1. Three-Phase AC machines
Resource 7
Three-Phase Synchronous Machines

2. Three-Phase AC Machines Resource 7
Three-Phase Synchronous Machines
Aim
To understand the construction and operation of a three-phase synchronous machine

3. Objectives Three-Phase AC Machines Resource 7
Three-Phase Synchronous Machines
Objectives
To be able to describe the construction of the stator
To be able to describe the construction of a salient pole rotor
To be able to describe the construction of a cylindrical rotor
To be able to describe the operation of a synchronous machine as a generator
To be able to calculate synchronous speed and terminals voltage
To be able to describe the operation of a synchronous machine as a motor

4. Stator Construction Stator is identical to the induction motor
Laminated low silicon steel rings joined together
Slots insulated with Mylar
Example of 36 slot stator with 3 coil conductors per slot, 12 slots per phase

5. Stator Construction Stator frame
Stator is identical to the induction motor
Laminated low silicon steel rings joined together
Stator slots with insulator
Slots insulated with Mylar
Example of 36 slot stator with 3 coil conductors per slot, 12 slots per phase
Slot insulator inserted by hand

6. Stator Construction Stator frame
Stator is identical to the induction motor
Laminated low silicon steel rings joined together
Stator slots with insulator
Slots insulated with Mylar
Coil
Example of 36 slot stator with 3 coil conductors per slot, 12 slots per phase
Slot insulator inserted by hand
Coils inserted by hand

7. Stator Construction Coils can be placed in single or double layers
Stator slot

8. Stator Construction
Single layer
Coil
1 coil arm per slot
Stator Slots

9. Stator Construction Double layer Stator Slots Coil
2 coil arms in each slot

10. Stator Construction
Stators can be very large

11. Rotor Construction
Two types of rotor
Salient Pole
Cylindrical

12. Rotor Construction Salient Pole
Difference between pole face curvature and stator creates non-linear variation in flux across pole face
Non-linear variation in flux across pole face produces sinusoidal change in the induced EMF

13. Rotor Construction Cylindrical
Difference in coil spacing creates non-linear variation in flux around the rotor surface
Non-linear variation in flux around rotor surface produces sinusoidal change in the induced EMF

14. Rotor Construction Cylindrical
Difference in coil spacing creates non-linear variation in flux around the rotor surface
Non-linear variation in flux around rotor surface produces sinusoidal change in the induced EMF

15. Operation as a Synchronous Generator
Two pole cylindrical rotor example S N A B B’ C C’ A’
Field produced on rotor by dc current through slip rings
Rotor field is turned at 3000rpm by a prime mover
EMFs induced in stator coils with frequency of 50Hz
Magnetic Flux distributed around rotor produces sinusoidal variation in induced EMF
Phase coils separated by 120o causes delay between phase EMFs

16. Operation as a Synchronous Generator
Two pole cylindrical rotor example
Period = 20ms
Field produced on rotor by dc current through slip rings C A B
Rotor field is turned at 3000rpm by a prime mover
EMFs induced in stator coils with frequency of 50Hz
Magnetic Flux distributed around rotor produces sinusoidal variation in induced EMF
Phase coils separated by 120o causes delay between phase EMFs
Delay between phases = 20/3 = 6.667ms
6.667ms

17. Calculations Synchronous speed Induced EMF
fS = supply frequency required
RPM
p = pole pairs
Induced EMF
Volts per phase
Φ = flux per pole set by rotor current
z = conductor in series per phase

18. Operation as a Synchronous Generator
Generated EMF relationship
Open circuit
stator
EMF
The open circuit EMF generated depends upon
saturation
Rotor speed
Rotor current
Relationship between open circuit stator EMF and rotor current is a straight line until the steel begins to saturate when it becomes non-linear.
linear
Rotor current

19. Operation as a Synchronous Motor
Two pole cylindrical rotor example S N A B B’ C’ A’ NR NS
Stator field rotates at 3000rpm from 50Hz supply
Rotor field must be locked on to stator field speed
Motor runs a synchronous speed whatever the mechanical load provided rotor field is strong enough
NR = NS
This is impossible within an induction motor as there wound be no induced currents to cause rotation
This motor runs at synchronous speed hence the name – SYNCHRONOUS MOTOR

20. Operation as a Synchronous Motor
Two pole cylindrical rotor example
Stator field rotates at 3000rpm from 50Hz supply
Rotor field must be locked on to stator field speed
Motor runs a synchronous speed whatever the mechanical load provided rotor field is strong enough
Rotor Speed (NR)
NR = NS
This is impossible within an induction motor as there wound be no induced currents to cause rotation NS
This motor runs at synchronous speed hence the name – SYNCHRONOUS MOTOR
Load Torque

21. Operation as a Synchronous Motor
The V-curve
The rotor current can be adjusted to vary the power factor of the stator
Unity power factor is achieved when stator current is at its minimum
This machine can be used to correct power factor of induction motors when connected in parallel