Confused about DC motor neutral plane from theory to real world observations.
Confused about DC motor neutral plane from theory to real world observations.
(OP)
Can someone tell me why books say the neutral plane is between the field poles and the brushes are there too for DC motors, but every real world DC motor I've seen, the brushes are always over the stator fields and not between them like books portray?





RE: Confused about DC motor neutral plane from theory to real world observations.
Bill
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"Why not the best?"
Jimmy Carter
RE: Confused about DC motor neutral plane from theory to real world observations.
Muthu
www.edison.co.in
RE: Confused about DC motor neutral plane from theory to real world observations.
RE: Confused about DC motor neutral plane from theory to real world observations.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Confused about DC motor neutral plane from theory to real world observations.
RE: Confused about DC motor neutral plane from theory to real world observations.
It is due to geometry. Imagine two coils, one pole part, undergoing commutation under interpoles. Where do they meet up? Under main pole.
The very idea of using interpoles is to have a stable & steady neutral axis from no-load to full load. The case where you have seen the brushes being advanced to achieve good commutation for a specific load is the classic case of a non-interpole machine. Now imagine doing all that moving brush back and forth for varying loads. That's why interpoles are used. To avoid such a moving back & forth.
Muthu
www.edison.co.in
RE: Confused about DC motor neutral plane from theory to real world observations.
RE: Confused about DC motor neutral plane from theory to real world observations.
Anyways, tried to help. You can take it or leave it.
Muthu
www.edison.co.in
RE: Confused about DC motor neutral plane from theory to real world observations.
Just to be sure we understand the neutral point:
Consider a simple armature. The conductor goes from a commutator segment to the core. It is wound around a rotor segment several times and returns to the next commutator. When that pole is midway between the field poles and is travelling parallel to the lines of flux, so that no voltage is developed in the coil, it is at the neutral point.
What is important is the position of the rotor pole.
Trying to draw the commutator offset in a simple drawing may add unnecessary complication.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Confused about DC motor neutral plane from theory to real world observations.
Have a look then at all the brushes being in the wrong locations compared to real world DC motors:
Wrong locations:
http://electriciantraining.tpub.com/14177/img/1417...
http://www.electrical4u.com/images/COMMUTATION-IN-...
http://circuitglobe.com/wp-content/uploads/2015/12...
http://autosystempro.com/wp-content/uploads/2013/0...
How brushes are actually located:
http://eternal.searchprodev.com/wp-content/uploads...
One of my personal electric motors:
"The books are to demonstrate the theory in as simple and clear a way as possible.
Just to be sure we understand the neutral point:
Consider a simple armature. The conductor goes from a commutator segment to the core. It is wound around a rotor segment several times and returns to the next commutator. When that pole is midway between the field poles and is travelling parallel to the lines of flux, so that no voltage is developed in the coil, it is at the neutral point.
What is important is the position of the rotor pole.
Trying to draw the commutator offset in a simple drawing may add unnecessary complication."
THANK YOU SIR!!! That is exactly what I needed. I gave you a star. :)
RE: Confused about DC motor neutral plane from theory to real world observations.
This link is a good example. You can see how cluttered and, dare I suggest, confusing, the drawing would become if the commutator segments were shown in their actual positions.
[linkhttp://www.electrical4u.com/images/COMMUTATION-IN-...]Link[/link]
This drawing illustrates the brush shorting two commutator segments as it passes from one to the other. The coil that is shorted is not cutting any lines of force and the voltage is zero.
Note that in the interest of simplicity, the offset due to armature reaction is not shown in this drawing.
Bill
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"Why not the best?"
Jimmy Carter
RE: Confused about DC motor neutral plane from theory to real world observations.
http://www.precisionmicrodrives.com/uploads/media_...
Every three pole armature I found shorts two comm bars with one armature pole is facing directly perpendicular to a magnet. Is this a special case where they cannot avoid shorting a the coil when its at maximum generated voltage? Or do I need to look at the fact that the coil of wires will be parallel to the flux lines produced by the stator magnets? Or perhaps the flux lines are distorted and they accounted for it?
Here is a picture of what I am talking about:
RE: Confused about DC motor neutral plane from theory to real world observations.
Anyone else have direct experience with these motors?
Bill
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"Why not the best?"
Jimmy Carter
RE: Confused about DC motor neutral plane from theory to real world observations.
You can see two comm bars shorted by that brush and the coil that connects to those two bar.
RE: Confused about DC motor neutral plane from theory to real world observations.
I agree with you the depictions are not always correct in this respect. It's not too surprising that depictions in textbooks are different from the physical reality. Often a 3-phase stator is depicted as 3 "coils"…. it does not come close to the physical layout. I've had more than one new EE graduate express surprise/bafflement when I tried to describe the layout of coils set into slots and wired in pole-phase groups. Bill mentioned, sometimes there is need to omit certain details to keep things manageable. And we cannot assume that everyone who writes about motor theory on the internet has studied their physical construction.
Attached is a figure from EASA dc motors document which correctly captures the position of the brushes and matches edison's description very well. It is a 4-pole motor with stator poles are located at 12:00/3:00/6:00/9:00 on the figure. They tried to depict a diamond shape rotor coil and had to pivot it out so we could see the shape of the diamond in the figure… but in reality of course the coil sides (coil legs) are positioned axially and would only appears as a point on this figure … I have drawn in circles to show where the coil sides would be.
In the top figure the coil center is at 10:30 and the associated coil sides are located half a pole pitch away at 9:00 and 12:00. Rate of change of flux thru is maximum and current is flowing. The two leads of the coil are connected to two adjacent commutator segments which are generally (*) near the center of the coil (10:30 in this case).
The bottom figure represents coil moved to neutral plane with coil center (and associated commutator segments) at 12:00 and the coil sides are located half a pole pitch away at 10:30 and 1:30. Rate of change of flux thru coil is minimum/zero at this location and this is approximately where commutation should occur…which is why stationary brushes would have to be positioned at approx 12:00 (and 3/6/9) so this coil will be shorted when the brush overlaps the two associated segments and then reverse when the brush ends up on the other side of these segments.
Of course as you pointed out, we don't guess about neutral position.
* There are a large variety of winding configurations possible so I'm a little cautious about generalizations.
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(2B)+(2B)' ?