In ac machines, why is the stator winding for phase-b placed 120 degree ahead of phase-a whereas the phasors for phase-b (such as Vb) lag behind the corresponding phasors for phase-a ?

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electrical

Dear fairooz, I don't now with reference to what u are asking. in case of generator, rotor rotates and as the field windings are on rotor, field is said to be rotating or rotating magnetic field. since the flux cuts the stationary conductors emf is induced in the stationary windings.and...
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Dear fairooz, I don't now with reference to what u are asking. in case of generator, rotor rotates and as the field windings are on rotor, field is said to be rotating or rotating magnetic field. since the flux cuts the stationary conductors emf is induced in the stationary windings.and emf phase sequence depends on how 3 phase windings are placed.if they are 120 degree apart then only it satisfies a 3 phase system otherwise it is not.leading or lagging of phasors depend on field rotation (if field is rotating) .in case of synchronus motor if we give 3 phase balanced supply rmf produced in stator if and only iff windinds are pl aced in 120 apart otherwise not.stator field is now rotating but rotor flux is stationary.so no motion of rotor in synchronus motor. read less
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I have very good subject knowledge in all the electrical subjects..i can cover to gate level.. And i can teach mathematics and java too.

Basic concept in polypahse AC machine is to generate RMF. The rmf can be developed in any machine except single pahse machine . if you increase the no of pahses strength of the rmf will increase but there more advantage if you use for 3 phase system instead of 2phase system.. As compared to other ez...
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Basic concept in polypahse AC machine is to generate RMF. The rmf can be developed in any machine except single pahse machine . if you increase the no of pahses strength of the rmf will increase but there more advantage if you use for 3 phase system instead of 2phase system.. As compared to other ez 4,5 phase system strength of rmf increase but very less value .. That requires details derivation... The steangth of rmf decides the stator rmf which will incfluence the rotor rmf ie speed of rotor .. So from the advantage of 3 phase system compared to other phase system the widings in eachpahse are diaplced 360/3 = 120 degress .. Let me know if you have more doubts? read less
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Tutor

It is all reference system. Its like if you are seeing rotor from outside then if you watching it moving clockwise. Then if we imagine you are standing on rotor, then u ll feel the surroundings are moving anticlockwise, think in this context. IT is what happens in actual.
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In ac machine, rotating magnetic field (rmf) is required. By using 3 phase ie 120 degree displacement rmf is produced. now, we place Va-Vb-Vc in this sequence because Va start from zero.
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Electrical Engineering ,gate

Its not always necessary that phase b is lag...it is just an examples we saw in books...for machines to rotate there should always a condition...that it rotates from leading axis to lagging axis.....
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What you are saying about laging behind is a 3 ph voltage .. one phase voltage lags the other bt 120.. and physically windings are kept 120 apart to produce rmf.. Dont confuse with mechanical design and 3 phase supply
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Student's Badhsha

in order to produce rotating magnetic filed b/w rotar and stator
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Electrical Enthusiastic

to produce a constant rotating magnetic flux the manufacturer designed with 120 degrees phase shift
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