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Alternators and
Synchronous Motors
Amit Mishra
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Tuesday, November 02, 2010 Ch. 14 Alternators and Synchronous Motors 2
Topics to be DiscussedTopics to be Discussed
Electro-mechanical Energy-conversion(EMEC) Machines.
Power Considerations for
A Generator.
A Motor.
Power Flow Diagram of
A Generator.
A Motor.
Construction of Alternator
Stator.
Rotor.
Cylindrical or Non-Salient Rotor.
Salient or Projected Pole Rotor.
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Tuesday, November 02, 2010 Ch. 14 Alternators and Synchronous Motors 3
Rotating Magnetic Flux due to Three-Phase Currents.
EMF Equation.
Equivalent Circuit of
A generator.
A Motor.
Synchronous Motor.
Constant Speed Operation.
Effect of Change in Mechanical Load.
Effect of Change in Excitation.
Synchronous Condenser.
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Tuesday, November 02, 2010 Ch. 14 Alternators and Synchronous Motors 4
Electro-mechanical Energy-conversion
(EMEC) Machines
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Tuesday, November 02, 2010 Ch. 14 Alternators and Synchronous Motors 5
AC and DC
Generators and Motors
When the electrical system is characterized by direct
current, the machines are called dc motors and dc
generators.
Similarly, if the electrical system is characterized by
alternating current, the machines are called ac
motors and ac generators.
Basically, the ac machines are not different from the
dc machines.
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Tuesday, November 02, 2010 Ch. 14 Alternators and Synchronous Motors 6
Electrical Rotary Machine It has two parts :
(1) the fixed part, called the stator, and
(2) the moving part, called the rotor.
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Tuesday, November 02, 2010 Ch. 14 Alternators and Synchronous Motors 7
Generator Action
Whenever the conductors of a coil cut across the
magnetic flux (or are cut by it), an emfe is induced
in it.
This emf can supply a current i to an electricalload.
Thus, an electrical powerei is generated.
This is the Generator Action.
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Tuesday, November 02, 2010 Ch. 14 Alternators and Synchronous Motors 8
Motor Action
Whenever current-carrying conductors of a coil areplaced (perpendicularly) in a magnetic flux, a force
is experienced by each conductor.
This gives rise to an electromagnetic torque Xe.
This torque can rotate a mechanical load at anangular speed .
Thus, a mechanical powerXe is generated.
This is the motor action.
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Tuesday, November 02, 2010 Ch. 14 Alternators and Synchronous Motors 9
Both the generator action and motor action go hand
in hand in an electro-mechanical energy-conversionmachine.
Only the direction of power flow decides whetherthe machine is a generator or a motor.
eei X [!
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Tuesday, November 02, 2010 Ch. 14 Alternators and Synchronous Motors 10
Power Considerations for a Generator
Large-size generators are usually called
alternators.
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Tuesday, November 02, 2010 Ch. 14 Alternators and Synchronous Motors 11
Current-carrying coil in the armature kept in magnetic
field experiences a torque,X
e.
This is the reaction torque and it opposes the applied
torque Xm.
In addition, there is also a frictional torque Xfthat
opposes the applied toque Xm.
m e fX X X!
m e fX X X!
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Tuesday, November 02, 2010 Ch. 14 Alternators and Synchronous Motors 12
Output Side I
fris the resistance of the coil, the current i inthe circuit is given as
L
L
ei e ir iRr R! !
2 2 2
orLei i r i ei i r iV ! !
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Tuesday, November 02, 2010 Ch. 14 Alternators and Synchronous Motors 13
Power Flow Diagram
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Tuesday, November 02, 2010 Ch. 14 Alternators and Synchronous Motors 14
Power Considerations for a Motor
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Tuesday, November 02, 2010 Ch. 14 Alternators and Synchronous Motors 15
The conductors of the coil are cut through the
magnetic field.
An emfe is induced.
This is the reaction emfand it opposes the applied
voltage V.
The difference ofVand emfe causes current i to
flow in the coil.
V ei V e ir r
! !
2Vi ei i r !
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Tuesday, November 02, 2010 Ch. 14 Alternators and Synchronous Motors 16
Output Side The motor converts electrical powerei into
mechanical powerXe.
A portionX
f is lost in overcoming the friction. The remaining part Xm is supplied to the
mechanical load.
e f mX [ X [ X [ !
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Tuesday, November 02, 2010 Ch. 14 Alternators and Synchronous Motors 17
Power Flow Diagram
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Tuesday, November 02, 2010 Ch. 14 Alternators and Synchronous Motors 18
How a Generator Differs
from a Motor1. EMF e and current iare in the same direction
in generator, but opposite in motor.
2. Torque Xe and rotation are opposite in ageneratorbut in same direction in a motor.
3. Torque Xe and Xf are in same direction in a
generatorbut opposite in a motor.
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Tuesday, November 02, 2010 Ch. 14 Alternators and Synchronous Motors 19
Synchronous Machines
The rotor speed is exactly fixed by the supply
frequency.
They are synchronized with supply frequency.
The other type of ac machines are called
asynchronous (orInduction) motors.
The synchronous generator(also called alternator) is
the workhorse of the electric power industry.
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Tuesday, November 02, 2010 Ch. 14 Alternators and Synchronous Motors 20Next
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Tuesday, November 02, 2010 Ch. 14 Alternators and Synchronous Motors 21
General Characteristics of the Synchronous
Machine The dc field circuit (on the rotor), is a rotatingelectromagnet controlled by the dc exciting current.
The power supplied to the dc field circuit does notenter directly into the energy-conversion process.
The armature circuit is placed on the stator and carriesthree-phase currents.
The flow of real power through the system isdetermined by the mechanical input because themechanical system exchanges real power only.
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Tuesday, November 02, 2010 Ch. 14 Alternators and Synchronous Motors 22
Synchronous Speed
For two-pole machine, one revolution of rotor represents 360mechanical degrees, and one cycle of emf represents 360
electrical degrees.
If the machine has 4 poles, one cycle of emf would be
generated when the field structure (on the rotor) rotates
through one-half revolution only. Thus, in a 4-pole machine two cycles of emf is generated when
the rotor completes one revolution.
If the machine has Ppoles, the number of cycles of emf in one
revolution will be P/2.
Thus,
2e m
PU U !
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Tuesday, November 02, 2010 Ch. 14 Alternators and Synchronous Motors 23
If the rotor has a speed ofns revolutions per second,
the frequency fof the induced emf would be
2s
Pf n!
Synchronous speed(expressed in
revolutions per minute) is
2 120
60s
fN f
P P! v v !
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Tuesday, November 02, 2010 Ch. 14 Alternators and Synchronous Motors 24
What can be the maximum speed of a synchronous
machine in India ?
Ans. 3000 rpm. A machine cannot have less than 2
poles.
What can be the maximum speed of a synchronous
machine in USA ?
Ans. 3600 rpm.
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Tuesday, November 02, 2010 Ch. 14 Alternators and Synchronous Motors 25
Example 1
A six-pole ac generator is running and producing
voltage at a frequency of 60 Hz.
Calculate the revolutions per minute of the
generator. If the frequency of the generated voltage is required
to be decreased to 20 Hz, how many poles would be
needed on the generator, if it still runs at the same
speed ?
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Tuesday, November 02, 2010 Ch. 14 Alternators and Synchronous Motors 26
Solution
120 120 60
6
s
fN
P
v! ! ! 1200 rpm
120 120 20
1200
fP
N
v
! ! ! 2
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