Module-4 Single phase Induction motor - PowerPoint Presentation

Slide1

Module-4

Single phase Induction motor

By

Prof. Arjun Kumar

Asst. professor

Electrical Engineering

B.C.E.

B

hagalpur

Slide2

Introduction of 1-ph. Induction Motor

A single-phase induction motor comprises a single-phase distributed winding on the stator and

normal

squirrel-cage rotor.

A

vailability of

a wide variety of small-size motors of fractional kilowatt ratings

.

M

otors

are employed in

fans, refrigerators

, mixers, vacuum cleaners; washing machines, other kitchen equipment, tools, small

farming

appliances

, etc

.

B

ehaviour

of

single-phase

induction

motor can be explained on the basis of double-field revolving

theory and cross magnetic field theory.

Single phase induction motors are simple, robust, reliable and cheaper for small ratings. They are available up to 1 KW rating

Slide3

CUT SECTION VIEW

Slide4

T

ypes

of single phase induction motor

Split phase induction

motor

Capacitor

start 

induction motor

Capacitor

start capacitor run induction

motor

Shaded

pole induction

motor

Permanent

split capacitor motor

Slide5

Constructional features

Fig.4.1 single phase I.M

Slide6

Main parts of

Single ph. I.M.

Single ph. I.M. having

two main

parts:

Stator

Rotor

Stator:

stator

is a stationary part of induction

motor.

A single phase AC supply is given to the stator of single phase induction

motor. In this machine stator having two windings one is main winding and other is auxiliary windings.

Rotor:

rotor

is a rotating part of an induction motor. The rotor connects the mechanical load through the shaft. The rotor in the single-phase induction motor is of squirrel cage rotor

type

.

Slide7

Constructional View

Fig.4.2 constructional view of 1-ph.I.M.

Slide8

Double revolving field theory

Fig.4.3 single phase I.M

Slide9

Figure 4.3 gives the schematic diagram of a single-phase induction motor with one stator winding and a squirrel-cage rotor. The winding is distributed in space so that the space fundamental of

mmf

is the most dominant component of the actual

mmf

distribution. When the winding carries a sinusoidal current, it produces a

sinusoidally

space distributed

mmf

whose peak value pulsates with time. As seen from the axis of the winding, the

mmf

at any angle

is

---------(4.1)

where

is the angle measured from the winding axis.

Now

------(4.2)so that the mmf has both space and time distribution expressed as --------(4.3)This equation can be trigonometrically manipulated into the form ----(4.4)

 

Slide10

Equation (4.4) tells us that a pulsating single-phase field can be considered as superposition of two rotating fields rotating at synchronous speed (

elect. rad/s) in opposite directions:

The forward rotating field,

----(4.5)

Backward rotating field,

---(4.6)

Both these fields have an amplitude equal to

where

is the maximum value of the pulsating

mmf

along the axis of the winding. The splitting of a single pulsating field into two rotating fields rotating in opposite directions

 

Slide11

T

wo rotating

fields rotating equivalent of a pulsating field

Fig.4.4

Slide12

Rotor slip with respect to two rotating field

Fig.4.5

Slide13

Fig.

4.5

shows the forward and backward rotating fields along with the rotor which is rotating at speed

in the direction of the forward field.

The slip of the rotor with respect to the forward rotating field

is then

rotor slip with respect to the backward rotating field

is

 

Slide14

Torque slip characteristic of single winding 1

ph

I.M

Slide15

Equivalent circuit of single

ph

I.M

Slide16

The performance of a single-phase induction motor can be obtained by analysis of the circuit model of the motor.

The air-gap powers for the forward and backward fields are given by

Air-gap power for backward field

Here,

main winding current

and

are the real parts of the complex number impedances

and

 

Slide17

The torques produced by the two fields can be expressed as

Resultant torque developed is

 

Slide18

Rotor copper-loss corresponding to forward field=

Total rotor copper-loss

=

 

Slide19

Split phase 1-ph I.M.

Slide20

Slide21

Pulsating fields

and

can be divided into two balanced sets of opposite phase sequence

=

=

The inverse of the relationships can be expressed as

 

Slide22

Split phase 1-ph I.M.

When a motor is provided with two windings, even though these are excited from the same voltage (supply being single-phase), the currents in the two windings can be made out-of-phase by adjustment of the impedance of the auxiliary winding in relation to the main winding. As a result

and

constitute an unbalanced field set with 90° elect. space-phase

relationship.

The two symmetrical components now being unequal

The forward rotating field is made stronger than the backward rotating field resulting in the net production of starting torque.

 

Slide23

Resistance split 1

ph

I.M

.

M

otor employs an auxiliary winding with a higher

R

/

X

ratio as compared to the

main

winding.

High

R

/

X

ratio

of auxiliary winding is

achieved by using a smaller number of turns of thin wire for the

auxiliary

winding.This difference in the R/X ratio causes the auxiliary winding current to lead the main winding current by angle Fields created by the two currents also have a phase difference of a thereby constituting an unbalanced field system. The result is the production of the starting torque 

Slide24

Resistance split 1

ph

I.M.

Slide25

Application of Resistance split 1

ph

I.M.

It has a low starting current and moderate starting torque

.

It is used

for

easily

started loads and typical

applications.

fans, saws, grinders, blowers, centrifugal pumps,

office

equipment

, washing

machines etc.

available in the range of 1/20 to 1/2 kW.

Slide26

Capacitor-start Motor

Slide27

Capacitor-start Motor

It

uses the capacitor only for the purpose of starting

.

The

capacitor value

is

usually

so chosen as to give

a

= 90° elect

.

The

starting torque

is high.

capacitor need only be short-time rated

Slide28

Capacitor-start Motor

Slide29

Application of Capacitor-start Motor

Motor

has a high starting torque and therefore is used for hard starting

loads.

Typical application in compressors

, conveyors, pumps, certain machine tools, refrigeration and air-conditioning

equipment.

Available

up to sizes as large as 6 kW.

Slide30

Two value capacitor motor

Slide31

T

wo value capacitor motor

2-value capacitor motor not only uses a capacitor for starting but also

continuous

(run

) operation

.

The capacitor used permanently is called the

run capacitor,

the use of which improves

the

motor

running

performance.

At starting

(

both

capacitors

in circuit) where

>

90° elect. so that when the start-capacitor is disconnected becomes 90° elect. 

Slide32

T

wo value capacitor motor

Slide33

Application of two value capacitor motor

It combines the advantages of capacitor-start and permanent-capacitor

motors and

is used for hard to start

loads.

It

gives a high power factor and efficiency under

running

conditions

.

applications

in

refrigerators, compressors and stockers.

Slide34

Equivalent circuit parameter

A

single-phase induction

motor

can

to imagined to be consisting of two motors, having a common

stator winding

but with their respective rotors revolving in opposite

directions.

Each

rotor

has resistance and reactance half the actual rotor values

.

= resistance of stator

winding

= leakage reactance of stator

winding

X

= total magnetizing reactance

= resistance of the rotor referred to the stator

= leakage reactance of the rotor referred to the stator 

Slide35

Transformer eq.

ckt

of Single phase I.M

Rotor is stationary

Not showing forward and backward field rotor circuit

Slide36

Rotating field equivalent

ckt.of

1-ph I.M with rotor stationary

Rotor stationary

Showing effect on equivalent

ckt

. by forward and backward field.

Slide37

Rotating field equivalent

ckt.of

1-ph I.M with rotor running

Rotor is running

Showing effect on equivalent

ckt

. by forward and backward field.

Slide38

No-load and blocked rotor test of 1 ph. I.M.

Aim to determine

the equivalent circuit parameters of a single phase induction motor by performing the

No-load and blocked rotor test .

Circuit diagram as:

Slide39

No load Test

Motor is running at no load condition at rated voltage supply.

Motor consumes rated core loss and friction and

windage

loss.

Negligible

ohmic

loss.

=

No load rated voltage

=

No load stator current

=

No load losses.

 

Slide40

No load test

=

/

reactance

+X

X=

=

 

Slide41

No load

Test experimental procedure

C

ircuit

connections are made as per the circuit

diagram.

Variac

(auto transformer) is set to zero output voltage position before starting

the test.

Gradually increasing the voltage across the 1-

ph

I.M.

Motor is freely running at no load.

Variac

is varied slowly, until rated voltage is applied to

motor.

Reading of ammeter , voltmeter and wattmeter is taken when reaching rated voltage across 1-

ph

I.M.Variac is brought to zero output voltage position after test.Switch off the supply.

Slide42

Blocked rotor test.

Rotor blocked condition (stationary)

Motor consumes

ohmic

losses

Core loss is negligible

Reading take place when rated current flowing.

=

Applied voltage

= I

nput rated current

=

Input power.

 

Slide43

Blocked rotor test

=

/

Blocked rotor resistance,

Blocked rotor reactance,

=

=

/2;

=(

-

)