47435760 01 Basic Electric Machines

8/11/2019 47435760 01 Basic Electric Machines

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Basic Electric MachinesOutside part of machine doesnot move, is stationary

Is called stator

Held stationarythrough vibrationdamping

attachments

Central part ofmachine rotates

Is called Rotor

Shaft willrotate withinsome form ofbearing.


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Lorentz force on a conductor:

Open righthand rule

Bil F

Source: T. Wildi, Electr ical Machin es, Drives and Pow er Systems , 5th Edition, Prentice-Hall, 2002

Basic Electric Machines


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A pole is a magnetic pole, that is north orsouth.

Must always have a north and a matchingsouth pole so poles always are in pairs.

On next slide is a two pole or one pole-pairDC machine.

Commutators are mechanical switchesthat change direction of current.

Basic Electric Machines


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South pole North pole

Magnetic flux

north to south

X

South pole North pole

elevation

plan

Rotation path

Conductor withcurrent out of page

Conductor withcurrent into page

Apply Lenzs Law open right hand rule

Rotates aboutaxis

Force

Force

Basic DC Machines


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Southpole X

Force

Northpole

X

X

Angular velocity

Zero Force

X

Basic DC Machines


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Southpole X

Northpole

X

X

Zero Force

X

+V

Brush andgap sized toavoid shortcircuit.

X

Commutator


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The power flows in electric machines arereversible.

To operate machine as motor supply electric

power to get mechanical power. To operate as generator supply mechanicalpower to generate electrical power.

To operate DC machine as generator remove

DC voltage supply and externally rotate shaft Conductor moving through (cutting lines of)

magnetic flux induces voltage and/or current.

Commutator


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Southpole X Northpole

XX

X

+

VT

X

C C

Terminal

voltageVT

Time in positionsNot to scaleC

C

ExternallyRotate rotor

D

D

D

D

E

A

B

E

AB

E

E

F

F

G

G

H

H

I

I

J

J

A

A BB

/2

90

3 /4

135

-0.1

181

-0.1

179

5 /4

225

3 /2

270

7 /4

315

2

360

0

0 /4

45

F

F

G

G

X HH

XI

I

J

J

XXX

DC Generator Voltage Plot


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Southpole

VT

+

Northpole

PrimeMover

shaft

Prime moverrotates shaft

Commutator

Rotate on shaft, rotor

Brushes

Fixed to casing, stator

Conductor,

rotor, rotateswith shaft

DC Generator Plan View


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Southpole

VT

+

Northpole

PrimeMover

shaft


Commutator


Brushes

Fixed to casing, stator

Conductor,

rotor, rotateswith shaft

Replace commutatorwith two solidconducting rings

Generator Plan View


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Terminalvoltage

VT

Time in positionsNot to scaleC

C

D

D

E

E

F

F G

G

H

H

I

I

J

J

A

A

B

B

/2

90

3 /4

135

-0.1

181

-0.1

179

5 /4

225

3 /2

270

7 /4

315

2

360

0

0 /4

45

No commutator so when direction of induced voltage (current)

changes direction the terminal voltage must also change direction.

The terminal voltage induced isan AC waveform

The frequency of the inducedvoltage is equal to the rotationspeed in revolutions persecond.

AC Generator Voltage Plot


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Synchronous Machines connectedto electricity system

The electricity system has a constant frequencyof 50 [Hz].

So (two pole, single pole-pair) synchronous

motor can only rotate at 50 revolutions persecond. = 3000 [rpm] If synchronous machine is operated as a

generator, connected to the electricity system

then the prime mover must rotate the shaft at3000 [rpm]. Generated AC voltage must be at 50 [Hz].


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It is more economic to operate and constructsynchronous machines inside -out compared to DCmachines.

The magnetic poles are placed on the rotor and so rotatewith the shaft.

The conductor remains stationary, in the stator. For voltage to be induced in a conductor the conductor

must be moving relative to the lines of magnetic flux.

The AC voltage is induced as the lines of magnetic fluxare moving as the magnetic poles are rotated on therotor.

Synchronous MachinesConstructions


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Synchronous Generator Plan View

Northpole

VT

+

South

pole

PrimeMover

shaft


Magnetic poles


Conductor,

fixed to stator. Magnetic core toprovide flux path.Is on stator


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Field winding

In the majority of situations it is moreeconomic to induce the magnetic poles

electro-magnetically. True for both DC machines andsynchronous machines.


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ElectromagnetismMagnetic field produced by a solenoid: ni

l Ni B


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Field winding

In the majority of situations it is moreeconomic to induce the magnetic poleselectron-magnetically.

The current used to induce the magneticpoles (flux) is called the field current.

The winding (coil or solenoid) is called the

field winding. Controlling the field current also allowscontrol of the magnetic flux density.


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Armature winding

The conductor carrying the current thatpasses through the lines of magnetic fluxis called the armature winding.

Thus the current is called the armaturecurrent.

The magnitude of the armature currentcontrols the force (Lenzs law) and so thetorque exerted on the shaft.


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Synchronous Generator Plan View

Northpole

VT

+

South

pole

PrimeMover


Conductor,fixed to stator.

Magnetic core to provideflux path. Is on stator

+

Vf

Armature winding

Armature currentMagnetic core for

field winding,rotates with shaft

Field winding

Field current

I f

I a


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Synchronous Motor Plan View

VT

+

PrimeMover


Magnetic core to provideflux path. Is on stator

+

Vf

Armature winding


field winding,rotates with shaft

Field winding

Field current

I f

I a

VT


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Synchronous Motor

If remove DC voltage supply to field winding then no fieldcurrent flows. If no field current then no force exerted on field winding

(rotor) so shaft will stop rotating. There is an AC current flowing through the armature

winding. This armature current will induce a magnetic flux. The current is AC and so the magnetic flux induced will

be varying (moving). The flux will pass around the magnetic core including the

field core. So the field winding will be in a moving magnetic field.

The conductor will be cutting lines of magnetic flux. A voltage will be induced across the field winding


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Synchronous Machine Plan ViewMagnetic core to provideflux path. Is on stator

Armature winding


field winding

Field winding

Field current

I f

I a

VT

V voltmeter

Zero field

current so rotor(shaft) will notrotate


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Transformer model

The armature winding is stationary on the stator. An AC voltage applied across the armaturewinding.

The rotor is stationary.

An AC voltage is induced across the rotor (field)winding. Both windings are stationary so this machine is a

transformer.

The armature winding on the stator is theprimary winding of the transformer. The rotor winding is the secondary winding of

the transformer.


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Transformer Machine ModelMagnetic core toprovide flux path.Is on stator

Armature winding

Magnetic corefor field windingRotor winding

VT

Vvoltmeter

Zero field

current so rotor(shaft) will notrotate


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Induction Motor (Machine)

The field (rotor) winding has a voltage inducedacross it.

If the field (rotor) winding is short-circuited then a

current will flow around the field winding. This rotor current is flowing through themagnetic field induced by the armature currentflowing in the winding on the stator.

Lenzs Law so force is exerted on rotorconductor, rotating the shaft. This is called an induction motor.


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Induction MotorMagnetic core toprovide flux path.Is on stator

Armature winding

Magnetic corefor field winding

Rotor windingVT

A Ammetershort-circuit

Rotor currentso rotor (shaft)will rotate


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Induction Motor

Want high rotor current for high torque. Recall transformer turns ratio

Keep large turns ration N 1 > N 2 Small voltage in rotor winding but large current

The force Lenzs law and so the torque depends onthe current

ratioturnsa N

N

i

i

V

V

2

1

1

2

2

1


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Induction MotorMagnetic core toprovide flux path.Is on stator

Armature winding

Magnetic corefor field winding

Rotor windingVT

Rotor currentso rotor (shaft)will rotate

Shortcircuit onrotor

47435760 01 Basic Electric Machines

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