DC motor brush question

[hotrodjohn71]

I suppose this question is not technically an electronics question, I wonder If I may be permitted to ask a question about the brushes on a DC motor, a motorcycle starter motor to be exact?
Thank you

 

[BobK]

Certainly, there is plenty of discussion about DC motors here.

 

[hevans1944]

I wonder If I may be permitted to ask a question about the brushes on a DC motor, a motorcycle starter motor to be exact?

It has always been my observation that it is easier to ask for forgiveness than to ask for permission. Ask away @hotrodjohn71.

 

[hotrodjohn71]

Thank you so much

Im working with a 12volt electric motorcycle starter. Battery is good at 13 volts and charging system working fine.
I have a voltage drop intermittantly at hot startups to around 8 volts which is causing problems with my solenoid hold in circuit to the starter gear as this system has a totally seperate starter and soleniod.

The problem doesnt usually show up on cold starts as I have about 10 volts durring cranking and it starts up fine. This intermittant problem occurs after its been shut off after a good ammount of run time and allowed to heat soak.

I have (atleast Im pretty sure) isolated the problem to the starter which I have disassembled and checked with my ohm meter regarding the armature and brush housing.

My first question is this: The brushes look OK, dont seem too short, but will a motor run differently as a brush gets shorter besides the spring pressure factor? in other words, as long as there is some brush material contacting the commutator, will the motor run the same. (I ask not because I am too cheap to buy new brushes, but am just trying to understand how the motor works.. but I am cheap though )
And if there is an adverse effect from shorter brushes, would that tend to draw more current or less.

Second question pertains to the commutator. considering that the brushes were good, if I have a commutator worn - where the micah is keeping the brushes from fully contacting (but enough that the starter works most of the time) would this situation cause an overdraw in current or underdraw.
And if I have a commutator where the grooves are good and well defined but they have carbon buildup between the bars from the brushes, would this tend to cause a current overdraw or underdraw on cranking?

 

[Minder]

Mica was an older material for separating segments, now material that wears with the comm is used.
Armatures that have mica that are turned down or reconditioned have to be undercut with a fine hacksaw blade, it is fairly easy to make one from broken blade.
If the brushes are kept from good contact, it is a high resistance situation and not good, build up between segments is fairly rare as the material generally wears the same rate as the comm.
Brushes do have a manufacturers recommended wear length, if very short, it would be advisable to replace them, starter brushes have a very high copper content so not any old brushes can be used.
If the comm is well worn, it may be worth a skim on a lathe and undercut.
M.

 

[hotrodjohn71]

Hi Minder, thank you for your reply.
Would a high resistance situation cause more current draw?

 

[duke37]

Make sure that the brushes are free to move in their holders. If the brushes do not fit properly, then the commutator will be damaged. If you remove the brushes, then put them back where they came from to maintain the bed-in.
The comm should be chocolate coloured and there should be no sign of arcing between segments.

 

[Minder]

Hi Minder, thank you for your reply.
Would a high resistance situation cause more current draw?

No generally the opposite.
Is this a series (field) wound starter or a P.M. field?
M.

 

[twister]

Most starter problems are the result of the armature dragging on the field due to worn bushings. If the amps drawn by the starter are high that is your problem. Buy a rebuilt starter or you can rotate the bushings 180 degrees.

 

[Bluejets]

Seems a waste of time assuming the starter is at fault. Best move would be to get the battery load tested, i.e. eliminate first link in the chain. Then look for any hot connections between the battery and the start solenoid etc etc .