Electrical Machines-||

Synchronous Motors

R.Prabhu132912

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Motor Types3 Phase1 Phase

• Reluctance• Hysteresis• Permanent Magnet• Inductor

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Characteristics• High operating efficiency• Smooth constant starting & accelerating torque• Versatile power factor control• Constant speed• Considerably more expensive than induction motors• Zero starting torque

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Torque Angle

No Load

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Torque Angle

Full Load

Torque AngleDependant on:• Load torque• Excitation

• The magnetic link between the stator & the rotor can be thought of as a connecting spring.

• The excitation can be used to strengthen the spring

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If the load Torque fluctuatesOr

Changes rapidly• The magnetic “Spring” will bounce• Causing large fluctuations in supply current• Amortisseur windings are added to the rotor• Also known as hunting

Torque Angle05/01/2023Prabhu mike

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Damper Windings

• When relative movement between the stator and the rotor poles occurs

• Voltage is induced into these windings.

• Subsequent induced magnetic field tends to slow movement and act like a “shock absorber”

• Can be used to aid starting in a simular way to that of the squirrel cage conductors

Similar to the squirrel cage found in induction machines

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StartingZero starting Torque

Number of methods:• Pony Motor• Low Frequency

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StartingLow Frequency

• The rotor’s excitation windings are short circuited• Act like a wound rotor induction motor• Supply is applied at reduced voltage & frequency• Rotor builds up speed• Excitation is then applied to windings and rotor locks in

Tumit 3 and the Shoalhaven hydro schemes use this system

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Motor Uses•Can be used as a standard motor similar

to an induction motor

•Main use as a power factor correction device

As an induction machine is cheaper it is seldom used just as a motor

Sometimes called a rotary capacitor

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Power Factor Correction

As we increase the excitationThe

stat

or c

urre

nt w

ill d

rop

And the Power Factor Improves

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Power Factor Correction

If we continue to increase the excitation

The

stat

or c

urre

nt w

ill in

crea

se

And the Power Factor decreased

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Power Factor Correction

These curves are known as “Vee Curves”

Curve A = Stator Current

Curve B = Power Factor

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Power Factor CorrectionThese curves are only applicable for a

set load torque

A different load will produce a different set of curves

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Power Factor Correction

Lagging

Unity

Leading

Under Excited Over Excited

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•Thank You For Listening …….

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