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in case of three phase induction motor,when it is connected
to three phase supply,why the magnetic field is rotets
around the armeture?
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Answer Posted / manav
The three phase windings of stator of a 3-ph ind. motor are
placed 120 deg space displaced.
Also the supply given to these is a 3-ph supply with time
lag of 120 deg b/w each phase.
thus, the expressions for mmf of each phase at any instant
at an angular position $ in air gap is given by:
Fr=Fm*cos(wt)*cos($)
Fy=Fm*cos(wt-120)*cos($-120)
Fb=Fm*cos(wt+120)*cos($+120)
adding all these three gives the resultant:
F=Fr+Fy+Fb= (3/2)*Fm*cos($-wt)
This is like a travelling wave [F=Fm*cos(kx-wt)] that
rotates with speed w.
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