Synchronous motors operate at a constant synchronous speed determined by the number of poles and frequency of the power supply. They have high efficiency and provide smooth constant torque but are more expensive than induction motors. The rotor is either wound similarly to an induction motor or contains permanent magnets. Methods to provide starting torque include using a pony motor, applying a reduced voltage and frequency to the rotor windings to make it operate like an induction motor initially, or using damper windings. Synchronous motors are commonly used for power factor correction by varying the field excitation to control the phase relationship between voltage and current.

1. Electrical Machines
LSEGG216A
9080V

2. Synchronous Motors
Week 14

3. 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

4. Motor Types
3 Phase
1 Phase
• Reluctance
• Hysteresis
• Permanent Magnet
• Inductor

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

6. Stator
Same as an induction motor’s stator
Some books may call this the Armature

7. 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

8. 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
120f
N sync = = Nrotor
P

9. Torque Angle
No Load

10. 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.
• The excitation can be used to
strengthen the spring
Full Load

11. 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

12. 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

13. Starting
Zero starting Torque
Number of methods:
• Pony Motor
• Low Frequency

14. Pony Motor
Starting
An auxiliary smaller motor is used to spin the main motor
up to or near Synchronous speed

15. 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

16. 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

17. Power Factor Correction
And the Power Factor Improves
The stator current will drop
As we increase the excitation

18. Power Factor Correction
And the Power Factor detieriates
The stator current will increase
If we continue to increase the excitation

19. Power Factor Correction
Curve A = Stator Current
Curve B = Power Factor
These curves are known as
“Vee Curves”

20. Power Factor Correction
These curves are only applicable for a
set load torque
A different load will produce a different set of curves

21. Power Factor Correction
If the bride is over excited she will
Unity
lead you to the marriage bed
Lagging
Leading
Under Excited Over Excited

22. 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?

23. Power Factor Correction
PF = 0.9 Lagging
Stator Current
38% X 100A = 38A

24. 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°
450 x sin 49.5°=
450kVA
342kVar

25. 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
tan-1 x 286/382.5 = 36.8°
292.5 + 90 = 382.5kW
PF = 0.8 Lag
90 x tan 31.8° =
292.5kW 31.8° 55.8kVar
342 – 55.8 = 286kVar
49.5° 90kW
450kVA
342kVar