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