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Commutator marks in DC Motors
- Thread starter sidpred
- Start date
- Moderator
- #2
The first thing that comes to mind is a mismatch between rotor slots and field width and or spacing.
Energize the field with 120 VAC.
The voltage at the brushes should be zero or very close to zero.
Slowly turn the rotor while monitoring the armature voltage.
Any variation of the voltage at the brushes indicates some issue or imbalance of the rotor.
Energize the field with 120 VAC.
The voltage at the brushes should be zero or very close to zero.
Slowly turn the rotor while monitoring the armature voltage.
Any variation of the voltage at the brushes indicates some issue or imbalance of the rotor.
Probable causes:
1) Incorrect brush alignment / off neutral. This can be because of the physical positioning of the brush contact face relative to the commutation zone, or it can be a result of dissymmetry in the magnetic field (interpole or main pole winding is exhibiting shorted turns, usually).
2) Incorrect interpole strength. Again, a "shorted turn" result, OR a mix-up in the sequencing of the shim pack between the interpole body and the frame cylinder. Most manufacturers recommend having magnetic material near the frame and nonmagnetic material near the pole body. it can also be a result of having the wrong (or at least different) air gaps between interpoles and armature surface.
3) Inappropriate brush grade. Brush is not capable of carrying the required current density, resulting in arcing as the bar leaves the brush face (so for argument purposes, the original post photo means the comm surface should be moving from top to bottom).
4) Low spring pressure. As brush meets the bar edge (in this case, surface moving from bottom to top), the edge of the bar causes the brush to "jump" away from the surface. Insufficient spring tension means the brush loses contact - creating an arc to the bar. The localized heating from the arc eventually destroys the film, appearing as a "burn" mark.
5) Sparking from some contamination. This may be airborne (chemical or moisture), particulate, or something embedded in the brush itself. Bottom line - it requires a larger voltage to "push" current across the brush-to-bar film - which in turn means a higher chance for arcing and eventual burning.
Just so you know - the burn is a direct result of an electrical arc producing erosion of the film (and, if allowed to progress far enough, the bar itself). It may be under the brush a bit, instead of the "streamers" right at the edge that you might have been expecting. But it is present.
1) Incorrect brush alignment / off neutral. This can be because of the physical positioning of the brush contact face relative to the commutation zone, or it can be a result of dissymmetry in the magnetic field (interpole or main pole winding is exhibiting shorted turns, usually).
2) Incorrect interpole strength. Again, a "shorted turn" result, OR a mix-up in the sequencing of the shim pack between the interpole body and the frame cylinder. Most manufacturers recommend having magnetic material near the frame and nonmagnetic material near the pole body. it can also be a result of having the wrong (or at least different) air gaps between interpoles and armature surface.
3) Inappropriate brush grade. Brush is not capable of carrying the required current density, resulting in arcing as the bar leaves the brush face (so for argument purposes, the original post photo means the comm surface should be moving from top to bottom).
4) Low spring pressure. As brush meets the bar edge (in this case, surface moving from bottom to top), the edge of the bar causes the brush to "jump" away from the surface. Insufficient spring tension means the brush loses contact - creating an arc to the bar. The localized heating from the arc eventually destroys the film, appearing as a "burn" mark.
5) Sparking from some contamination. This may be airborne (chemical or moisture), particulate, or something embedded in the brush itself. Bottom line - it requires a larger voltage to "push" current across the brush-to-bar film - which in turn means a higher chance for arcing and eventual burning.
Just so you know - the burn is a direct result of an electrical arc producing erosion of the film (and, if allowed to progress far enough, the bar itself). It may be under the brush a bit, instead of the "streamers" right at the edge that you might have been expecting. But it is present.
- Moderator
- #4
Yes, general causes of sparking but mostly not applicable to sparking at every second bar.
Look for something inducing an AC ripple on the generated DC.
THe spacing for bar burning is going to be related to the number of coils per slot. If the burn is every second bar, then the commutator most probably has
(BARS) = 2 x (SLOTS)
However, I did see the same thing once, when (BARS) = 4 x (SLOTS) and only every second bar was equalized.
(BARS) = 2 x (SLOTS)
However, I did see the same thing once, when (BARS) = 4 x (SLOTS) and only every second bar was equalized.
- Moderator
- #6
Does that make the commutator burning a design issue?
Rule of thumb for sparking/arcing at the commutator.
Move the brushes to cover the sparks.
So when we cover those sparks may we expect sparking at every second alternate bar?
Or, do we split the difference and go with minimum sparking at each bar?
Additionally, I would check the tightness of the screws holding the main and auxiliary poles and measure the air gap between the rotor and all poles on the stator. If any screw is loose or the air gap is uneven, it can create an uneven electromagnetic field and such phenomena on the commutator.
Good luck.
Good luck.
If the brush "span" is wrong (the brush contact does not span enough bar surface in circumferential direction), we can end up with one or both of the following.
1) The brush face area may be less than ideal, which translates to high current density - which in turn tends to start burning the brush at either the first or last edge to contact a given commutator bar. As the brush material burns, less face is available and up goes the current density some more. Eventually, a point is reached where no matter what happens, an arc develops and the film (and maybe the copper itself) starts burning.
2) The brush has a "dead band" where commutation is concerned because there isn't a smooth transition from one conducting coil to the next. When this happens, the voltage at the line of initial (or final) contact becomes too high and creates an arc. Think about it like stepping off a landing in a stairway. Ideally, you'd like to go from the landing to the ground in small increments. But if there are no actual steps, that first stride is going to make you pay by jumping the whole distance.
General rule of thumb is that the brush face should contact enough bars at the same time to span all of the bars for one slot, plus one-half of the next bar. So, for a 2-coil-per-slot design, the brush should touch 2.5 bars. Similarly, 3-coil-per-slot would touch 3.5 bars, etc.
Last thing - paraphrasing NEMA: "... successful commutation means that the level of sparking does not result in excessive brush or commutator maintenance." It says nothing about eliminating sparking altogether - although that would be the "perfect" condition (i.e. "black" commutation).
The "burn every second bar" is not - strictly speaking - a design issue. The design issue is going to be having the rigging off neutral (for whatever reason) or having insufficient brush cross-section for the current (e.g. overload).
1) The brush face area may be less than ideal, which translates to high current density - which in turn tends to start burning the brush at either the first or last edge to contact a given commutator bar. As the brush material burns, less face is available and up goes the current density some more. Eventually, a point is reached where no matter what happens, an arc develops and the film (and maybe the copper itself) starts burning.
2) The brush has a "dead band" where commutation is concerned because there isn't a smooth transition from one conducting coil to the next. When this happens, the voltage at the line of initial (or final) contact becomes too high and creates an arc. Think about it like stepping off a landing in a stairway. Ideally, you'd like to go from the landing to the ground in small increments. But if there are no actual steps, that first stride is going to make you pay by jumping the whole distance.
General rule of thumb is that the brush face should contact enough bars at the same time to span all of the bars for one slot, plus one-half of the next bar. So, for a 2-coil-per-slot design, the brush should touch 2.5 bars. Similarly, 3-coil-per-slot would touch 3.5 bars, etc.
Last thing - paraphrasing NEMA: "... successful commutation means that the level of sparking does not result in excessive brush or commutator maintenance." It says nothing about eliminating sparking altogether - although that would be the "perfect" condition (i.e. "black" commutation).
The "burn every second bar" is not - strictly speaking - a design issue. The design issue is going to be having the rigging off neutral (for whatever reason) or having insufficient brush cross-section for the current (e.g. overload).
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