Name :- Smit Shah -140410109096 Rawat Suraj-140410109085

S.Y electrical 2 Sem 4

Subject:-AC Machines

Topic :- Type of Single phase induction Motor…..

INTRODUCTION

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 or starting winding.

Operating principle

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 so we will discuss the double revolving theory only.

Double revolving field theory

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

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

The interaction between the fields and the current induced in the rotor bars generates opposing torque.

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Resultant flux φr = 2 x =

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Starting torqueThe single-phase motor starting torque is zero of the

pulsating single-phase magnetic flux.

The starting of the motor requires the generation of a rotating magnetic flux similar to the rotating flux in a three-phase motor.

Two perpendicular coils that have currents 90° outof-phase can generate the necessary rotating magnetic fields which start the motor.

Therefore, single-phase motors are built with two perpendicular windings.

The phase shift is achieved by connecting – a resistance, – an inductance, or – a capacitance in series with the starting

winding.

Most frequently used is a capacitor to generate the starting torque.

Single-phase induction motors are classified and named according to the method employed to make them self-starting.

(1) Split-phase induction motor : ( started by two phase motoraction through the use of an auxiliary or starting winding)

(2) Capacitor motors : ( started by two phase motor actionthrough the use of an auxiliary and a capacitor)(i) Capacitor-start induction run motor(ii) Capacitor run induction motor(iii) Capacitor-start and Capacitor run induction motor

(3) Shaded-pole induction motor :(started by the motion of themagnetic field produced by means of shading coil aroundA portion of the pole structure)

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Split phase induction motorTwo windingMain winding &

auxiliary winding

90 degree phase angle

Main winding – low resistance and high reactance

Aux winding- high resistance and low reactance

Switch = motor pickup its 75 % of its rated speed.

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Working

Is lags by V because low reactance and high resistance

Im lags V by very large angle due to high reactance

Starting torque proportional to sin α

Disconnect switch after reach its 75 % of rated speed

Centrifugal switch- operation It is connected in series with the auxiliary

winding and mounted on the rotor shaft. When the motor started and reaches 75% of synchronous speed, CFS is mechanically opened and in running condition auxiliary winding remains cutout from the circuit.

Therefore, the motor runs only with main winding.

Characteristics1.Starting torque – 1.5 times to 2 times the full load torque.

2. Starting current – 6 to 8 times the full load current.

3. Operating power factor – 0.55 to .65.

4. Efficiency – 60-65%

5.Power rating-(1/20 to 1/2kW)

6. Speed range- 2875 rpm to 700 rpm

Applications It has low starting current and moderate starting

torque. It is used for easily started loads and typical applications include fans, saws, washing machines, grinders, blowers centrifugal pumps, oil burners, domestic refrigerators etc.

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Capacitor induction motor1. Capacitor start motor2. Permanent capacitor motor3. Capacitor start capacitor run motor

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Capacitor start motorElectrolytic

capacitor 90degree phase

angle Im and Is Im lags V due to

high reactance of main winding

Due to capacitor Is lead V

Im and Is angle 90 High starting torque Apps: Lathes,

drilling machiine,fan etc

Characteristics1.Starting torque- 3.5 to 4.5 times thefull load torque

2. α-varies from 30˚ to 80˚-Startingtorque can be improved

3. Power factor varies from 0.65 to 0.75

4. Efficiency– 65 to 70%

5. Power rating –1/10 to 3/4 kW

Requirements of capacitorElectrolytic capacitors are used as starting

capacitors ,value around 50μF to a few hundred micro-farads.

Capacitor designed for extremely short time duty service and is guaranteed for not more than 20 periods of operation/hr, each period not to exceed 3 seconds.

ApplicationsThis motor has high starting torque and therefore

is used for hard start loads such as pumps, compressors, refrigeration units, conveyors, certain machine tools etc.

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Capacitor start, capacitor run motor

Characteristics1.Starting torque – 3 to 3.5 times the full load torque.

2. Efficiency – 60% approx.

3. Power factor-0.9 approx.

4. Power rating- 1/8 to 3/4 HP

5. Pull out torque- up to 260% ofrated torque

Applications It combines the advantages of capacitor start and

permanent-capacitors and is used for hard to start loads such as compressors, refrigerators, pumps, conveyors etc.

These motors are often employed requiring a quite operating motor such as in hospitals, studios.

Shaded-pole induction motor

Theory of operationSmall motors requiring only small starting torque

may be made self-starting by using low resistance copper bands called shading bands.

The emf induced in the shading band causes a current to flow in the band. This current opposes the change of flux to which it is due and thus there is a time lag between the fluxes in the shaded and unshaded portion of the pole.

The flux in the shaded portion of the pole lags the flux in the other portion. The results is like a rotating field moving in the direction from the unshaded to the shaded portion of the pole and a starting torque is produced.

Characteristics1.Low starting torque – 0.4 to 0.8 times the full load

torque.

2. Efficiency-35% approx.

3. Powerfactor-0.45 approx.

4. Pull out torque- 15% of full load torque.

5.Power rating-1/300 to 1/20 HP (2.5W to40W)

ApplicationsBecause of their poor starting torque, low power

factor and poor efficiency such motors are only suitable for low power applications such as for toys, portable fans , electric clocks, hair dryers, slide projectors, humidifiers, small business machines such as photo copying machines, vending machines, advertising displays etc.