Transcript Electrical Machines

Electrical Machines
LSEGG216A
9080V
Synchronous Motors
Week 14
Introduction
• 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 Types
3 Phase
1 Phase
• Reluctance
• Hysteresis
• Permanent Magnet
• Inductor
Characteristics
•
•
•
•
•
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High operating efficiency
Smooth constant starting & accelerating torque
Versatile power factor control
Constant speed
Considerably more expensive than induction motors
Zero starting torque
Stator
Same as an induction motor’s stator
Some books may call this the Armature
Rotor
• 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 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
Called “Excitation”
• Rotor operates at synchronous speed
120 f
= Nrotor
N sync 
P
Torque Angle
No Load
Torque Angle
Torque Angle
Dependant on:
• Load torque
• Excitation
• The magnetic link between the
stator & the rotor can be thought
of as a connecting spring.
Full Load
• The excitation can be used to
strengthen the spring
Torque Angle
If the load Torque fluctuates
Or
Changes rapidly
• The magnetic “Spring” will bounce
• Causing large fluctuations in supply current
• Amortisseur windings are added to the rotor
• Also known as hunting
Amortisseur Windings
Similar to the squirrel cage found in induction machines
Also Known as “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
Starting
Zero starting Torque
Number of methods:
• Pony Motor
• Low Frequency
Pony Motor
Starting
An auxiliary smaller motor is used to spin the main motor
up to or near Synchronous speed
Starting
Low 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 Uses
• Can be used as a standard motor similar to an
induction motor
As an induction machine is cheaper it
is seldom used just as a motor
• Main use as a power factor correction device
Sometimes called a rotary capacitor
Power Factor Correction
The stator current will drop
And the Power Factor Improves
As we increase the excitation
The stator current will increase
Power Factor Correction
And the Power Factor detieriates
If we continue to increase the excitation
Power Factor Correction
Curve A = Stator Current
Curve B = Power Factor
These curves are known as
“Vee Curves”
Power Factor Correction
These curves are only applicable for a
set load torque
A different load will produce a different set of curves
Power Factor Correction
If the bride is over
Unityexcited she will
lead you to the marriage bed
Lagging
Under Excited
Leading
Over Excited
Power Factor Correction
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 Correction
PF = 0.9 Lagging
Stator Current
38% X 100A = 38A
Example
• 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
450 x cos 49.5=
292.5kW
49.5
450kVA
450 x sin 49.5=
342kVar
Example
• 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
292.5 + 90 =
382.5kW
tan-1 x 286/382.5 = 36.8
PF =
0.8 Lag
90 x tan 31.8 =
292.5kW
49.5
31.8
55.8kVar
342 – 55.8 = 286kVar
90kW
450kVA
342kVar