Induced emf

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SEE 3433 ELECTRICAL MACHINES
Induced emf Developed torque Magnetization curve
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Induced emf
Regardless of operation, emf is always induced in
armature circuit when there is rotation
For a conductor of length l, moving at a speed v
in magnetic field intensity B, the induced
voltage is given by
X X X X X X X X X X X
X X X X X X X X X X
X X X X X X X X X X X
ea B v l
Induced emf ea B?v l
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Induced emf
4
Induced emf
l
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Induced emf
l
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Induced emf
where ? B A and
which gives
This is an induced voltage for a single turn. If
there are N turns with a parallel path,
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Developed torque
For a conductor of length l, carrying current i
in magnetic field intensity B, the torque
developed is given by
Fc B i l
X X X X X X X X X X X
X X X X X X X X X X
X X X X X X X X X X X
Force produced, F B?l i
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Developed torque
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Developed torque
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Developed torque
T F r
l
Ia
Ia
T2c 2 B l Ia r
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Developed torque
This is torque for a single turn.
If there are N turns with a parallel path,
Similar to the constant obtained in induced emf !
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Magnetization curve
Is a plot of the induced emf vs If on an open
armature circuit, at a given rotor speed
Armature circuit
Field circuit
Induced emf Ea K?? ?
At a given speed and K, the emf induced depends
on ?
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Magnetization curve
How does ? vary with the field current?
Flux will increase with field current - but not
necessarily linear!
Flux path produced by field stator core ? air
gap ? rotor core ? airgap ? stator core
At low ?, core reluctance is small most of MMF
drop appear across air gap consequently
relation between ? and field current is almost
linear (due to the airgap)
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Magnetization curve
How does ? vary with the field current?
As field current increases, so too ? - some part
of the core (especially the rotor teeth) will
saturate ? Relation between ? and Ifield is no
longer linear
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Magnetization curve
?1
?1 gt ?2 gt ?3
Since for constant speed Ea ? ? the curve can be
represented by Ea vs If