single phase induction motor1

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8/10/2012 Single phase induction motor 1


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Single Phase Induction Motor

The single-phase induction machine is the most

frequently used motor for refrigerators, washing

machines, clocks, drills, compressors, pumps, and so

forth.

The single-phase motor stator has a laminated iron

core with two windings arranged perpendicularly.

One is the main and The other is the auxiliary winding orstarting

winding


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This single-phase

motors are truly two-

phase machines.

The motor uses asquirrel cage rotor,

which has a laminated

iron core with slots.

Aluminum bars are

molded on the slots andshort-circuited at both

ends with a ring.

Stator with laminated

iron core Slots with winding

Bars

Ring to short

circuit the barsStarting winding

+

_

Main winding

Rotor with

laminated

iron core+

_

Figure 7.42 Single-phase

induction motor.


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Figure 7.10 Squirrel cage rotor


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The single-phase induction motor operation

can be described by two methods:

Double revolving field theory; and

Cross-field theory.

Double revolving theory is perhaps the

easier of the two explanations to understand

Learn the double revolving theory only


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7.6.1 Double revolving field theory

A single-phase ac current supplies the main windingthat produces a pulsating magnetic field.

the pulsating field could be divided into two fields,which are rotating in opposite directions.

The interaction between the fields and the currentinduced in the rotor bars generates opposing torqueSo the resultant torque is equal zero.


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The interaction betweenthe fields and the currentinduced in the rotor barsgenerates opposing

torque. Under these conditions,

with only the main fieldenergized the motor willnot start

However, if an external

torque moves the motor inany direction, the motorwill begin to rotate.

Starting winding

Main winding

+t-t

Main winding flux

Figure 7.43 Single-phase motor main winding

generates two rotating fields, which oppose and

counter-balance one another.


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*Tomake the single phase

induction motor self start: By adding extra winding to the main winding .

These two winding (main winding and auxiliary

winding )are perpendicular . And are connected inparallel with the single phase supply.

To produce torque to rotate the motor there must be

phase angle between the current down by main

winding (Im)and the current drown by the startwinding.

To make this there are many methods:


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Split phase machine: Main winding has low

resistance and highreactance .

Auxiliary winding hashigh resistance and lowreactance.

To increase the resistanceof the start winding

connect resistance inseries with the startwinding.


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Is lags volt by small angle butIm lags volt by larger angle sothere is larger phase angle

between the two currents sothe torque of split phasedepend on sin(alpha)

The center fugal switch isdisconnect the starting

winding after the motorreaches to 70 to 80 percentageof its full load speed.

Center fugal switch

connected in series with thestarting winding.


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We can use electromagnetic relay

instead of center fugal switch:

The relay coil us connected to themain winding

The pair of contacts witch is

normally open is included in the

startingwinding. At start Im is very larg the relay

contacts close and allow to Is to

flow and the motor start as usual .

After speed reaches to 7percentage of the full load speed

Im drop to low value enough to

open the contacts of the relay.


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*capacitor start induction-run motors: At this type we connect

capacitor in series with thestarting winding and alsocenter fugal switch.

the capacitor connected out of

the motor. The phase angle at this type

reaches to 80 degrees at(200W,50Hz motor) but at splitphase type phase angle reaches

to 30 degrees. This means thatthe torque developed in case ofcapacitor start type is twice thevalue at split phase type.


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*capacitor start capacitor run:

We get greater phase angle and greater

torque than capacitor start by adding

capacitor in parallel with the capacitor inthe previous type .


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Forward

rotating field

Xsta/2

Vsta

Ista

Rrot

(1-spos

)/(2spos

)

Rsta/2

Rc/2 X

m/2

Xrot/2 R

rot/2

Reverse

rotating field

Xsta/2

Rrot(1-sneg)/(2sneg)

Rsta/2

Rc/2 Xm/2

Xrot/2 R

rot/2

Ipos

Ineg

Figure 7.44 Equivalent circuit of a single-phase motor in running condition.


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7.6 Single Phase Induction Motor

The results of the calculations are:

Input power:

Developed or output power:

*

stastain IVS

neg

negrot

pos

posrot

devs

sR

s

sRP

1

2

1

2

22

negpos II


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7.6 Single Phase Induction Motor

nm 400rpm 410rpm 1780rpm

400 600 800 1000 1200 1400 1600 18000

100

200

300

400

500

Pmech nm W

Pmot_dev nm W

Pmech nrated W

nm

rpm

Pdev_max

nmax

Operating Point

Pmech_max

Stable

Operating

Region

Figure 7.47 Single-phase motor mechanical output power and electrically

developed power versus speed.


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Operating Point

nm 0.1rpm 1rpm nsy

0 500 1000 15000

2

4

6

8

10

12

Tsm nm N m

Tm nm N m

T nm N m

Tnom

N m

nm

rpm

Starting winding

is disconnected

Main winding

generated torque

Combined main and

starting windings

generated torque Figure 7.51Torquespeed

characteristic of

a small single-

phase induction

motor.


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A less effective but more economical method usingshaded pole motors

The motor has two salient poles excited by ac

current.

Each pole includes a small portion that has a short-circuited winding. This part of the pole is called theshaded pole.

The main winding produces a pulsating flux thatlinks with the squirrel cage rotor.

This flux induces a voltage in the shorted winding.


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The induced voltage produces a current in the

shorted winding.

This current generates a flux that opposes the mainflux in the shaded pole (the part of the pole that

carries the shorted winding).

The result is that the flux in the unshaded and

shaded parts of the pole will be unequal.

Both the amplitude and the phase angle will be

different.


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These two fluxes generate an unbalanced rotatingfield. The field amplitude changes as it rotates.

Nevertheless this rotating field produces a torque,which starts the motor in the direction of the shadedpole.

The starting torque is small but sufficient for fansand other household equipment requiring small

starting torque.

The motor efficiency is poor but it is cheap


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The motor has twosalient poles excitedby ac current.

Each pole includes asmall portion that hasa short-circuitedwinding.

This part of the poleis called the shadedpole

Main winding

Shorted

coil

Unshaded

pole

Unshaded

pole

Shaded

pole

Shaded

pole

Shorted

coil

Squirrel cage

rotor

Figure 7.52 Concept of single-

phase shaded pole motor.


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Figure 7.53

Shaded polemotor for

household fan.

single phase induction motor1

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