MAGNETIZING CURRENT&

EFFECT ON SYNCHRONOUS MOTOR

SEMINAR PROJECTON

COLLEGE OF ENGINEERING & TECHNOLOGYNAME: ADITYA PRASAD MISHRAREGD NO:0901106227BRANCH: 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 curveII. Non linear curveo 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 voltage1.44[3.14(fkN)]

Where, f= frequency k=winding factor N=no. of turns.

o Constant terminal voltage implies constant air gap flux.

o Synchronous 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 oneo 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 (orActual Power) in the forward direction, where he most wants his load totravel, is KW.Unfortunately, he wouldget a tremendous backache, so his shoulder height adds a little ReactivePower, 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. . .

Thank you