Generator Basics
Basic Power Generation
•
Generator Arrangement
•
Main Components
•
Circuit
– Generator with a PMG
– Generator without a PMG
– Brush type
– AREP
•
PMG Rotor
•
Exciter Stator
•
Exciter Rotor
•
Main Rotor
•
Main Stator
•
Laminations
•
VPI
Generator Arrangement
• Most modern, larger generators have a stationary
armature (stator) with a rotating current-carrying
conductor (rotor or revolving field).
Revolving
field coils
Armature
coils
Main Electrical Components: Cutaway
Main Electrical Components: Diagram
Circuit: Generator with a PMG
• As the PMG rotor rotates, it produces AC voltage in the PMG stator.
• The regulator rectifies this voltage and applies DC to the exciter
stator.
• A three-phase AC
voltage appears at the
exciter rotor and is in
turn rectified by the
rotating rectifiers.
• The DC voltage
appears in the main
revolving field and
induces a higher AC
voltage in the main
stator.
• This voltage is sensed by
the regulator, compared to
a reference level, and
output voltage is adjusted
accordingly.
Circuit: Generator without a PMG
•
As the revolving field rotates, residual magnetism in it produces a
small ac voltage in the main stator.
•
The regulator rectifies this voltage and applies dc to the exciter
stator.
•
A three-phase AC voltage
appears at the exciter
rotor and is in turn
rectified by the rotating
rectifiers.
•
The magnetic field from
the rotor induces a higher
voltage in the main stator.
•
This voltage is sensed by
the regulator, compared to
a reference level, and
output voltage is adjusted
accordingly.
Circuit: Brush Type (Static)
• DC voltage is fed
directly to the main
revolving field through
slip rings.
• Power source for the
main revolving field
can be very large and
expensive.
• Requires brush
maintenance.
• Common in variable
speed applications.
External
Source
(+)
Stator (armature)
(-)
Rotor (field)
Slip rings
AC out
Circuit: AREP
• Auxiliary winding
regulation excitation
principle.
• Secondary winding in
the main stator
provides power to the
voltage regulator.
PMG Rotor
• Is a field that induces voltage
in the PMG stator.
• Poles are permanent magnets.
• Mounted on the shaft with the
main rotor.
• Optional (benefits to be
discussed later).
PMG Stator
• Is an armature that provides power
to the regulator
• Induced by the PMG rotor.
• Typically has random-wound coils
in a laminated steel core.
• Various configurations:
– Wound cores in a frame
– Wound cores with no frame
– Combined with the exciter stator in
one frame
• Mounted on an end bracket
(opposite side of prime mover).
• Optional
Exciter Stator
• Is a field that induces voltage in
the exciter rotor.
• Typically powered by the regulator.
• Typically has random-wound coils
in a laminated steel core.
• Various configurations:
– Wound cores in a frame
– Wound cores with no frame
– Combined with the PMG in one
frame
• Mounted on an end bracket
(opposite side of prime mover).
Exciter Rotor
• Is an armature that provides
rectified power to the main
rotor (revolving field).
• Induced by the exciter stator.
• Three-phase high frequency
AC output.
• Typically has random-wound
coils in a laminated core.
• Mounted on the shaft with the
main rotor.
Rectifier
• Mounted on the exciter (as in
previous slide) or PMG.
• Has diodes that full wave rectify
the three-phase (three separate
voltage) exciter armature AC
current to DC before it enters
the main rotor.
• Leads connect to the main field
(rotor)
Resultant DC wave
Rectifier
Main field
Exciter
armature
Main Rotor
• Is a field that induces voltage
in the main stator.
• Powered by the exciter rotor.
• Connected to the (+) and (-)
rotating rectifier terminals.
• Coils are connected in series
around a core.
– Laminated core is typical
– Solid core with large rotors
• Current flow is directed in a
clockwise and CCW rotation to +
create north and south poles.
• Pressed on a shaft.
N
S
N
To the rotating
rectifier assembly
S
-
Main Rotor: Types
• Cylindrical
• Salient
Main Rotor: Layout
Main rotor (field)
Exciter rotor
(armature)
Rectifier
Self excited
Main rotor (field)
PMG
rotor
(field)
Exciter rotor
(armature)
Rectifier
PMG excited
Main Rotor: Frequency, RPM, Pole #
Frequency = RPM × number of poles
120
If you have a prime mover that runs at 1000
RPM and you wanted 50 Hz, you would need a
generator with how many poles?
50 Hz × 120
1000 RPM
= 6 poles
Main Rotor: Frequency, RPM, Pole #
# of
Poles
RPM
Hz
4
1800
60
4
1500
50
6
1200
60
6
1000
50
8
900
60
8
750
50
10
720
60
10
600
50
Main Rotor: Damper Cage
• Also called “Amortisseur windings.”
• Copper bars through the pole faces
and shorted together by the end
plates.
• Standard for all but traction
generators, solid rotors.
• Has a very short time constant
(effect expressed in datasheets as
X”d).
– Helps with parallel operation
– Helps with load-induced harmonics
(non-linear loads).
• Helps reduce initial voltage dip
during motor starting.
Main Rotor: Magnetism
• Magnetic flux paths (i.e. flow of magnetism) for a
generator operating at 0.8 PF
Main Stator
• Is the main armature,
the component that
delivers power.
• Windings (copper
conductors) are either
form-wound coils or
random-wound coils
fitted in core slots.
• Core is laminated
steel housed in a
metal frame.
• Typically has three
phases (three
separate windings).
Main Stator: Coils and Slots
• The number of turns and cross section are specific to
each frame size, slot combination or design, and voltage.
• Coils typically span into two slots in the core, so there are
two coils per slot.
• Pitch = (span -1) × the number of rotor poles / total # of
slots.
Main Stator: Coil Types
Form wound
Random wound
Main Stator: Coil Types
Random wound
Advantages, disadvantages, applications
discussed later.
Form wound
Main Stator: High-Voltage Coil
• Use with voltages above 6000 V
• Has conductive and semi-conductive tape
Main Stator: Coil Connection
• Series circuit - coils
connected one after
another.
– Voltage additive for
each coil.
– Current capacity is that
of any one coil
• Parallel circuit - coils
connected in parallel
– Voltage across group is
voltage across any one
coil.
– Current capacity is
additive for each coil.
Main Stator: Three Phase
• Three windings.
• For each phase, there
is one group (one or
more coils) for each
rotor pole.
– A group is
interconnected
– Can be considered as
one large coil.
• The leads are typically
wye (star) connected.
The neutral is usually
connected to ground or
brought out with singlephase loads.
2 poles
6 groups
2 coils/group
(12 coils)
series connected
Main Stator: Three Phase (cont.)
• As the rotor
rotates, three
separate
voltages are
created at the
stator
terminals.
Main Stator: Three Phase (cont.)
N
S
4 poles
12 groups
4 coils/group
(48 coils)
S
N
1-9 span
2/3 pitch
Wye vs. Delta
Dilbert’s Renewable Energy Idea….
Other Considerations: Laminations
• Magnetic cores
(stacks) used in
manufacturing
generators are
typically made
from thin steel
sheets called
laminations.
• Reduce losses
due to stray
currents.
Other Considerations: VPI
• Vacuum pressure
impregnation
• A polyester or epoxy
resin is applied to
windings.
• Provides mechanical
strength, heat
transfer, dielectric
strength and
environmental
protection.
• A bake cycle after
VPI hardens the
resin.
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