single phase series motor

8/2/2019 Single Phase Series Motor

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Single phase Series Motor


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Ordinary dc series motor when energized from ac supplywould operate inefficiently because of

1. Inferior commutation. Sparking at brushes is excessive

2. poor p.f, due to large reluctance of the field and armaturewinding.

3. Increased iron losses in poles and yoke. So Efficiency is low.

To overcome these difficulties following modifications are made1. poles and yoke are laminated to reduce eddy current losses.

Core is made up of material having low hysteresis loss.

2. No of armature conductors is increased to give requiredtorque with low flux.

3. Field winding is provided with less no turns. Area of fieldpoles is increased, so that flux density is reduced. Pf getsimproved.

4. To reduce effects of armature reaction, threre by improvingcommutation and reducing armature reactance,

compensating winding is provided.


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EMFs produced in a pulsating Field

Rotational EMF: Due to armature rotation in themain field as in DC machines

Transformer EMF: Due to the fact that armaturewinding is linked by alternating field produed byfield winding.

Here field winding is considered as primary andarmature winding as secondary


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Rotational EMF


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Er and are in time phase

If field flux is alternating quantity, then Er is also alternating.

freq of Er = freq of field flux = Freq of Field current.


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fr depends upon armature speed and f depends on supply frequency.

T

T

T

Magnitude of Er changes with fr and hence with armature speed . But freq of

Er is constant = field freq


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If angle of brush separation is less than 180, but centre line of brush separation

coincides with flux axis then


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If armature magnetic axis or brush axis is at 90 degrees from fieldflux axis, only rotational emf Er will appear across the brushes, but no

transformer emf Et will appear across the brushes.

Transformer EMF

Now brushes are shifted by 90so brush axis coincides with feld axis then Et will

appear across the brushes, but rotational emf will be zero.

Transformer emf Et is always independent of armature

speed.


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T

T

T

a

T/a

T


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Magnitude of Et depends on supply freq f and is independent ofrotational freq fr.

Et lags field flux by 90 degrees.


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General Case


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Torque


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Commutation in dc machines


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Commutation in ac machines

Three emfs induced in the coil undergoing

commutation in ac machines are

1. Reactance EMF

2. Rotational EMF

3. Transformer EMF


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Reactance emf e1


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Methods of Improving commutation

Current in the coil undergoing commutation =Ert/Zsc

Design considerations:--reactance emf Lc T*T

so single turn coils are used,

--Er, Et can be reduced by reducing flux per pole &turns per coil

Use of compoles: toneutralize reactance emf.

to neutralize partly effect of both Et,Er.

Increase of Zsc

Use of discharge winding

I f Z


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Increase of Zsc:1. by using brushes with high contact resistance.

2. by connecting additional resistance between coils

and commutator segments


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Use of discharge winding

Characteristics of discharge winding


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Phasor diagram


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single phase series motor

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