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Old 29th Mar 2022, 5:02 pm   #21
Lucien Nunes
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Default Re: Cleaning motor commutators

Quote:
The failure was quite spectacular with lots of smelly smoke, and eventually the plug-in RCD tripped
Are you sure the smoke did not come from a suppression delta cap hidden in one of the end bells? To trip an RCD there must otherwise have been some kind of flashover perhaps starting with streamers around the comm, or leakage via a very heavy carbon buildup on the brush holder insulators. Worth poking it with a Megger at 250V to eliminate.

As per Joebog not all small moulded comms are intended to be undercut. If it is, clean out any carbon but only cut into the Micanite if the comm is heavily worn and the grooves are very shallow. After cleaning them, run some very fine glass paper over the comm one last time to remove any fine burrs that the process might have thrown up.

Small universal motors do not always achieve sparkless commutation. If fine sparking is confined to the brush trailing edges and uniform, it's normal. Problems like heavy, hot sparking, streamers or sparking in time with rotation can be indicative of shorted turns in the winding, which often first make themselves known by rapid brush wear and comm damage. They can also be caused by intermittent loss of contact with the brush at high speed if the comm is badly eccentric or damaged, in which case there is usually some localised pitting or burning instead of all over.

It is worth examining a commutator carefully before attempting to clean it, as the cleaning erases possibly useful evidence of any other problems within a motor. Its appearance is a versatile diagnostic tool. Good commutation should leave an even chocolate-brown film of oxide on the surface which is much more resistant to wear than bare copper. I have seen people with vintage generators etc obsessively cleaning the commutator in the belief that it should be bright metal, whereas doing so destroys its natural ability to self-protect.
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Old 29th Mar 2022, 5:21 pm   #22
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Default Re: Cleaning motor commutators

I was also a bit puzzled by the tripping RCD, as I couldn't see where the necessary current was escaping to. The machine isn't earthed, as was standard with prewar electric sewing machines, so any leaking current would have gone through me, and I didn't feel any sort of belt or even a tingle. All I can think of is that the mess on the commutator caused so much noise on the mains that the RCD tripped for that reason.

It hasn't tripped since.

The suppression caps are in the rheostat pedal rather than the motor. I actually replaced the original 1934 caps with modern X2s about 5 years ago.
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Old 29th Mar 2022, 5:44 pm   #23
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Default Re: Cleaning motor commutators

I have a VAX vacuum that's used for everything that the 'best' isn't and would occasionally cause the RCD to trip. Upon dismantling all I could find was a minor build up of carbon dust, cleaned everything up and it's been fine so far

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Old 29th Mar 2022, 7:23 pm   #24
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Default Re: Cleaning motor commutators

Nice machine and yes several type bearings were used, the clue is if the lubrication hole has a cotton wad in it. Cotton wad means grease.
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Old 27th Apr 2022, 12:33 am   #25
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Default Re: Cleaning motor commutators

As a teenager I was into competitve RC models. The accepted way to clean a commutator was a fibreglass pen, as Paul suggests. If it had become pitted, then a skim on a lathe (but obviously, this could only be performed a finite amount of times). WD40 was absolutely the worst substance that could go anywhere near brushes / comm. I don't remember anyone using lubricant of any kind - the softness of the brushes was self-lubricating.
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Old 28th Apr 2022, 5:05 pm   #26
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Default Re: Cleaning motor commutators

The thought of using WD40 on shellac is a bad idea, it could start dissolving it.
As for the RCD tripping on something without an earth, the only thing to cause that would be either a direct mains short or the live supply going somewhere without returning through the neutral.
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Old 28th Apr 2022, 5:11 pm   #27
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Default Re: Cleaning motor commutators

WD40 is only used to dampen a rag to clean off dust, but I take your point.

I have no idea why the RCD tripped. It seems completely counterintuitive.

The motor is now working normally without mystery trips.
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Old 28th Apr 2022, 5:17 pm   #28
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Default Re: Cleaning motor commutators

Paul, a damp cloth should be fine, saturation of the shellac is bad news.
It sounds like you had a carbon build-up between the segments, that would be enough to trip the RCD.
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Old 28th Apr 2022, 5:39 pm   #29
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Default Re: Cleaning motor commutators

That would just pass current between L and N though. There's no provision for an earth connection with these old machines, everything is just floating. That's why I always use it with an RCD - I don't want the earth connection to be my body if there's a fault.
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Old 7th Jan 2023, 4:23 pm   #30
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Default Re: Cleaning motor commutators

A quick followup to this old thread.

I've just tried to use the machine again after a few months and the RCD tripped immediately I touched the foot pedal. Another RCD was fine so I guess the RCD was just knackered, which does make sense. Into the WEEE it goes.
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