single types

7/30/2019 Single Types

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Classification of single-

phase induction motor(Starting methods of 1

induction motor)

1PRB/EEE Dept/SCE9/28/2011


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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 motor

action through the use of an auxiliary or starting winding)

(i) Resistance-start split-phase induction motor

(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 the

magnetic field produced by means of shading coil around

A portion of the pole structure)2PRB/EEE Dept/SCE9/28/2011


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Resistance-start split-phase induction motor

(temporarily split-phase)



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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 mechanicallyopened and in running condition auxiliary

winding remains cutout from the circuit.

Therefore, the motor runs only with main

winding.



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The basic Centrifugal Switch



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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

ApplicationsIt has low starting current and moderate

starting torque. It is used for easily started loadsand typical applications include fans, saws,

washing machines, grinders, blowers centrifugal

pumps, oil burners, domestic refrigerators etc.12PRB/EEE Dept/SCE9/28/2011


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In the case of split-phase motors that are hermetically sealed in refrigeration

units, instead of internally-mounted CFS, an electromagnetic relay is used.

As shown in fig., the relay coil is connected in series with main winding and

the pair of contacts which are normally open, is included in the starting

winding. During starting period, Im is large relay contacts close, thereby

allowing Is to flow and the motor starts as usual. After motor reaches 75% of

full load speed, Im drops to a value that is low enough to cause the contacts to

open.



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Reversal of direction of rotation may bereversed by reversing the line connections ofeither themain winding or the starting winding only when it is at

rest.

If reversal is attempted under running conditions,nothing will happen because under normal operating

conditions the starting switch (CFS or RELAY) is in

open position and the motor is running as a pure 1

induction motor and developing torque in the directionof rotation.

PRB/EEE Dept/SCE9/28/2011


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Capacitor start, induction run motor

(temporarily split-phase)



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Torque-speed characteristics



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Characteristics

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

2. -varies from 30 to 80-Starting

torque can be improved3. Power factor varies from 0.65 to 0.75

4. Efficiency 65 to 70%

5. Power rating 1/10 to 3/4 kW



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Requirements of capacitor

Electrolytic capacitors are usedas starting capacitors ,value

around 50F to a few hundred

micro-farads.

Capacitor designed forextremely short time duty

service and is guaranteed for

not more than 20 periods of

operation/hr, each period not toexceed 3 seconds.



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Reversal of direction of rotationIt may be reversed by reversing the

connections of one of the windings. The motoris first brought to rest for this purpose.

Applications

This motor has high starting torque andtherefore is used for hard start loads such

as pumps, compressors, refrigeration

units, conveyors, certain machine toolsetc.



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Capacitor run induction motor

(permanent split-phase ORsingle value capacitor motor)



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Phasor diagram



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Characteristics1.Starting torque-0.5 to 0.8 times the full load

torque.

2.Higher power factor-0.8 t0 0.953. Efficiency -55 to 65%

4.Smaller full load current

5.Power rating 1/20 to 3/4 HP



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Oil-filled or pyranol-insulatedfoil-paper capacitors are used as

running capacitors. Low value

of capacitance(2 to 20F) when

compared to the start capacitor.

This type is especially designed

for continuous duty rating.

Requirements of capacitor



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Reversal of direction of rotationIt may be reversed by reversing the connections

of one of the winding. Reversing operation can

be done under running condition is possible.

Applications

It is used for both easy and hard to start loads.Such motors are widely used in table and ceiling

fans, air circulators, hand blowers and other

portable tools etc. Because of the simplicity ofreversal, such motors are often used to operate

devices such as induction regulators, rheostats,

furnace valve control and arc welding control.PRB/EEE Dept/SCE9/28/2011


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Construction of ceiling fan

starting winding

Main winding

Squirrel cage rotor



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Capacitor start-capacitor run motor

(Two value capacitor motor)



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Phasor diagram



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Characteristics

1.Starting torque 3 to 3.5 times thefull 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% of

rated torque



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Reversal of direction of rotationThe direction of rotation of motor may be

reversed by interchanging the connections to

the supply of either the main winding or

auxiliary winding.

ApplicationsIt 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 etc.PRB/EEE Dept/SCE9/28/2011

S i i


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Shaded-pole induction motor



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Shading band

Main winding

Stator core

Rotor

Shaft



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Theory of operationSmall motors requiring only small starting torque may

be made self-starting by using low resistance copperbands 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 theshaded portion of the pole and a starting torque is

produced.PRB/EEE Dept/SCE9/28/2011

T d h i i


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Torque speed characteristics



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9/28/2011 PRB/EEE Dept/SCE 41

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 to

40W)


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Reversal of direction of rotationThe direction of rotation depends upon

the position of the shading coil on the

pole. i.e., unless the machine is

constructed so that the shading coil can be

shifted to the other side of pole, the

direction of rotation of such a machine

cannot be reversed.

A li ti


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Applications

Because 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 asphoto copying machines, vending

machines, advertising displays etc.

single types

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