Induced emf Developed torque Magnetization curve SEE 3433
ELECTRICAL MACHINES
Slide 2
Induced emf Regardless of operation, emf is always induced in
armature circuit when there is rotation Induced emf e a = B v l For
a conductor of length l, moving at a speed v in magnetic field
intensity B, the induced voltage is given by: e a = B v l X X X X v
+ea+ea
Slide 3
Induced emf
Slide 4
l
Slide 5
e a = 2 B m r l + e a In terms of flux per pole, where = B A
and l + + _ _ + e a
Slide 6
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,
Slide 7
Developed torque Force produced, F = B l i For a conductor of
length l, carrying current i in magnetic field intensity B, the
torque developed is given by: F c = B i l X X X X F i
Slide 8
Developed torque
Slide 9
x l i i
Slide 10
x l IaIa IaIa T = F r T c = B l I a r T 2c = 2 B l I a r In
terms of flux per pole, where = B A and
Slide 11
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 !
Slide 12
Magnetization curve Is a plot of the induced emf vs I f on an
open armature circuit, at a given rotor speed +Ea+Ea I f Field
current Induced emf E a = K (flux per pole) depends on field
(stator) current and hence MMF of the stator circuit K is a
constant depends on physical construction of the machine - angular
speed of the rotor At a given speed and K, the emf induced depends
on Field circuit Armature circuit
Slide 13
How does vary with the field current? 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) IfIf Flux will increase with field current -
but not necessarily linear! Magnetization curve
Slide 14
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 I field is no longer
linear IfIf Magnetization curve
Slide 15
Since for constant speed Ea the curve can be represented by E a
vs I f EaEa I field Magnetization curve 11 Reduced speed 22 33 1
> 2 > 3
