Transcript AC Generators

Lesson 30:
AC Generators I
1
Learning Objectives
• Understand the operation of a single phase two pole AC generator.
• Describe the operation of a simple AC generator.
• Identify and define the components of a three phase two pole AC
generator to include rotor, stator, armature. field windings, slip rings
and brushes.
• Understand the effects of applying a DC voltage power supply to a
two pole rotor's field windings via brushes and slip rings.
• Understand the induced effects that result from rotating the rotor's
electromagnetic field past the armatures (Faraday's Law).
• Given the armature coil sequence and their physical location, plot the
induced AC voltages for a three phase two pole AC generator as a
function of time and as phasors.
• Understand the relationship between the number of poles and rpm of
the rotor to the induced AC current's frequency.
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Producing Electricity
• A generator is a machine that converts mechanical
energy into electrical energy.
− Motors and generators perform exactly the opposite
function.
− However, motors and generator are essentially the same
device.
3
Advantages of AC Power
• Motors:
− AC is ‘natural’ for rotary motors.
• Voltage Transformation:
− AC transformers allow efficient changing of voltage to
enable better power transmission.
• Power Transmission:
− AC power can be transmitted hundreds of miles.
− DC transmission limited to ~1 mile.
4
Motor to Generator: Rotating DC
• Armature current (Ia) produces
force (Fd) in the armature causing
rotation.
• What if we remove the voltage
source (VT) and we provided the
torque?
Equivalent circuit representation
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Motor to Generator: Rotating DC
• What if we remove the voltage source (VT) and the torque
was provided?
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Basic Single-Phase AC Generator
• Turning the armature results in
induced emf (eAA ) across the
load (Faraday’s Law).
• The voltage eAA will be single
phase AC given as:
eAA = Vm sin t [V, volts]
• What determines ?  Rotor
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Three-Phase AC Generator
• What if we added two additional armature coils?
Single Phase
Three Phase
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Three-Phase AC Generator
• The three-phase generator has voltages as a function of time:
eAA  Vm sin  t
eBB  Vm sin(t  120 )
eCC   Vm sin(t  120 )
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Three-Phase AC Generator
• Phasor representation of the three-phase generator:
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E AA 
Vm
E BB 
Vm
ECC  
Vm
2
2
2
0
  120
  120
Phase Sequence
• The phase sequence is the time order in which the
voltages pass through their respective maximum
values.
• Phase sequence is important because it determines the
direction of rotation of a connected motor.
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Positive Phase Sequence (ABC)
• The ABC sequence or positive sequence, is produced when the
generator rotates counter-clockwise.
eAA  Vm sin  t
eBB  Vm sin(t  120 )
eCC   Vm sin(t  120 )
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E AA 
Vm
E BB ' 
Vm
ECC ' 
Vm
2
2
2
0
  120
  120
Negative Phase Sequence (ACB)
• The ACB or negative sequence, is produced when the generator rotates
clockwise.
eAA  Vm sin  t
eCC   Vm sin(t  120 )
eBB  Vm sin(t  120 )
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E AA 
Vm
ECC  
Vm
E BB 
Vm
2
2
2
0
  120
  120
Large AC Generator
• Unlike the generator model with a fixed magnetic field and rotating
armature, it is more practical to fix the armature windings and rotate
the magnetic field on large generators.
•
Rotating armatures require brushes and slip rings to conduct current from the
armature to the load.
• The fixed armature advantage is that the generated voltage can be
connected directly to the load with no slip rings or brushes.
• The voltage applied to generate the rotating field is a small DC
voltage called the field excitation voltage.
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Generator Stator
• The stator is the stationary part of induction motor. A stator
winding is placed in the stator of induction motor and the
three phase supply is given to it.
• Stator is slotted with integer multiple of 6 slots.
• Three pairs of slots contain identical coils of wire, each with
NS turns.
• These windings are called the armature.
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Generator Rotor
• Rotor contains rotating electromagnet called the field
winding and is connected to the mechanical load through a
shaft.
• The electromagnet is powered by a DC current via slip rings
and brushes.
• Unlike in the DC motor application, brushes are not
commutating and are not as subject to wear (less friction).
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Slip Rings
• Allow DC current to flow to the field windings on the rotor
to produce the magnetic field.
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Generator Output
• The amplitude of voltage output is a function of the
current supplied to the field windings.
• The stronger the current, the larger the magnetic
field, the larger the output voltage.
http://people.ece.umn.edu/users/riaz/animations/alternator.html
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Generator Frequency
• The frequency f (in Hz) of the AC voltage is a function of
speed of the rotor N (in RPM):
N = 60 f
[RPM]
• If the rotor contains multiple number of even poles (2, 4, 6,
etc.) then:
 2 
rotor  
 2 f (rad/sec)
 Poles 
120 f
NP 
(RPM)
Poles
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Synchronous Speed
• Synchronous Speed (speed of rotation of B) versus
Poles for a 60Hz Machine:
P
(poles)
2
4
6
8
10
N
(RPM)
3600
1800
1200
900
720
120 f
N
P
[RPM]
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Example Problem 1
a) For a 4 pole, 60 HZ generator, what is the speed in rpm of the rotor?
b) What would be the frequency of a 6 pole machine spinning at the
same rpm? 90 Hz
120 f
(RPM)
Poles
120(60 Hz )
NP 
=1800 RPM
4
a) N P 
b) N P 
120 f
(RPM)
Poles
N P ( Poles ) (1800RPM)(6)
f 
=
=90 Hz
120
120
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QUESTIONS?
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