Post on 24-Dec-2015
Three-Phase AC machines
Three-Phase Synchronous Machines
Resource 7
Three-Phase AC Machines Resource 7
Aim
Three-Phase Synchronous Machines
• To understand the construction and operation of a three-phase synchronous machine
Objectives
Three-Phase AC Machines Resource 7
Three-Phase Synchronous Machines
• 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
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
Stator Construction
• Slot insulator inserted by hand
• 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
Stator frame
Stator slots with insulator
Stator Construction
• Coils inserted by hand
Coil
• 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
• Slot insulator inserted by hand
Stator frame
Stator slots with insulator
Stator Construction
• Coils can be placed in single or double layers
Stator slot
Stator Construction
Single layer
Stator Slots
1 coil arm per slot
Coil
Stator Construction
Double layer Stator Slots
Coil
2 coil arms in each slot
Stator Construction
Stators can be very large
Rotor Construction
• Salient Pole
Two types of rotor
• Cylindrical
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
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
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
Operation as a Synchronous Generator
Two pole cylindrical rotor example
• Rotor field is turned at 3000rpm by a prime mover
SN
A
B
B’
C
C’
A’
A
A’
B
B’
C
C’
• 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
• Field produced on rotor by dc current through slip rings
Operation as a Synchronous Generator
Two pole cylindrical rotor example
A BC
• Delay between phases = 20/3 = 6.667ms 6.667ms
Period = 20ms
• 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
• Field produced on rotor by dc current through slip rings
Calculations
Synchronous speed
Induced EMF
fS = supply frequency required
p = pole pairs
Φ = flux per pole set by rotor current
z = conductor in series per phase
Volts per phase
RPM
Operation as a Synchronous Generator
Generated EMF relationship
• 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.
The open circuit EMF generated depends upon
saturation
Open circuitstatorEMF
Rotor current
linear
Operation as a Synchronous Motor
Two pole cylindrical rotor example
• 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
• Stator field rotates at 3000rpm from 50Hz supply
SN
A
B
B’C’
A’
NR
NS
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
Operation as a Synchronous Motor
Two pole cylindrical rotor example
• 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
• Stator field rotates at 3000rpm from 50Hz supply
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
Rotor Speed (NR)
NS
Load Torque
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