MadModder

The Shop => Electronics & IC Programing => Topic started by: speedibee on November 14, 2018, 10:52:31 AM

Title: motor brushes
Post by: speedibee on November 14, 2018, 10:52:31 AM

I repalced the brushes in my angle grinder , they only lasted about 15 seconds and there was nothing left just the springs . any ideas why

Title: Re: motor brushes
Post by: ddmckee54 on November 14, 2018, 11:27:47 AM

There could be a couple of possible reasons for this:

1) Did you use the exact replacement brush, or was it a hardware store "equivalent" that was the right size?

2) You could have a bad/rough spot on the commutator that basically ate the brushes while the motor was turning.  Angle grinders live in a fairly harsh environment so this is a good possibility.  Bet it made a helluva light show while it was running.

3) Could the brushes have been installed incorrectly?  Some brushes are set up to be installed in one direction only.  Installing them backwards will reduce their lifespan - but it shouldn't reduce it THAT much.

4) Bad/damaged brushes that were really-really soft and crumbly?  Not likely, but it COULD happen.

Don

Title: Re: motor brushes
Post by: PekkaNF on November 14, 2018, 11:40:14 AM

Also: Wrong type of brushes - automotive (low voltage) brushes are different from mains voltage brushes, but then commutator should be a write off as well.

Pekka

Title: Re: motor brushes
Post by: speedibee on November 14, 2018, 11:44:10 AM

they are cheapo's off ebay , amazingly after scrapping 3 sets the comm  looks like new

Title: Re: motor brushes
Post by: John Rudd on November 14, 2018, 11:50:40 AM

Brush composition is determined by application...Spring pressure is another important factor..
Your best bet is go to a brush maker, tell them the make/model etc....

These people for example...
https://www.carbonbrush.co.uk/

Title: Re: motor brushes
Post by: picclock on November 19, 2018, 04:43:30 AM

It may be that the armature has a fault, such that when the brushes contact a faulty shorted winding the excess current flow causes the brushes to heat and disintegrate.
I have used carbon rods from a U2 battery to make temporary brushes for a hoover, and they lasted quite a few months before I got around to replacing them with the proper spares. So even the wrong carbon should last a reasonable time.

Best Regards

picclock

Title: Re: motor brushes
Post by: Doc on November 19, 2018, 08:55:48 AM

One other thing that could happen is the insulator material between the com. plates should be below if they are flush or slightly above they can cause the brushes to arc and fail after just a short time.

Title: Re: motor brushes
Post by: Arnak on November 23, 2018, 11:43:51 AM

Hi,

Carbon Brushes are made specifically for the motor that they are being used in and come in different degrees of hardness.

Carbon brush grades are only designed to carry a specific current.  If you increase the current density then you increase the temperature at which the carbon brushes operate.  If your current density is too low then you get rapid brush wear.  Taken to extreme, this can mean that your brushes could either dust or wear out in days.

So cheap, incorrect brushes can wear out or damage the commutator if they are too hard.

Make sure you buy from the motor manufacturer or a known brand equivalent.

Martin