Single Phase Induction Motor - RKDF University single phase IM.pdfThe single-phase motor stator has a laminated iron core with two windings arranged perpendicularly. 1. One is the

Single PhaseInduction Motor

Dr Sanjay Jain

Department Of EEEX

Application - The single-phase induction machine is

the most frequently used motor for refrigerators

washing machines clocks drills compressors

pumps and etc

1048707

The single-phase motor stator has a laminated iron

core with two windings arranged perpendicularly

1 One is the main and

2 The other is the auxiliary winding or starting

winding

The single-phase induction motor operation

can be described by two methods

ndash Double revolving field theory and

ndash Cross-field theory

bull Double revolving theory is perhaps the

easier of the two explanations to understand

bull Learn the double revolving theory only

Double revolving field theory

bull A single-phase ac current supplies the main

winding that produces a pulsating magnetic

field

bull Mathematically the pulsating field could be

divided into two fields which are rotating in

opposite directions

bull The interaction between the fields and the

current induced in the rotor bars generates

opposing torque

Double revolving field theory

bull Each of the rotating fields induces a voltage in the

rotor which drives current and produces torque

bull An equivalent circuit similar to the equivalent

circuit of a three phase motor can represent each

field

bull The parameters of the two circuits are the same

with the exception of the slip

Double revolving field theory

bull The two equivalent circuits are connected in series

bull Fig shows the equivalent circuit of a singlephase

motor in running condition

bull The current power and torque can be calculated

from the combined equivalent circuit using the Ohm

Law

bull The calculations are demonstrated on a numerical

example

AC Motorbull An AC motor is an electric motor driven by an alternating current (AC)

Types of AC Motor [depending on the type of rotor used]

1 The first type is the induction motor or asynchronous motor

This type relies on a small difference in speed between the rotating magnetic field and the rotor to induce rotor current

2The second type is the synchronous motor

This type does not rely on induction and as a result can rotate exactly at the supply frequency or a sub-multiple of the supply frequency

Types of Single Phase Motors Are[construction amp method of starting]These motors are designed to operate from single phase supply are manufactured in large no of types for the use in home offices factories etc small motors come in Kilo Watt power ratings

bull Induction Motors

bull Repulsion Motors

bull AC Series Motors

bull Un-exited Synchronous Motors

Double Revolving Field Theory

1 If the rotor is given an initial rotation in either direction the torque due to the rotating field acting in the direction of initial rotation coil will be more than that due to the other rotating field and the motor will develop a net positive torque in the same direction as the initial rotation

2 Thus the motor will keep running in the direction of initial rotation3 According to this theory any alternating quantity can be resolved into

two rotating components which rotate in opposite directions and each having magnitude as half of the maximum magnitude of the alternating quantity

3In case of single phase induction motors the stator winding produces an alternating magnetic field having maximum magnitude of Φ1m

4According to double revolving field theory consider the two components of the stator flux each having magnitude half of maximum magnitude of stator flux ie (Φ1m2)

5 Both these components are rotating in opposite directions at the synchronous speed Ns which is dependent on frequency and stator poles

bull Let Φf is forward component rotating in anticlockwise direction

bull while Φb is the backward component rotating in clockwise direction

bull The resultant of these two components at any instant gives the instantaneous value of the stator flux at the instant

bull So resultant of these two is the original stator flux

fig shows the stator flux and its two components Φf and Φb At start both the components are shown opposite to each other in the Fig1(a) Thus the resultant ΦR = 0 This is nothing but the instantaneous value of the stator flux at start After 90o as shown in the Fig 1(b) the two components are rotated in such a way that both are pointing in the same direction

Hence the resultant ΦR is the algebraic sum of the magnitudes of the two components So ΦR = (Φ1m2) + (Φ1m2) =Φ1m This is nothing but the instantaneous value of the stator flux at θ = 90o as shown in the Fig 1(c) Thus continuous rotation of the two components gives the original alternating stator flux

bull Both the components are rotating and hence get cut by the motor conductors

bull Due to cutting of flux emf gets induced in rotor which circulates rotor current

bull The rotor current produces rotor flux

bull This flux interacts with forward component Φf to produce a torque in one particular direction say anticlockwise direction

bull While rotor flux interacts with backward component Φb to produce a torque in the clockwise direction

bull So if anticlockwise torque is positive then clockwise torque is negative

At start these two torque are equal in magnitude but

opposite in direction

Each torque tries to rotate the rotor in its own

direction

Thus net torque experienced by the rotor is zero at

start And hence the single phase induction motors

are not self starting

Torque speed characteristicsbull The two oppositely directed torques and the resultant torque can be shown effectively

with the help of torque-speed characteristics

Starting methods

o single-phase capacitor-start motors

o capacitor-run motors

osplit-phase motors

o shaded-pole motors

o small poly phase induction motors

Single Phase Induction Motor

bull The single-phase motor starting torque is zero

because of the pulsating single-phase magnetic flux

bull The starting of the motor requires the generation of a

rotating magnetic flux similar to the rotating flux in

a three-phase motor

bull Two perpendicular coils that have currents 90deg outof-

phase can generate the necessary rotating

magnetic fields which start the motor

bull Therefore single-phase motors are built with two

perpendicular windings

The phase shift is achieved by

connecting

ndash a resistance

ndash an inductance or

ndash a capacitance

in series with the starting winding

bull Most frequently used is a capacitor to

generate the starting torque

Capacitor start induction motor

Capacitor run motors

Split phase motor

Shaded pole motors

Blocked Rotor test for Induction Motorbull Blocked rotor test is conducted on an induction motor It is also known as short circuit test or

locked rotor test or stalled torque test

bull From this test short circuit current at normal voltage power factor on short circuit total leakage reactance starting torque of the motor can be found

bull The test is conducted at low voltage because if the applied voltage was normal voltage then the current flowing through the stator windings were high enough to over heat the winding and damage them

bull The blocked rotor torque test is not performed on a wound rotor motors because the starting torque can be varied as desired However blocked rotor current test is conducted on squirrel cage rotor motors

some one can hold this shaft to block the rotation

0-10A

0-300V

Method

bull In the blocked rotor test the rotor is locked

bull A low voltage is applied on the stator terminals so that full load current flows in the stator winding

bull The current voltage and power input are measured at this point

bull When the rotor is stationary the slip S=1

bull The test is conducted at 14th the rated frequency as recommended by IEEE

bull This is because the rotors effective resistance at low frequency may differ at high frequency

bull The test can be repeated for different values of voltage to ensure the values obtained are consistent

bull As the current flowing through the stator may exceed the rated current the test should be conducted quickly

bull By using the parameters found by this test the motor circle diagram can be constructed

No Load Test of Induction Motorbull By the name no load test it means that there is no load- that is load is zero But it is exact

opposite No load means infinite load test It is because in no load there is NO load and no load means it is open circuit Open circuit means infinite resistance

bull But that was the case in transformers where you can open circuit the low voltage of transformer and obtain readings But how will you do that in Induction motor How can you make infinite resistance at load side

bull If slip=0 then Rl will be infinite And you can make slip zero by making synchronous speed Ns equal to actual speed N

bull So slip will be zero Load resistance will be infinite

No load at output of motor0-300V

0-10A

Method

bull Connect the circuit

bull Supply the rated voltage to induction motor keep it running

bull The current drawn by motor is quit low

bull Take care of the voltmeter should be of voltage ratings of induction motor amp the ratings of ammeter should be low because the current drawn by motor is very small

bull Take the readings of voltmeter ammeter and wattmeter