01 - Basic Electric Machines
Transcript of 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 of machine rotates
Is called Rotor
Shaft willrotate withinsome form of bearing.
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Lorentz force on a conductor:
Open righthand rule
Bil F !
S ource: T. Wildi, El ectrica l Machines, Drives and Power Systems , 5th Edition, Prentice-Hall, 2002
Basic Electric Machines
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A pole is a magnetic pole, that is north or south.
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-pair DC machine.Commutators are mechanical switchesthat change direction of current.
Basic Electric Machines
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South pole North pole
M agnetic fluxnorth to south
X
South pole North pole
elevation
plan
Rotation path
Conductor withcurrent out of page
Conductor withcurrent into page
A pply Lenzs Lawopen right hand rule
Rotates aboutaxis
Force
Force
Basic DC Machines
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Southpole X
Force
Northpole
X
X
A ngular 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 mechanical
power to generate electrical power. To operate DC machine as generator remove
DC voltage supply and externally rotate shaftConductor 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
Terminalvoltage
VT
T ime in positionsNot to scaleC
C
ExternallyRotate rotor
D
D
D
D
E
A
B
E
A
B
E
E
F
F
G
G
H
H
I
I
J
J
A
AB
B
/ 2
90
3 / 4
13 5
-0 .1
18 1
- 0 .1
17 9
5 / 4
22 5
3 / 2
2 7 0
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3 15
2
3 6 0
0
0
/ 4
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F
F
G
G
XHH
XI
I
J
J
XXX
DC Generator Voltage Plot
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Southpole
VT
+
Northpole
PrimeMover
shaft
Prime mover rotates shaft
Commutator Rotate on shaft, rotor
Brushes
Fixed to casing, stator
Conductor,rotor, rotateswith shaft
DC Generator Plan View
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Terminalvoltage
VT
T ime in positionsNot to scaleC
C
D
D
E
E
F
FG
G
H
H
I
I
J
J
A
ABB
/ 2
90
3 / 4
13 5
-0 .1
18 1
- 0 .1
17 9
5 / 4
22 5
3 / 2
2 7 0
7 / 4
3 15
2
3 6 0
0
0
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No commutator so when direction of induced voltage (current)changes direction the terminal voltage must also change direction.
T he terminal voltage induced isan A C waveform
T he frequency of the inducedvoltage is equal to the rotationspeed in revolutions per second.
A C Generator Voltage Plot
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This electric machine with solid rings is called asynchronous machine.
The induced voltage frequency must be thesame as the speed of rotation.
The frequency is synchronous with the speed of rotation.
W hen used as a motor the synchronousmachine can only rotate at the frequency of the A C voltage supply.
The speed inflexibility results in synchronousmachines being used mostly as generators.
Sy nchronous Machines
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Synchronous M achines connectedto electricity system
The electricity system has a constant frequencyof 5 0 [Hz].So (two pole, single pole-pair) synchronousmotor can only rotate at 5 0 revolutions per second. = 3000 [rpm]If synchronous machine is operated as agenerator, connected to the electricity systemthen the prime mover must rotate the shaft at3000 [rpm].Generated A C voltage must be at 5 0 [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 A C voltage is induced as the lines of magnetic fluxare moving as the magnetic poles are rotated on therotor.
Sy nchronous MachinesConstructions
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Synchronous Generator Plan View
Northpole
VT
+
South
pole
PrimeMover
shaft
Prime mover rotates shaft
M agnetic poles
Rotate on shaft, rotor
Conductor,fixed to stator. M agnetic 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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ElectromagnetismM agnetic field produced by a solenoid: n i
l
Ni Q
Q
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F ield 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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A rmature 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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Sy nchronous Generator Plan View
Northpole
VT
+
South
pole
PrimeMover
Prime mover rotates shaft
Conductor,fixed to stator.
M agnetic core to provideflux path. Is on stator
+
Vf
A rmature winding A rmature current
M agnetic core for field winding,
rotates with shaft
Field winding
Field current
I f
I a
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Sy nchronous Motor Plan View
VT
+
PrimeMover
Prime mover rotates shaft
M agnetic core to provideflux path. Is on stator
+
Vf
A rmature winding A rmature current
M agnetic core for field winding,
rotates with shaft
Field winding
Field current
I f
I a
VT
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Sy nchronous 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 A C current flowing through the armaturewinding.
This armature current will induce a magnetic flux. The current is A C 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.T he conductor will be cutting lines of magnetic flux.
A voltage will be induced across the field winding
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Sy nchronous Machine Plan ViewM agnetic core to provideflux path. Is on stator
A rmature winding A rmature current
M agnetic core for 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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Trans f ormer model The armature winding is stationary on the stator. An A C voltage applied across the armature
winding. The rotor is stationary. An A C voltage is induced across the rotor (field)winding.
Both windings are stationary so this machine is atransformer.
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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Trans f ormer Machine ModelM agnetic core toprovide flux path.Is on stator
A rmature winding
M agnetic corefor field winding
Rotor windingVT
Vvoltmeter
Zero field
current so rotor (shaft) will notrotate
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Induction Motor M agnetic core toprovide flux path.Is on stator
A rmature winding
M agnetic corefor field windingRotor windingVT
A A mmeter short-circuit
Rotor currentso rotor (shaft)will rotate
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Induction Motor
W ant high rotor current for high torque.Recall transformer turns ratio
Keep large turns ration N 1 > N 2Small voltage in rotor winding but large current
The force Lenzs law and so the torque depends onthe current
rat ioturnsa N
N
i
i
V
V
2
1
1
2
2
1
!!!!
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Induction Motor (Machine)
The field (rotor) winding has been short circuitedso no need for slip rings could short circuit therotor winding on the rotor.
No electrical connection between the rotor andthe outside.Brushes are a mechanically weak point due tofriction.
So induction motors that do not need brushesare mechanically simpler and more reliable thanDC or synchronous machines.
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Induction Motor M agnetic core toprovide flux path.Is on stator
A rmature winding
M agnetic corefor field windingRotor windingVT
Rotor currentso rotor (shaft)will rotate
Shortcircuit onrotor