Amadeal VM25L Uneven Motor Brush Wear

[Richard Kirkman 1Participant @richardkirkman1]

i was machining some cast iron on Saturday when the mill suddenly went pop.

It tripped the power, skipping the garage breaker and tripping the house breaker instead.

Once I had sorted all that out, I found the fuse had blown, so I replaced it.

I then found that the mill would not start. Multimeter out, the motor was still getting voltage to it so the controller was okay.

So I took the motor brushes out to have a look- one had broken completely, and the other was looking fine.

I have ordered replacements from amadeal so will be able to test in a few weeks when I am home again.

Is uneven brush wear common in DC motors? Or does this look like a faulty brush?

It is a 2014 model and I think I’m the 3rd owner, don’t know how much it’s been used, but minimal use from me.

I did notice a while ago that the mill did not run in reverse at the same speed as it did forwards, could this have been related to the brushes? About a month ago it wasn’t wanting to start immediately, but once it warmed up it seemed to run just fine.

Any thoughts?

Thanks,

Richard

 

[noel shelleyParticipant @noelshelley55608]

Looking at the old brushes it looks as though they are contaminated – oil ? The brushes need to be clean, run free in their holders and long before they get in this state changed OR they will wreck the commutator and then you will need a new rotor as well. Noel.

[JasonBModerator @jasonb]

My Warco lathe ones went like that after just over a year, the replacements I got from them for about a fiver have been fine for the last 14yrs,

Give Amadeal a call for a spare set.

The DC Brussed machines both lathe and mill run slower in reverse

[KenLParticipant @kenl]

Those brushes in the photos look to be quite badly worn and the one that is broken probably became too short to properly locate in the brush holders causing it to tilt over and break.

Hopefully a new set of brushes will get it going again but I would also want to clean and undercut the commutator to clear any burning and remove carbon deposits from the commutator slots.

If you feel competent to do so I suggest thst you remove the rotor, chuck it in the lathe and carefully clean (or indeed skim the surface if any burning is present) the commutator followed by cleaning out the slots that divide the copper to remove carbon deposits which will cause shorting if not removed. This can be done by using a piece of HSS (hacksaw blade?) to scrape the slots.

[Richard Kirkman 1Participant @richardkirkman1]

Noel- Thanks, always good to know. Unsure about contamination, I’ll give everything a good clean before I put the new ones in. I have no reference for what they used to be like with the machine being pre-owned. This was my first time looking at the brushes. With the motor being up at the top of the machine, I don’t think oil would accidentally be put there, unless oiling the motor bearings, but that seems unlikely too.

Jason- thank you. That’s reassuring to hear about reverse. Did your lathe also blow a fuse when the brushes went? It felt strange that it blew and didn’t blow the plug fuse, yet tripped the house.

I already ordered some directly from amadeals website, only £9 with free postage so not too bad. A preferable cost compared to a new motor or control board!

Kent- Good idea with the commutator, I do have a lathe so I will consider this, I imagine a very very light cut just to clean the surface? The brush that is still intact is currently 12mm long, from my research the originals are 3.5x10x17mm. So it has only worn 5mm since the mill was made in 2014.

[Richard Kirkman 1]

On Richard Kirkman 1 Said:

Kent- Good idea with the commutator, I do have a lathe so I will consider this, I imagine a very very light cut just to clean the surface? The brush that is still intact is currently 12mm long, from my research the originals are 3.5x10x17mm. So it has only worn 5mm since the mill was made in 2014.

Just to clarify Richard:

I wouldn’t “skim” the commutator unless there is damage or burning to the surface. You can use some fine abrasive cloth to get a clean surface. When I was an apprentice we were all given a “com brush” which was a very tightly bound bundle of glass fibres about 6″ long – I’ve still got one somewhere. You had to press the end grain hard against the surface of the commutator whilst the motor was running. These brushes worked wonders to keep the surface clean. Bear in mind that this was on very large 110v and 400v DC motors of 19th century vintage – whilst they were running and in service. Can you imagine the H&S implications in this day and age?????

It’s important though to make sure the insulation between the segments is clean and does not protrude above the copper segments, this will lead to arcing and commutator damage.

It’s not so much the amount the old brush is worn, more the percentage length of unsupported brush protruding from the brush holder. If more is protruding than is captive the brush will become unstable and tilt when the motor spins, often causing the brush to chip at the end or break.

[Richard Kirkman 1Participant @richardkirkman1]

Thanks Kent, I will have a proper look at the commutator and review it when I get home next.

Thanks for the brush explanation. It’ll be interesting to see how they wear over the coming years, assuming I keep the mill that long!

[Richard Kirkman 1]

On Richard Kirkman 1 Said:

…Did your lathe also blow a fuse when the brushes went? It felt strange that it blew and didn’t blow the plug fuse, yet tripped the house.

…

Wouldn’t worry me. The internal fuse blew before the plug fuse, the latter being intended to protect the house wiring, the risk being it will overheat and burn the building down.

Trips serve a different purpose; RCDs protect people by detecting a small current imbalance between Live and Neutral, perhaps <30mA, which implies an accidental earth leak.

Depending on the circumstances and how fast acting each protective device is, you can get trip only, trip plus plug fuse, trip plus internal fuse, trip plus all fuses, or one or more fuses blown without a trip.    Main thing is that at least one device operates to protect the machine, avoid a house fire, and saves the operator from a fatal shock.

I guess the brush broke and shorted to the metal frame of the motor.  The short blew the machines internal fuse.  But, because the motor is earthed, the short also caused an imbalance between L and N that was detected by the trip, and it cut the power.

Not sure what causes uneven brush wear, which I’ve seen a few times.   I think if the brush material or spring is slightly different one of a pair might spark a little more than the other.  Once a brush starts to erode, I suspect the imbalance might cause faster wear on that brush, especially if the motor is worked hard.  Not a quick process, my examples had all done at least a couple of year’s work.

Dave

[JasonBModerator @jasonb]

I don’t think mine blew any fuses, the motor was stopping and starting and generally running erratically. Most likely the short brush that had got hot and caused the spring to soften was making intermittent contact.

 

Your broken bits most likely shorted something.

[BazyleParticipant @bazyle]

Uneven wear is common as manufacturing and assembly tolerances are loose. Slight misalignment of the guide and its dimensions, different spring rates, and that little copper braid can interfere. When you fit the new ones make sure they are not sparking or they and the commutator will soon die.

Avoid using abrasive paper to clean things as the particles will remain to continue grinding. The glass fibre brush mentioned above was to avoid this. You can get them in a pencil like form for cleaning printed circuit boards.

[Richard Kirkman 1Participant @richardkirkman1]

Great insight from all. Thank you!

I’ll have a look into glass fibre brush pencils, looks like they’re only a few quid on eBay.

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