Bench grinder wont start, just hums

Calahonda52 · 19-01-2026 04:59PM #1

This is whats showing at the base. I bought it in 2009 and it worked once then, took it out today and it just hums..

Took off one of the discs and got it up to speed with a cordless drill but no joy.

I suspect its the " black" thing, any thoughts on:

1: how I can test it

2: get a replacement.

Thanks as always

10-10-20 · 19-01-2026 05:06PM

Hi ya,

Black thing is just a noise snubbing capacitor (class C). It's highly unlikely to be the issue. You could try without it (pull one leg off using the spade connector), but it should remain on the circuit to catch most of the EMF emitted by the motor. I can see the motor coil in the background it it looks 'unburnt', so should be good.

Try rotate the grinder in case the commutator is stuck, corroded or dirty.

cml387 · 19-01-2026 05:26PM

If you spin it while it hums will it start rotating?

Calahonda52 · 20-01-2026 09:00AM

Thanks guys

Took off one of the grinding wheels and got it up to speed with a cordless drill but no joy.

I remember my father had a big circular saw which had 3 belts from motor to blade spindle with the same issue so he took off one belt and would wind a bit of rope around the pulley and get it going and then turn on the power.

same idea as this, which also had: Seagull outboard

IIRC it was a capacitor issue

Escapees · 20-01-2026 10:42AM

https://www.boards.ie/discussion/comment/124162366#Comment_124162366

Is it not a starting capacitor, which usually is the issue when ac motors can't start?

blackbox · 20-01-2026 11:09AM

When unplugged it should rotate perfectly freely when spun by hand and continue rotating for a while.

If not, the grease has probably dried up in the bearings.

Escapees · 20-01-2026 11:23AM

https://www.boards.ie/discussion/comment/124163878#Comment_124163878

Just to confirm, the capacitor is definitely not a snubbing cap as it's wired in series. The datasheet indicates it's typically used as a start or run cap for motors. If there's no other cap, I'd assume it's a start cap (even if it does seem on the small side) and therefore the source of the issue.

Screenshot_2026-01-20-10-08-16-366_com.google.android.apps.docs~2.jpg

10-10-20 · 20-01-2026 11:55AM

https://www.boards.ie/discussion/comment/124163878#Comment_124163878

Oh crud, I missed the black wire from the motor and thought it was directly on the L/N mains in. 😐️

That's what you get when you hire randomers off the 'net. 😙

I have a start cap here on my desk, except it's 10uF, I'd send it but it's way too big.

Escapees · 20-01-2026 12:44PM

https://www.boards.ie/discussion/comment/124164115#Comment_124164115

Ah I see now what you saw! You had me second-guessing myself big time!! :)

Calahonda52 · 21-01-2026 09:11AM

https://www.boards.ie/discussion/comment/124164004#Comment_124164004

Much obliged, whats the link for that stuff pls: Am I looking in China?

Escapees · 21-01-2026 10:54AM

It looks like China alright, as tried the likes of Farnell and RS out of curiousity but no luck. It's available for around a fiver to a tenner, but the killer is the delivery time of a month. You could just swap it with a different form (CBB60 instead of CBB61) but I'd recommend not fecking about and just replace like for like so that there's no gotchas. So go for CBB61 SH 2.5uF (>=400V) with spade terminals. E.g.

https://www.ebay.com/itm/195713755582

Escapees · 21-01-2026 11:19AM

https://www.boards.ie/discussion/comment/124166733#Comment_124166733

Era, life's too short... If you're comfortable with stripping wires and using screw terminal connectors, then you can go for this option with flying leads and have them in a week:

https://www.amazon.ie/DollaTek-CBB61-Conditioner-Running-Capacitor/dp/B07HBKVXLP/

Calahonda52 · 21-01-2026 11:24AM

Many thanks,

ordered

Escapees · 21-01-2026 11:49AM

Bit late but...

https://www.amazon.ie/PATIKIL-Ceiling-Capacitor-Conditioner-Capacitors/dp/B0C9T9C7G2

Amazon is a pain for searching!!

Calahonda52 · 21-01-2026 11:55AM

No worries, many thanks, given that I didn't use it for 15 years, as Johnny Logan might say, whats another 2 months

Escapees · 21-01-2026 12:15PM

Well that confirms that it's almost definitely the capacitor so, as they dry out and fail with time...

Calahonda52 · 21-01-2026 12:24PM

https://www.boards.ie/discussion/comment/124166943#Comment_124166943

Thanks, a bit like ourselves then !!

Over and out and many thanks for your insights

Calahonda52 · 25-02-2026 08:49AM

it arrived this morning, spades different size and fixing screw on different side but should be doable

Escapees · 25-02-2026 10:17AM

https://www.boards.ie/discussion/comment/124263580#Comment_124263580

Ah, that's annoying. If the spades are too wide, you could probably snip an edge. Regardless, try to roughly test the cap out before worrying too much about the mods in case the cap was never the issue all along!