Position of brushes on the commutator in a DC generator

[Axe199]

i am having a hard time understanding why do we place the brushes on the Magnetic neutral axis, the textbook simply says, because this is where the current reversal takes place, is the point where emf from different meet is the same as the point of current reversal?

 

[Merlin3189]

If you look at the coil in the magnetic field, you want the current to flow through it in a certain direction in order for the force to turn it in the wanted direction. When the coil gets to be perpendicular to the magnetic field, that's when we want the direction of the force to change. (The side of the coil which was going up, has reached the top and now must come down, etc.) So that's when we want the current to change which way it goes round the coil.
So the brushes must be positioned so that they change connections to the coil, when the coil is perpendicular to the magnetic field.

See this video animation.

 

[abdo799]

I think the motor and generator will differ in the position of the brushes

 

[sophiecentaur]

I think the motor and generator will differ in the position of the brushes

Yes. The optimum phase for switching would probably be slightly different. To minimise the current during switching, it would not be far from '90degrees', for both, though.

 

[abdo799]

I still don't know why we put the brush at the point where the current is 0, we need max output so we should put it where the emf is max right?

 

[sophiecentaur]

I still don't know why we put the brush at the point where the current is 0, we need max output so we should put it where the emf is max right?

Faulty logic there, I'm afraid. When you have max emf, you want to be making use of it all the time. You use it for as long as it is in the same direction - i.e. until it is near zero. The direction of the emf will change after the zero crossing so that's when you want to switch over the connection. Then the current will be flowing in the same direction as before, through your external circuit - whichever angle the commutator is at.