Magnetization curve

SEMINAR PROJECT
ON

MAGNETIZING CURRENT
&
EFFECT ON
SYNCHRONOUS MOTOR

COLLEGE OF ENGINEERING & TECHNOLOGY
NAME: ADITYA PRASAD MISHRA
REGD NO:0901106227
BRANCH: ELECTRICAL ENGINEERING

Contents:

1. Definition of magnetizing current.
2. Properties
3. B-H curve
4. Magnetization principle
5. Synchronous motor relation

MAGNETIZING CURRENT:

 MAGNETIZES THE MATERIAL.
 ORIENTS THE DOMAIN OF THE
MAGNETIC MATERIAL.
 STORES ENERGY IN THE FORM OF
MAGNETIC FIELD.

PROPERTIES OF MAGNETISING
CURRENT:
1. Lags behind the voltage by 90 degrees phase
shift:
The voltage mentioned here is the
magnitude,obeying KVL.

V= -N*d(flux)/dt
Or v= -N*d(L/N*i)/dt ...as N*flux = L*i
Or v=-L*d(i)/dt
Taking i=i1 sin(wt)
we have : v=-Liw sin(wt+Pi/2)
...neg. sign shows direction of the drop

2.Ideally in phase to the the flux,it produces.
 Ideal means the domain produces dipoles in
proportional to the input i.e. the current.
 Magnetic saturation & Retentivity absent.
( Hence the magnetization curve is linear)
 Practically, the curve exerts non linearity owing to:
1. Saturation effects.
2. Retention property of the magnetic dipoles
to lie on the easy axis of magnetisation.

B~H CURVE OR
MAGNETIZATION CURVE

o B~H curves are of two types:
I. Linear curve
II. Non linear curve
o We will consider both of them in ideal and
actual cases.

LINEAR CURVE (IDEAL)

o Ideal case of study, i.e. magnetic saturation is
neglected.

o Flux varies linearly, with respect to magnetizing
current.

o So, waveforms are identical as flux is
proportional to magnetizing current.

o Also true for non-magnetic materials.

NON LINEAR CURVE(IDEAL)

o Occurs due to saturation of magnetic materials.

o The magnetic dipoles formation rate reduces as
current increases.

o Effects is production of harmonics, especially
the 3rd ones.

o Flux & current waveforms are always in same
phase.

continued. . .

o Magnetic saturation doesn’t result in power loss,
it merely distorts current waveforms.

o 3rd harmonic is neglected as these cannot be
represented for mathematical analysis.(study of
phasors)

ACTUAL CASE
o Magnetization curve involves hysteresis.

o Hysteresis caused by retention properties of
magnetic dipoles.

o Retention means the presence of flux even in the
absence of magnetizing current.

o Exists in anisotropic materials, where dipoles align
in the easy axis of magnetization.

Continued…
o Hysteresis effect gives rise to energy loss.

o Flux lags the current, hence exciting current
doesn’t lag the voltage by 90 degrees.

o Component which is in phase with flux
represents magnetizing part & component in
90 degrees phase shift with flux gives core loss

o A sinusoidal current produces a flat topped
wave.

PRINCIPLE OF MAGNETIZATION

o It occurs due to the magnetic dipole
moments of a compound bar magnet.
o Magnetic moment inside a bar magnet is
influenced by:
o intrinsic movement of unpaired
electrons.
o orbital movement of electrons around
nucleus.
o intrinsic movements of protons.

The reason a piece of magnetic
material spontaneously divides
into separate domains, rather than
exist in a state with magnetization
in the same direction throughout
the material, is to minimize its
internal energy

continued. . .
o Dipoles get aligned due to mechanical energy,
transformed from electrical energy.

o As the current increases, dipoles alignment
rate decreases due to crowding, thereby
requiring more electric energy.

o When there is no current applied, dipoles
arrange themselves n an energetically
favorable direction of spontaneous
magnetization.

MAGNETIZING CURRENT IN
SYNCHRONOUS MOTOR
o Air gap flux=terminal voltage
1.44[3.14(fkN)]
Where, f= frequency
k=winding factor
N=no. of turns.
oConstant terminal voltage implies
constant air gap flux.
oSynchronous motor draws
magnetizing current to setup the flux
in the magnetic circuit of the device.

 Synchronous m/c: double excited.
1. field winding: d.c. excited
2. armature winding: a.c. excited.

Field current sufficient to generate the
flux

o Magnetizing current from ac source is zero.

o Normal excitation leads to unity power factor.

Field current lesser than normal one
o allows ac source to give magnetizing current.

o Under excitation it leads to lagging power
factor.

field current greater than normal current

o Motor is over excited.
o Excess flux is neutralized by armature
winding.
o Demagnetizing current (leads voltage by 90
degrees) is drawn from ac source.
o Operates under leading power factor.

Powerfactor: physical analogy

Mac here is dragging a heavy load . Mac’s Working Power (or
Actual Power) in the forward direction, where he most wants his load to
travel, is KW.
Unfortunately, he would
get a tremendous backache, so his shoulder height adds a little Reactive
Power, or KVAR.
The Apparent Power Mac is dragging, KVA, is this “vectorial
summation”
of KVAR and KW.

POWER FACTOR VARIATION IN A SYNCHRONOUS
MOTOR

o Inverted V graph shows the variation of
power factor w.r.t field current at const load.

o The compounding curve justifies the
magnetizing current needed to balance the
operation of a motor.

CONTROL OF POWER FACTOR BY SYNCHRONOUS
MOTOR
o Power factor control decides the losses & real
power needed to fetch a load.

o Poor power factor means more real power
supply to meet a load demand.

o Synchronous power factor can operate at
leading power factor, hence improves power
factor of the system.

Continued. . .

o This reduces the KVA demand of load &
hence :
i. Less KVA supplied from the supply.
ii. Less current implies less losses.
o Special type of synchronous motor is
used, synchronous condenser.

NOTE
The magnetizing property of magnetic materials sets up an open field
for storage of energy in magnetic form. This is vital because
electromagnetic device works on the conversion of electrical energy
into mechanical energy & vice-versa.
ELECTRICAL ENERGY MAGNETIC ENERGY MECHANICAL ENERGY
Studying the magnetic energy we need to learn, what actually creates
this energy & how it is stored??? This is what we need to explore.
The main reason for the control of power factor of a synchronous motor
is this magnetization.
Therefore, to have power factor control we need to have basic idea for
the magnetizing current & then atomic view (electrons protons)
responsible for the game. . .

Magnetization curve

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