Why are the motor brushes not placed in the magnetic neutral axis?

[janu203]

In given figure of interpoles connection for a DC Machine, why the carbon brushes connected to the commutator are not placed in the Magnetic neutral axis? The given position will induce more sparking.

Interpoles solution is from following website
https://www.quora.com/Electrical-Machines-What-do-interpoles-do-in-DC-motors

 

[Babadag]

See:
https://en.wikipedia.org/wiki/Armature_(electrical_engineering)

 

[Baluncore]

Viewed from the side, it is clear that the motor commutator and brushes are mounted further along the shaft, outside the magnetic circuit of the field and armature. The design of the magnetic circuit aims to reduce the air gaps between the poles and the armature. The brushes are surrounded by large air gaps which implies low magnetic fields.

It is unsafe to assume that more sparking would result from the presence of a magnetic field. Magnetic fields can be an advantage in “blowing out” electric arcs that form when contactor switches are opened with an inductive load.

 

[jim hardy]

How certain are you that sketch is correct?

http://www.engineersparadise.com/sixcms/media.php/1466/2007Neutral-zone.pdf

http://www.wmea.net/Technical%20Papers/Setting%20Neutral%20via%20AC%20Curve%20Method%20on%20DC%20Machines%20-%20Flander.pdf

 

[sophiecentaur]

On the motors that I have seen, there gap between the field magnet and the commutator core appears to be a small as possible. I would have assumed that the external field would be small in the region of the brushes. Also, I think the field would be not change much and always be in the same direction (which is what the brushes are there for).
But looking at the OP again, why would the local field around the brushes be responsible for 'inducing sparks'? An emf is induced around a whole loop, not a point.

 

[jim hardy]

In given figure of interpoles connection for a DC Machine, why the carbon brushes connected to the commutator are not placed in the Magnetic neutral axis? The given position will induce more sparking.

Interpoles solution is from following website
https://www.quora.com/Electrical-Machines-What-do-interpoles-do-in-DC-motors

Always cross check anything you get from internet.

But looking at the OP again, why would the local field around the brushes be responsible for 'inducing sparks'? An emf is induced around a whole loop, not a point.

Bravo Sophie !

@janu203

Think about what's happening at the commutator.
As the brush slides along it short circuits adjacent commutator segments. That connects some coils in parallel.
So it's important that the coils connected to those segments are experiencing minimum rate of change of flux at instant of commutation.. Else there'd be significant voltage between adjacent segments.
Where does a sinewave have its minimum rate of change ? At the peaks where its slope goes through zero. That's why commutation is counter-intuitively set to occur at instant of maximum flux linkage which is its minimum rate of change.
Always keep in the back of your mind that derivative-integral relationship between flux and voltage. It gets you out of a lot of paradoxes.

I hope that word picture helps you form a mental image that leads you to the right formulae. That's the start of understanding... .From an ancient magazine -

observe brushes under interpoles...

old jim