “run” windings?

Answer 2

Start = Smaller wire, higher resistance, lots of turns, closer to outside of stator

Run = Larger wire, low resistance, fewer turns, deep in stator

Question 3

At what speed does the centrifucal switch disconnect the start winding from the line? Why does it do this?

Answer 3

75% of synchronous speed. Starting winding is not rated for continuous duty

Question 4

How do you reverse the direction of rotation for a single phase AC motor?

Answer 4

interchange start OR run leads

start is standard

Question 5

What would happen to a capacitor start motor if the centrifugal contacts welded closed during start?

Answer 5

Slower speed/burnout

Question 6

What would happen to a capacitor start motor if the centrifugal contacts remained open during starting?

Answer 6

No start / high current

Question 7

What advantages are gained from a “permanent capacitor” single phase induction motor?

Answer 7

Quiet running speed / good operating PF

Question 8

What is another name commonly applied to the AC series motor?

Answer 8

Universal Motor

Question 9

What are some common applications of of a single phase AC series motor?

Answer 9

Drills, Vacuums, Reciprocating Saws

Question 10

How would you reverse the direction of rotation for an AC series motor?

Answer 10

Interchange armature leads

Question 11

Describe constructional features of a shaded pole AC motor stator

Answer 11

Shading coils placed on one side of wound stator poles

Question 12

Describe constructional features of a shaded pole AC motor rotor

Answer 12

Regular squirrel cage rotor (high resistance)

Question 13

How could you reverse the direction of rotation for a shaded pole type AC motor?

Answer 13

Change the orientation of the stator

Question 14

How is a single phase AC synchronous motor constructed?

Answer 14

Shading coil type stator with hardened steel rotor

Question 15

what is the main difference between the performance between a split phase induction motor and a capacitor start induction motor?

Answer 15

Capacitor start has a greater starting torque

Question 16

What is the main function of the capacitor in the capacitor start induction motor?

Answer 16

Improve the starting torque

Question 17

List two methods of controlling the speed on a single phase universal motor?

Answer 17

1. Series resistor

2. Electronic control

Question 18

what is the main advantage of a universal single phase motor?

Answer 18

High starting torque

Question 19

which single phase motor has the best speed regulation?

Answer 19

Synchronous motor

Question 20

which motor has the worst speed regulation from no load to full load?

Answer 20

Universal motor

Question 21

What are two advantages and disadvantages of Shaded pole motors?

Answer 21

Advantages: Cheap & simple
Disadvantages: Poor torque, inefficient

Question 22

Name two applications of shaded pole motors

Answer 22

Bathroom fans, small appliances

Question 23

Main parts of single phase AC motors?

Answer 23

• Stator
  - main/run winding
  - aux/start winding
• Rotor
  - squirrel cage
• Centrifugal switch
• End bells/bearings
• Nameplate

Question 24

What are the 4 main types of single phase motors?

Answer 24

1. Resistive start / Induction run
2. Capacitor Start
3. Capacitor start / capacitor run
4. Capacitor Run

Question 25

Which winding current is more out of phase of the voltage in a resistive start / induction run motor? what is the typical angle between the two winding currents?

Answer 25

Run current lags the supply voltage more than the start.

Run & start are typically 30-50° phase displaced