l6 alternators and synchronous motors

8/8/2019 L6 Alternators and Synchronous Motors

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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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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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Electro-mechanical Energy-conversion

(EMEC) Machines

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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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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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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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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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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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Power Considerations for a Generator

Large-size generators are usually called

alternators.

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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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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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Power Flow Diagram

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Power Considerations for a Motor

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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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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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Power Flow Diagram

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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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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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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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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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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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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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Click

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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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Solution

120 120 60

6

s

fN

P

v! ! ! 1200 rpm

120 120 20

1200

fP

N

v

! ! ! 2

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l6 alternators and synchronous motors

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