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Three-Phase AC machines
Resource 7
Three-Phase Synchronous Machines
Three-Phase AC Machines
Resource 7
Three-Phase Synchronous Machines
Aim
• To understand the construction and operation of a three-phase synchronous machine
Three-Phase AC Machines
Resource 7
Three-Phase Synchronous Machines
Objectives
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To
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To
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describe the construction of the stator
describe the construction of a salient pole rotor
describe the construction of a cylindrical rotor
describe the operation of a synchronous machine as a generator
calculate synchronous speed and terminals voltage
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
• 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
• Stator is identical to the
induction motor
• Laminated low silicon steel
rings joined together
Stator frame
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
Stator Construction
• Coils can be placed in single or double layers
Stator slot
Stator Construction
Single layer
Coil
1 coil arm
per slot
Stator Slots
Stator Construction
Stator Slots
Double layer
Coil
2 coil arms in
each slot
Stator Construction
Stators can be very large
Rotor Construction
Two types of rotor
• Salient Pole
• 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 nonlinear 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 nonlinear 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
• Field produced on rotor by dc current
through slip rings
• Rotor field is turned at 3000rpm by a
prime mover
A’
A’
BB
N
• EMFs induced in stator coils with
frequency of 50Hz
• Magnetic Flux distributed around rotor
produces sinusoidal variation in induced
EMF
• Phase coils separated by
delay between phase EMFs
120o
causes
C
C
S
C’
B’
C’
B’
A
A
Operation as a Synchronous Generator
Two pole cylindrical rotor example
• Field produced on rotor by dc current
through slip rings
Period = 20ms
C
A
• 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
B
Calculations
Synchronous speed
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
Operation as a Synchronous Generator
Generated EMF relationship
The open circuit EMF generated depends
upon
Open circuit
stator
EMF
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
Operation as a Synchronous Motor
Two pole cylindrical rotor example
NS
A’
• 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
B
N
S
C’
B’
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
A
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
• This motor runs at synchronous speed
hence the name – SYNCHRONOUS MOTOR
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