Symptom of worn-out Brushes?

[jeffreyclay]

Hi friends. I have a Quickie Rhapsody with new MK batteries. Last month I had to replace the left worm gearbox due to a gear missing 2 teeth. That cured my mechanical "lurching" but I have an intermittent electrical lurch now. I suspect the brushes in the left motor and I'll give you the symptoms for your analysis. When acting up it has a very "choppy" supply of current allowing only a slow, spastic turning of wheel. The controller "sees" this and limits the speed of the right wheel. If I was to back-up a few feet it self corrects and I can ride forward at any speed I choose for a short period of time. Ambient temperature has a big effect. If I go outdoors and stop under even a shade tree on a 90f degree day for 15 minutes, it is guaranteed to act up. If I return to my cool basement, 70f, it will function perfectly for hours. I pulled the motor, stripped it down for cleaning and dropped in a used set of brushes that cleared it up for a month. I mic'd one of the brushes (6mm X10mm X17mm) and discover it's a popular size for GE brand motors in hand tools like circular saws and hand grinders. Do you think a new set will do the job? The brush face isn't perfect but I'd say the commutator is reasonably smooth. Any tips other than assemble clean & dry?

[woodygb]

New brushes can't harm ...but buy the correct sort.

http://store.eurtonelectric.com/brushes.aspx

[ex-Gooserider]

Agreed, get the right brush that's made for the motor... Also note that if the commutator is not very smooth, it will rapidly eat the new brushes.... Grooves and ridges running around it aren't a big problem, but any sort of crossways roughness needs to be fixed if you want the new brushes to last...

Run the new brushes gently for a while - I've seen many suggestions to get the wheel off the deck and just run with no load at different speeds, and periodically reversing directions for 20-30 minutes, followed by a couple weeks of GENTLE use. This will help the brush faces wear to match the surface of the commutator so that you have a large electrical contact area instead of a small one.

Also look at replacing any bearings while you have the motor apart... BM suggests using rubber sealed (-RS) bearings as opposed to open or metal (-ZZ) shielded.

ex-Gooserider

[dewaj]

Almost seems as if a motor winding has broken or shorted. True, it's relatively rare, but still happens. It can be perfectly fine when cold but become apparent as the motor warms up. In the few cases where this has happened I discovered that the voltage across the motor would jump as the damaged winding connected. But this was with plain-old, fixed-voltage DC, not PWM, and with an hefty, old, Simpson analog meter. The Fluke's readings weren't stable and masked the jump when it occurred.

There's a related story, rather long and likely severely boring, behind this which involved heavy-handed "repairs" that broke things, like these motors, further. For another time, perhaps.

[dewaj]

I wish there was an easy way to get more clean air into the motor. Carbon buildup inside the motor assembly means that the motor is not getting enough oxygen to burn off the carbon coming off the brushes.

[Burgerman]

If everything is running well, thereshould be absolutely miniscule amounts of carbon dust. Oxygen isnt your issue. Brushes shouldnt arc. Carbon shouldnt burn up. But I have seen this happen on 2 pole motors. Where you tip out a cupful of carbon. Because the brushes overheat under heavy loads. They have too much current going through them. Hence 4 pole better... Never saw one of those screw up a set of brushes. Unless bad bearing or something making commutator not run true.

[dewaj]

I'm pretty certain that the carbon burns off as the brushes wear and it requires oxygen. But I only have my direct observation and experience to rely upon and not anyone's scientific data.

[Burgerman]

We describe brushes as made of carbon. But in reality they are made of many things including carbon. And unless they are overloaded, or bouncing on a non round commutator, or a worn non concentric bearing, they should barely arc at all. It takes a very long time to wear down a correctly running motor brush. And the deposits are basically not black, or shouldnt be, as it should be dust particles of the brush itself. A very fine grey dust. Not burned carbon as soot.

Typical motor brush manufacturer.

Our electrical carbon brushes are made from low friction conductive carbon materials including copper graphite, silver graphite, natural graphite, electro graphite, resin bonded carbon and pitch bonded carbon.

The combination of this wide range of specialist materials and our highly experienced local technical support network allows us to match brushes and holders to customer equipment for optimum performance with minimal maintenance requirements.

Brush rush

As the commutator and brushes move relative to each other, they wear, releasing fine particles of dust into the motor. As carbon makes up most brush material, the dust tends to be conductive; as it accumulates, it can cause problems, such as slot packing. Slot packing is the dust particles filling the commutator slots and connecting both sides of the segments that are the copper ends of the armature windings where the brushes rub, basically shorting them together. Ideally, the centrifugal force of the rotating shaft will keep the slots clean, but any type of humidity or too slow a shaft speed will contribute to dust accumulation. Motor speed should be a minimum of 1500 rpm to keep the slots clean.

Excessive dust can also cause the brushes to stick or bind in their holders, which can lead to arcing or open circuit failures.

Sparking is another common problem on electric motors. Small sparks are yellow in appearance, whereas large sparks are white. White sparks are detrimental to good motor performance. Sparking can be caused by numerous conditions like motor overload, vibration, high humidity, worn brushes and worn commutators. When checking for sparking or arcing, a good visual inspection of the commutator and brushes will have to be performed.

[jeffreyclay]

Thanks to all who replied. When I mentioned the brushes looked to be the same as some found in GE pm motors I was only referring to the size. I have found a company that sells for powerchair use. Interior of motor looks to be really nasty with black carbon dust! I hope a thorough brushing and wash in Hexane will clean it up. While waiting for the brushes my Joy stick buttons became unresponsive. I had cleaned the circuit board and carbon coating on the silicone membrane only last month. I may have scrubbed too vigorously on the carbon so I added aluminum pads to the rubber. I had some HVAC duct connection tape but the self adhesive did't grip the silicone very well. I used "Weldwood" contact cement as directed by the label instructions and received a good bond. Control buttons react very smartly. I'll see if I can record some video of how the motor reacted before and after the brush change so you can see the symptoms.
l]

[jeffreyclay]

I pulled the motor to give it a cleaning before the new brushes arrive. I quickly saw the reason for the jerky application of power when I removed the brushes to allow one end of the motor to separate from the magnet body. (See photo) Jeez, I have no idea how that happened as it was running fine for a couple of months after I bought the chair to this condition a week ago! I had a junk motor on the shelf and robbed a brush set from it and things were back to normal.
I also learned a few things that might be helpful to those of you who delve into disassembly of your motors. I marked each end cap to body for an index in assembly. I later discover the crinkle black paint will completely hide your permanent marker stripe once dry. This allowed me to reassemble the motor with the magnet body reversed. Not being aware of any consequences I installed the motor in the chair. Well the motor ran backwards. I was about to pull the motor and go through the whole process again until it hit me. The controller simply reverses polarity to change motor direction, so will I. I used a small, thin piece of steel to open the pin's latch for the motor's power in the 4 pin plug and reinserted them backwards. Done in 3 minutes! And about the long fasteners holding the motor's end caps on. The magnets will keep that long screw jumping from one side of threaded hole to the other. Use a dental cleaning tool or the like to guide the 1st one to align and start perhaps 2-3 threads. Do the same for the other side using the available gap then as you begin to tighten, shuffle the body to make sure you've engaged the alignment notch on each end cap. It's a job for 3 hands but 1 person can do it with patience.

[woodygb]

Make sure that the contact area shown in the pic is clean and the cap is screwed down fairly tight.... or the brush spring , wire etc will get very hot due to this bad connection between the motors power wire and the brush / armature.