How Does a 3 Phase Motor Works

8/3/2019 How Does a 3 Phase Motor Works

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How does a- 3 phase motor works?

3 phase AC voltagewill create rotatingmagnetic field , whichwill sweep the rotorconductor and a force=BIL Sin Q shall beexperienced by rotor.And rotor shall start

rotating in thedirection of magneticflux.


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How does a 3- phase motor works

At the moment of startbecause rotor isstationary therefore fieldcreated by rotor current

shall be stationary henceno emf shall be inducedin stator winding and noback emf, therefore full

starting current limited bystator impedance shallflow through the motorstator winding.


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What happens when motor picks up speed?

As rotor start rotating due totorque experienced by rotorbar due to sweeping flux,magnetic field created by rotorcurrent shall also rotate along

with rotor and this rotating fieldwill cause a back emf in thestator winding. The magnitudeof the back emf shall be directlyproportional to speed of rotor.

That is why as rotor picks upspeed stator current startdiminishing due to rising backemf.


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What happens when motor is near to synchronous speed

When motor is near tosynchronous speed backe.m.f will limit the stator

current to no load current.This no load current isused to meet frictional andwind losses plus current

required to createmagnetic plus in iron ofstator winding.


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What happens when motor is exposed to load

When load is applied to motor ,it will slow down the rotor hencefield created by rotor shall alsoslow down and hence back emfshall reduce and this will cause

stator current to increase andthat in turn will increase rotorcurrent and hence torqueapplied to rotor. thus as forevery action there is opposite

reaction as per Newton's law.So increase in load will causemore current to flow throughstator winding.


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What happens when excessive load is applied?

As the load grows so doesthe stator current. Somotor can not limit statorcurrent at its own. if no

protection is there the

excessive current eithermay damage the rotorcoupling. If rotor couplingis strong then it will burn

the stator winding/rotorbars due to excessivecurrent.


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What happens if rotor is fed by external DC current

If we switch off the power to

stator and inject dc currentthrough slip rings to rotor &rotate the rotor nearsynchronous speed by

external device then dccurrent flowing through rotorwinding shall create amagnetic field which will rotatewith rotor. This rotatingmagnetic field when cut bystator winding will induce a 3phase induce voltage in stator.

+

-


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What happens when generator is loaded

When load is connected

across stator a current willflow and it will create its

own magnetic field. Thismagnetic field will cause atorque in opposite direction

of prime mover. As perNewton's law equalamount of torque shall beexerted by prime mover.This is how conversion ofmechanical energy toelectrical energy takesplace

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-

Load


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V-BUS

I

LOAD ANGLE


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What happens when excitation to rotor is increased

When excitation to rotor is

increased and load anglebetween v-bus and v-genis kept same (same activepower), the extra fieldcurrent will create strongermagnetic field, rotating withrotor and cut the statorwinding, hence magnitudeof generator voltage willrise . Since load angle isconstant therefore V-gen

will rise by increased IXdrop. Ix drop is nothing butMVAR . So more MVARshall be generated byraising excitation current.

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-

Load


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VG1

VG2

VG3

ig3

ig2

ig1

Load angle

Thus we see that for same active power( load angle) , as weIncrease excitation following will occur1. Generator voltage shall rise.2. MVAR shall increase.3. Active power shall remain same. I,e V-grid x ig cos phi

V grid


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What happens when phase sequence is changed?

As the phasesequence of appliedvoltage is changeddirection of rotation ofmagnetic flux shallreverses , hence sodoes the force appliedon the rotor bar,

hence rotor startsrotating in reversedirection.

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How Does a 3 Phase Motor Works