Motor carbon brush sparking. Why it happens and how can I fix it?

Thread Starter

rambomhtri

Joined Nov 9, 2015
317
Hi, I have a 300W electric saw (small), and the motor carbon brushes start sparking "like crazy" whenever I turn it on. First, I'd like to know what can cause a spark (short circuit, simple friction between brush and metal???). The brushes are really easy to take out, so I did and they look fine, they don't look burn't or destroyed. What I don't know how it is is the commutator (or armature). This is basically my motor:


Should I sandpaper the carbon brushes to remove a bad "coat"? (there's still a lot to go, and the spring is strong and nice)
May be should I sandpaper the commutator and make it shiny and uniform?
Could it be an impossible to solve problem?
 

Alec_t

Joined Sep 17, 2013
11,296
Some sparking at the brushes is normal. Excessive sparking may be due to worn brushes, resulting in reduced spring pressure, or due to roughness of the commutator segments (test with a finger ..... with the power OFF!), or perhaps carbon dust between the commutator segments. If you do use sandpaper, use only the finest grade on the commutator. Don't be tempted to sand/reshape the brushes.
 

MaxHeadRoom

Joined Jul 18, 2013
20,703
Also circular arc following the commutator rotation can be a sign that there is a armature fault.
Another is if it is an old motor and the comm segments are separated by Mica instead of the wearable kind of material now used.
Max.
 

Thread Starter

rambomhtri

Joined Nov 9, 2015
317
It is quite old actually, may be from the 80's or so, but it's still perfectly fine, no reason to toss it. I actually took it apart almost completely like 2 years ago, made a nice clean up, but I think I didn't clean the commutator, at least thoroughly, and I didn't test it after the "restoration". I don't think there's any arching, I'm pretty sure it's the brushes only, nothing else, but the sparks are not normal I believe.
 

MisterBill2

Joined Jan 23, 2018
6,072
All brush type motors have some brush sparking because you are breaking an inductive circuit with current flowing in it. The one check you can easily do is to verify that there is at least a small depression between the commutator segments. But you will need to gain access to the commutator so that you can touch it. First, make sure the saw is disconnected from all power. Then, when you have access to the commutator, drag your finger nail across the part between two segments. There should be at least a small bit of a depression. If there is a bump then it is causing the brush to lose contact too early and make contact too late. In that case the depression needs to be added, a process described in detail by other folks, not me.
 

MaxHeadRoom

Joined Jul 18, 2013
20,703
The older comm's that had Mica separators, required undercutting after a long period of use, this often neccesary after skimming of the comm. We made our own from a fine tooth hack saw blade with the tooth set ground off flat.
And a handle and blade clamp made to keep the blade rigid..
If this is that old and has had quite some usage, this may be the problem.
Later comm's have a wearable insulation and do not need this procedure.
Excessive arcing is not normal.
Max.
 

MisterBill2

Joined Jan 23, 2018
6,072
How much sparking is "like crazy"? Since the TS is not an experienced motor service person we have no reference as to the actual length or intensity of the sparks, and thus no simple means of verifying that anything is even wrong. I have been using a half-inch drill motor to drive a drain snake for 30 years and indeed the brushes do spark, even a bit more than in forward, because of running the drill motor in reverse. If the saw motor in question turns freely then my advice is to use it and not be concerned, as long as there is no mechanical binding problem. Just be sure to keep the thing clean, since sawdust blocking the air flow could become an issue.
 

Tonyr1084

Joined Sep 24, 2015
4,538
It is quite old actually, may be from the 80's or so
GEE THANKS! I'm from the 80's or so too.

I've used one of those white PVC erasers to clean commutators before. They work quite well without removing any material. However, reading this thread has taught me a little about the sparking I've seen in my "Quite Old" experience. What I would call "Excessive arcing" would be a spark that follows the commutator a good eighth of a turn. That is one pizza slice when the pie has been cut into 8 pieces. And I've seen that rarely.

If you DO find a need to reduce the insulator between the contacts do so very carefully so as to not damage any of the windings. You COULD end up with a non-functional motor.
 

Thread Starter

rambomhtri

Joined Nov 9, 2015
317
GEE THANKS! I'm from the 80's or so too.

I've used one of those white PVC erasers to clean commutators before. They work quite well without removing any material. However, reading this thread has taught me a little about the sparking I've seen in my "Quite Old" experience. What I would call "Excessive arcing" would be a spark that follows the commutator a good eighth of a turn. That is one pizza slice when the pie has been cut into 8 pieces. And I've seen that rarely.

If you DO find a need to reduce the insulator between the contacts do so very carefully so as to not damage any of the windings. You COULD end up with a non-functional motor.
Hahaha, lemme rewrite that sentence:
It is quite old actually FOR A TOOL, from the 80s or so. God, if you were born in the 80s you're now in 2020 too young to even care about age. Remember, a person that in the 80s was 60 years old corresponds to someone today that is at least 70 years old. People get old later and later, so don't worry.

Yeah, I'm not an expert at all at motors, but I'm pretty sure that quantity of sparks are not normal. The things works, sure, but I don't think it's working as neat as it should be.
 
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MaxHeadRoom

Joined Jul 18, 2013
20,703
If the brushes have never been replaced, and it has had a fairly high rate of usage, it is likely they need replacing, especially if this is a Universal motor.
Also, due to the very high RPM for this type of motor, the bearings should be checked/replaced.
If the arc is bright blue and appears to follow the comm almost all the way around, it points to a short in the motor.
Max.
 

Thread Starter

rambomhtri

Joined Nov 9, 2015
317
If the brushes have never been replaced, and it has had a fairly high rate of usage, it is likely they need replacing, especially if this is a Universal motor.
Also, due to the very high RPM for this type of motor, the bearings should be checked/replaced.
If the arc is bright blue and appears to follow the comm almost all the way around, it points to a short in the motor.
Max.
It's been used but not heavily used. The carbon brushes are not at all wasted or fully used. I already said that there's still a lot of carbon brush to go. That's why I asked if it could be a "coat" in the carbon surface, or simply dirt in the commutator. The arcing is definitely blue, I guess, or orange? I'm not sure now, and I don't have access to it right now.
To discard a short, I could simply test the impedance between opposite sides in the commutator and check that the value is low, and also that it doesn't short with any other point, right?
 

MaxHeadRoom

Joined Jul 18, 2013
20,703
In general, the comm of a motor is self cleaning as the carbon tends to be incandesced this is generally seen as sparking.
A dull colour is generally normal, the sign of stress is deep comm wear or ridges etc.
The sparking should be mild and red/orange in colour, if blue and appears almost all around the comm, it can indicate a short circuit somewhere.
The conclusive test for a armature, short etc, is an instrument called a Growler, measures voltage produced across each comm section when the armature is under test.
Max.
 

MisterBill2

Joined Jan 23, 2018
6,072
It is not likely that there is a short in the armature, and I would not let that bother me anyway. The TS did state that the brushes were good and did not look worn. A gentle cleaning of the commutator will not hurt anything, just don't leave any abrasive or conductive stuff on it after cleaning. That means don't use steel wool to clean it. And do wipe it clean with a fresh wipe after removing the glaze. And I am not sure that an ohm meter check will tell much about any shorted turns, because the armature winding resistance is fairly low.
 
When bad, they wear unevenly. Shiny and no pitting. A ceramic capacitor around 0.1 uF near the motor terminals might reduce the spark and EMI.

A vacuum cleaner repair store is a good source for brushes. You can file them to fit the holder. I'v even shimmed the holder for an automobile blower brush set. Look for adjacent shorts and clean the dust.
 

MisterBill2

Joined Jan 23, 2018
6,072
When bad, they wear unevenly. Shiny and no pitting. A ceramic capacitor around 0.1 uF near the motor terminals might reduce the spark and EMI.

A vacuum cleaner repair store is a good source for brushes. You can file them to fit the holder. I'v even shimmed the holder for an automobile blower brush set. Look for adjacent shorts and clean the dust.
The TS has stated that the brushes are in good condition and not worn.
 

Thread Starter

rambomhtri

Joined Nov 9, 2015
317
The TS has stated that the brushes are in good condition and not worn.
Yes, may be they are "damaged" in the contact surface, but they are almost new in terms of quantity, there's still a lot of brush to go, and plenty of force from the springs.
 
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shortbus

Joined Sep 30, 2009
7,691
there's still a lot of brush to go, and plenty of force from the springs.
How do you know that? Do you have new brushes and springs to compare them to?

One thing that no one has brought up is the position of the brushes. Due to manufacturing variations the brushes may not be exactly 180 degrees apart. Something that happens more often in a clamshell type tool housing. One brush being even a few degrees displaced will make it or both spark. This is due to the commutator segments still trying to pass the electric and being a small amount of a short circuit, from not being aligned.

I have an expensive electric impact wrench, that the rotation direction is changed by moving the brush holder end cap. If you slightly move the brush cap from it's proper position, you can make it spark much more than normal. And it is normal for a small amount of sparking in any brushed motor.
 
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