Carbon Brush Wear DC Motor

[Oeztuerk]

Dear Experts,

I Kindly request your Support for a Project which i Can Not unterstand what is Happening electrically.
I have a brushed dc motor in a fuel pump of vehichle. The Motor has two permanent Magnets and an Armature with flat Carbon Material commutator. It drives through its shaft an impeller and so pressures and pumps fuel.

The brushes are also carbon material located on flat carbon commutator. The problem is that they are and mostly minus side is getting worn at a small amounts of Pumps As premature means really Earlier.

All the mechanical and Material relevant Thinge are compared to good pumps and the result is same. The brushes are driven with Spiral Springs and there exists No difference with good pumps springs i.e. Material Force are Same.


The terminals of dc Motor includes a capacitor connected parallelly. The positive Terminal connected to a coil inductor and then inductor connected to positive brush. The Same is also for minus terminal.

At this Point i am a Little Bit suspicious about this lc circuit. Because the armature shaft has contact with pumps Housing and housing has contact with a rfi suppression cap which is connected to minus Terminal also.

Could you please Check this configuration and Tell me what Could cause such a wear especially on minus brush.

Thanks a lot.

[oldway]

Hello and first welcome on the forum.

Some corrections in your presentation:
The collector of the rotor is made of copper, not carbon.
The brushes are made of graphite or a graphite / copper mixture.

In a DC motor, this can cause excessive brush wear:
1) the quality and hardness of the brushes.
2) the brushes are not in the neutral line and there is sparking
3) the collector is defective or it is ovalized and must be rectified with a lathe
4) the rotor is not properly balanced
5) the bearing on the collector side is defective
6) a collector / winding connection is broken
7) the pressure on the brush (s) is incorrect. (too strong or too weak.)
8 ) brush blocked or not sufficiently free in the brush holder
9) the collector is dirty and contaminated with oil or grease.
10) isolating material (mica) between collector strips preventing good contact between brush and collector

[amyk]

There is a direct loss of carbon by vaporization at the arc spot, and this is most pronounced for the negative brush, which undergoes positive ion bombardment.

See attached for more information...

Lancaster, J. K., 'The Wear of Carbons and Graphites, Treatise on Materials
Science and Technology', Wear 13, 1979, 141-174

J. E. Thompson and M. J. B. Turner, "Mechanism of the wear of graphite sliding on steel with and without an electric current passing across the interface", Nature, 196 (1962) 329–331

[SeanB]

If new brushes, of the right composition ( as it is a fuel pump I guess 12V or 24V, so the brush material will be a copper powder loaded graphite) are wearing really fast, and with a spark, and the commutator bars are both shiny bright and clean, with a good undercut in the slots, then the most likely candidate for wear is a shorted winding on the motor rotor.

Only way to tell is to find a motor rewinder ( really only if the motor is no longer available as spare part, and there is no other one that can be fitted there) to check the armature on a growler to see if there is a short, and then to rewind it. Cost however will probably be more than a new pump though.

[oldway]

A permanent magnet motor with a short-circuited rotor (partially or totally) is difficult to turn by hand, it does not rotate freely like a normal motor.

Moreover, under tension it turns only very slowly and releases the magic smoke after a short time

[Oeztuerk]

Hello Everyone,

Thanks all for your comments.
I send you a small sketch of this system for a better explanation.
The DC motor has a LC circuit (as far as i know against voltage ripples and rfi protection).
Operating voltage of motor is 12,5 V. The configuration includes a ceramic capacitor connected as shown to the terminals and two suppressor chokes, one for each terminal and the other side of chokes are conneced to each brushes. Both commutator and brushes are made of carbon graphite material. The brushes are laying under two spiral springs, till now I have checked some good and bad pump motors, there is no material difference. The spring forces are also same. Armature is also checked for short windings but nothing abnormal. And almost all the time the minus brush has nearly double more wear rate in comparison to plus side.

One additional point, the armature shaft has direct contact with housing and housing has also contact with a suppression cap which is made of alu material. The suppression cap is connected to the minus pole.

An additional question, could any resonances be a reason for excessive brush wear in that LC circuit ? or did I make a Tesla circuit wrongly.

Could you please check this and share your opinions.

Thanks all again.

[floobydust]

Troubleshooting uneven carbon brush wear is very difficult.

First, look for mechanical differences (x,y,z direction) in the brush holder, springs, alignment etc.
The torque due to friction across the brush face must not move one brush differently compared to the other. Or tension (tug) from the brush wiring can make it crooked.
Sometimes mechanical engineers make a mistake in the brush holder design and mounting. Tightening the mounting screw can warp the holder.

Also there can be a mechanical resonance occuring which is like a squeal, this can cause rapid brush wear.
It is complex; changing spring tension or tighter tolerances seems could be a fix.
But this would be the reason here only if you have a mechanical asymmetry problem. I think both brushes would wear rapidly if there was a mech. resonance.


Electrically, your circuit may not have symmetry and could have an (electrical) resonance.

Having the EMI capacitor after the coils/springs/ferrite beads is not ideal for lessening brush arcing.
You would have a cap right across the brush take-off leads. The cap value may be too small.

I would measure dielectric strength from the armature shaft to the coils/commutator.
You probably have arcing/breakdown from the coils/commutator to the armature/laminations, which are grounded. I do not know if your commutator design has proper gaps, but I'm guessing the HV is arcing to the armature instead of the opposite brush. You could try measure motor-case return current/voltage.

If you can look at a motor running in air, to see the arcing pattern.

[Oeztuerk]

Hi Floobydust
I have measured insulation Résistance of armature to shaft under 500volts, the non Problematic Armaturen have bigger that 1Giga Ohm and the problematic Ones Have the insulation Résistance between 2-20 megaohms.can this be an issue or 2 megaohm is already big enough for a 12 Volt dc Motor ?
Thanks

[Oeztuerk]

I mean from Commutator Segments to shaft the insulation measured.

[oldway]

All this seems strange to me, I have a lot of experience with industrial and traction dc motors but I have never seen a dc motor with a graphite collector.

I wonder how it withstand to the centrifugal force ... 

A brush motor to pump fuel, this does not seem to me either a very good security choice. 

In order to diagnose commutation problems and excessive brush wear in DC motors, it is necessary to examine the condition of the collector, the state of the brushes and check the amount of sparks produced.

It is already difficult to make a correct diagnosis with all these informations at his disposal and on a traditional dc motor (as explained in a previous post, there are a lot of problems who can produce a bad commutation), then, remotely, on a special motor without information, it is truly the quadrature of the circle

A really unlikely idea, but in the absence of any other idea, I give it anyway: it is probably a permanent magnet motor, one of the magnets could have moved or would have been placed in an erroneous position.

[floobydust]

My understanding of automotive fuel pumps is the motor is completely immersed, the gasoline is the lubricant. There is no risk of explosion or fire because the mixture is too rich i.e. not enough air present. Fuel passes through the armature.

Arcing through air does not happen, just through the liquid dielectric (gasoline).

I think the armature insulation has failed, or water is present. 2-20MEG is a low reading on only the failed motors compared to 1GIG for a good motor.

At 12V this is not an issue, but the motor generates high-voltage back EMF during commutation. If it can arc from the coil/commutator to the armature to ground, then that current path might be causing problems. A winding may even be partially short-circuit if the insulation is failing in many places. The coating might be attacked by the fuel.

As oldway says, this is very hard to troubleshoot by just reading. You have to carefully go through a checklist.

[oldway]

It's very interesting, I did not know this technology at all ... 

I have never worked in the automotive field, only in industrial and power electronics.

[klaff]

What does the drive circuit look like? Just a switch to 12V DC? Any electronics and PWMing? High-side switch, low-side switch? Is the housing grounded to the negative supply voltage (as in typical automotive 12V negative ground arrangement)?

[Oeztuerk]

Hello all, Thanks for the comments.
It is directly connected to the dc and there is No Regulation or speed control. I found Some Information on the Web, it is written that if the inductance increased the minus Pole will wear more. My question again, how Can i measure this dc Motors induction, there exists a capacitor parallel connected to the Terminals , will it be wrong if i measure L directly from terminals with an LCR Meter or should i take capacitor out ?

[Gyro]

It's very interesting, I did not know this technology at all ... 

I have never worked in the automotive field, only in industrial and power electronics.

It's a very good reason for not letting your tank run dry!     I'm sure they must have thought of that one and included safeguards though.

[BradC]

It's very interesting, I did not know this technology at all ... 

I have never worked in the automotive field, only in industrial and power electronics.

It's a very good reason for not letting your tank run dry!     I'm sure they must have thought of that one and included safeguards though.

No issues as the tank isn't filled with air as it empties. It fills with petrol vapour. No air, no issue.

On the other hand, I powered up a second hand fuel pump one day just to see if it spins. It did spin, but it also spewed fire as it inhaled air to mix with the remaining fuel in the body to be ignited by the brushes.

That was temporarily exciting.

[Gyro]

Wow, that's more exciting than I thought! 

It does raise interesting thoughts about loose / leaky filler caps seals etc. I suspect that they have flame supression meshes (Davy lamp style) on the inlet to prevent flashback into the tank. A flame jet on the output would be pretty well supressed by the long thin fuel feed pipe. I'm impressed that you actually achieve flame though!

[BradC]

Wow, that's more exciting than I thought! 

It does raise interesting thoughts about loose / leaky filler caps seals etc. I suspect that they have flame supression meshes (Davy lamp style) on the inlet to prevent flashback into the tank. A flame jet on the output would be pretty well supressed by the long thin fuel feed pipe. I'm impressed that you actually achieve flame though!

I suspect I got very unlucky and had just the right conditions and amount of residual fuel in the pump. It was also a grade A dumb thing to do really. "Here, I have this fuel pump that still has some fuel in it. Hold my beer while I pick it up and put power on it to see what happens." It was a roller cell pump, so positive displacement. It was going to pump air no matter what.

You'd have to work pretty hard and in quite cool conditions to get enough air into the tank to even allow a flash back. The vapor pressure on petrol is high enough it almost always wants out rather than drawing in.

[Zero999]

I did a similar silly thing, with a fuel pump, when I was a teenager, except is wasn't that exciting. I tried to make a water pistol from an old fuel pump, I found discarded by the side of the road. Of course it worked for a bit, until the water corroded it and it ceased up, causing a huge current to flow and magic smoke to be emitted.