Electrical Machines

LSEGG216A9080V

Synchronous Motors

Week 14Week 14

IntroductionIntroduction• State the principles of operation of a synchronous motor.• Identify the main parts of a synchronous

alternator/motor.• List the methods used to provide the excitation of a

synchronous alternator/motor.• List the starting methods of synchronous

Motor TypesMotor Types3 Phase 1 Phase

• Reluctance

• Hysteresis

• Permanent Magnet

• Inductor

CharacteristicsCharacteristics

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

StatorStatorSame as an induction motor’s stator

Some books may call this the Armature

RotorRotor• Wound simular to a wound rotor induction motor

• When operating DC voltage is placed across this coil to produce an electromagnet

Some books may call this the Field Windings

Motor SpeedMotor Speed• Magnetic force is obtained from an external source

(In an induction machine rotor’s magnetism is induced from the stator)

• Rotor poles lock onto the RMF• Rotor operates at synchronous speed

P

f120N sync = Nrotor

Called “Excitation”

Torque AngleTorque Angle

No Load

Torque AngleTorque 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

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 AngleTorque Angle

Amortisseur Amortisseur WindingsWindings

• 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

Also Known as “Damper “ windings

StartingStartingZero starting Torque

Number of methods:

• Pony Motor

• Low Frequency

StartingStartingPony Motor

An auxiliary smaller motor is used to spin the main motor up to or near Synchronous speed

StartingStartingLow 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

Motor UsesMotor 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

Power Factor Power Factor CorrectionCorrection

As we increase the excitationThe

sta

tor

curr

ent

will

dro

p

And the Power Factor Improves

Power Factor Power Factor CorrectionCorrection

If we continue to increase the excitation

The

sta

tor

curr

ent

will

incr

ease

And the Power Factor detieriates

Power Factor Power Factor CorrectionCorrection

These curves are known as “Vee Curves”

Curve A = Stator Current

Curve B = Power Factor

Power Factor Power Factor CorrectionCorrection

These curves are only applicable for a set load torque

A different load will produce a different set of curves

Power Factor Power Factor CorrectionCorrection

Lagging

Unity

Leading

Under Excited Over Excited

If the bride is over excited she will lead you to the marriage bed

Power Factor Power Factor CorrectionCorrection

A motor has full load of 100A and an excitation current of 8A what will be:

• The stator current?• PF of the motor?

Power Factor Power Factor CorrectionCorrection

38%

PF =

X 100A = 38A

Stator Current

0.9 Lagging

ExampleExample• A load of 450 kVA operates at a power factor of 0.65

lagging. • An additional synchronous motor is added having an

input power of 90 kW and a maximum power factor of 0.85 leading.

• Determine reactive power and the overall power factor

450kVA

49.5

342kVar

450 x sin 49.5=

450 x cos 49.5=

292.5kW

ExampleExample• A load of 450 kVA operates at a power factor of 0.65

lagging. • An additional synchronous motor is added having an

input power of 90 kW and a maximum power factor of 0.85 leading.

• Determine reactive power and the overall power factor

450kVA

49.5

342kVar

292.5kW

90kW

31.8

90 x tan 31.8 =

55.8kVar342 – 55.8 = 286kVar

292.5 + 90 = 382.5kWtan-1 x 286/382.5 = 36.8

PF = 0.8 Lag