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I was reading book Electric Machinery by Chapman and in topic 'Armature reaction ' It is written that

"The magnetic neutral plane is defined as the plane within the machine where the velocity of the rotor wires is exactly parllel to magnetic flux lines ,so that E(induced ) in the conductors in the plane is exactly zero ".

This definition of MNP is very confusing for me because

1.I always consider that velocity of conductors is always perpendicular to magnetic field of stator because velocity is tangential to cylindrical surface and magnetic field is radial to cylindrical surface but after reading this definition I'm not sure whether I had this misconception or I am right?

2.and if velocity and magnetic field is always perpendicular the how such a plane (MNP ) is possible where magnetic field and conductors are parllel?

user215805
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  • Sanity check : the EMF is an AC waveform (in the windings; before the commutator in a DC machine). Therefore there must be a point where it crosses zero. The flux alternates, as the rotor wires pass N and S poles in turn. What happens approx halfway between N and S poles? –  Dec 14 '20 at 19:20
  • @Brian Drummond EMF is an AC waveform (before commutator ) because here we consider that B is sinusoidal in space but in case of DC machine we consider flat topped waveform of B and hence we get a flat topped(rectified due to commutator ) EMF but still isn't B is always perpendicular to velocity ? – user215805 Dec 14 '20 at 19:30
  • Flat top merely makes the AC waveform trapezoidal rather than sinusoidal. It still has zero crossings. –  Dec 14 '20 at 19:55

1 Answers1

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The actual air gap is smaller this so the field is almost perpendicular to the conductor movement around most of the rotor however at the top and bottom of this rotor there is a area where the direction of rotation is parallel to the magnetic field.

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At the top and bottom of this machine the rotor coils are moving horizontally. This means that they are not cutting the flux of the magnetic field so no EMF is induced. The MNA is the plane that cuts both of these points.This means that the MNA runs vertically through this machine.

RoyC
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  • Thanks well explained ! – user215805 Dec 14 '20 at 20:04
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    At the top and bottom of this machine the rotors are moving horizontally. This means that they are not cutting the flux of the magnetic field so no EMF is induced. The MNA is the plane that cuts both of these points. – RoyC Dec 14 '20 at 20:09
  • I always thought that at every point velocity is perpendicular to B and at the top and bottom end EMF is zero because of discontinuity at these points as Field shift from N to S but you cleared all my doubts , thanks again! – user215805 Dec 14 '20 at 20:19
  • I think you are confusing the main field and the armature field like I did in my first iteration of this answer. – RoyC Dec 14 '20 at 20:27