Brushes in an universal motor: opinion on their condition - Page 1
Forum section: electronics / repair

author: golden_labels
author_rank: Super Contributor
timestamp: May 21, 2024, 11:24:30 pm
content: I’m a newbie regarding electric motors. Worn out brushes I’ve seen previously were usually much shorter.(1)Would you consider these to be still fine?These come from an universal motor from a cheap paper shredder. Fixing it is likely not financially reasonable, but I won’t mind learning something.Also: yes, they were mounted perpendicular to their original profile.(1)I’ve just realized I forgot to take photos showing the brushes’ size. They are 4×5 mm, currently 10 mm in length. Certainly still pushing well against the commutator.

author: johansen
author_rank: Super Contributor
timestamp: May 22, 2024, 01:23:11 am
content: Whats the problem?

author: Jeff eelcr
author_rank: Frequent Contributor
timestamp: May 22, 2024, 01:41:22 am
content: Most pictures look strangely worn, 1 looks normal.Jeff

author: perieanuo
author_rank: Frequent Contributor
timestamp: May 22, 2024, 07:24:25 am
content: change them, pressure and shape taking rotor shape is the key

author: wraper
author_rank: Unknown
timestamp: May 22, 2024, 07:34:17 am
content: Looks weird, as if brushes were misaligned with commutator and one side has no wear at all, the same as 2/3 of commutator. Does the brush size match the slot size properly? And are the brush slots located correctly above the commutator?

author: golden_labels
author_rank: Super Contributor
timestamp: May 22, 2024, 08:19:48 am
content: Quote from: johansen on May 22, 2024, 01:23:11 amThe distal issue is overheating.Brushes are something I just noticed while cleaning the entire drive assembly. I don’t have any strong motivation to replace them: the entire shredder costed $20 when new. But I don’t mind learning something about evaluating the state of the brushes. All I have seen in my life lost most of their length, had obvious cracks etc. These here seem just weird.Quote from: Jeff eelcr on May 22, 2024, 01:41:22 amThe collage is a close-up, each photo about 5–6 mm in width. Perhaps the magnification make it look unfamiliar. The first picture is a general view of brushes and the commutator.Quote from: wraper on May 22, 2024, 07:34:17 amThe brushes look as if they were originally profiled to work with a completely different configuration. The wedged surface seems to be cut like that. It is mostly not touched. The commutator touches the brushes along the wider axis, at the tip. You can see a dip in that direction on the first, bigger photo.I see nothing wrong with slots.

author: coromonadalix
author_rank: Super Contributor
timestamp: May 22, 2024, 02:38:54 pm
content: or the rotor is not centred on the brush surface ??anyway its not normal, to have wear in non uniform fashion ?

author: wraper
author_rank: Unknown
timestamp: May 22, 2024, 07:55:33 pm
content: QuoteThen it's weird but brushes are barely worn. With more wear there will be full commutator coverage. IMHO leave it as is.

author: BrokenYugo
author_rank: Super Contributor
timestamp: May 22, 2024, 07:59:46 pm
content: Small cheap paper shredders are just prone to overheating in my experience, and the page ratings must be assuming something like newspaper, run it at half capacity max. If it takes at least a few batches of a few sheets in a row to trip the thermal overload and doesn't let the smoke out it's probably working correctly.

author: golden_labels
author_rank: Super Contributor
timestamp: May 22, 2024, 08:18:35 pm
content: Thank you all for confirming, that it’s not only my inexperience and there is something weird with those brushes.The wedged side of a brush is aligned with rotor’s axis:As for overheating (the original issue) of cheap shredders: it wasn’t doing this before. Now it’s failing to shred 2 sheets, inserted separately. Mother asked me to check it. It has some years, grease is everywhere but on the gears, a few pieces of paper mixed with grease stuck in gears and inside the motor. Before throwing it away I decided to clean and regrease the drive and blades. I found those weird brushes inside and considered that an opportunity to educate myself more.For the record: Tracer TRANIS40566.

author: wraper
author_rank: Unknown
timestamp: May 23, 2024, 03:25:06 am
content: I guess it's some oddball version of this that allows brush wearing the shape of commutator faster. Nice drawings BTW.