Bench grinder just humming

[Gragson]

Recently got a wet/dry bench grinder 2nd hand in for sale section, which worked fine.
Switched on today and just hums, no mechanical jams on the wheels, turn fine by hand.

Got the start cap out and has a little 5mm case crack but does not looked swelled or leaking/poor. Although didn't get any spark when took out and arched to discharge.

Tested it and the uf looks pretty much spot on.
Any ideas?

 

[brightspark]

for a couple of quid just try a new one

 

[Gragson]

for a couple of quid just try a new one

And that was going to be the next Q

Where's the best place to search for size and spec to fit - appreciate any links. Cheers

 

[brightspark]

And that was going to be the next Q

Where's the best place to search for size and spec to fit - appreciate any links. Cheers

just type in ebay stacks of sellers available

 

[DAPPH]

Whilst your waiting for the new one to arrive virtually any domestic appliance 25 micro farad motor capacitor should be Ok to do a test run
When ordering the new capacitor ensure it is the same dimensions or very close and the same capacitance number in micro farads .

Ensure it is the same sort of casing as some have 8mm screw spigot on the end for mounting it to a bulkhead /chasiss


This eBay one has the spigot on the end eBay number 291813939276

 

[waddycall]

Can you turn it on and spin it by hand to get it going (very carefully etc as a test)

 

[RichardM]

If you have a flourescent tube fitting the cap in that will probably do.

 

[Gragson]

Can you turn it on and spin it by hand to get it going (very carefully etc as a test)

I know what you mean, first thing I tried, it doesn't pick up, just stays humming.
I will order a new cap, all the ones I see are 4 spade, I only want 2 spade connector on the cap

 

[waddycall]

I know what you mean, first thing I tried, it doesn't pick up, just stays humming.
I will order a new cap, all the ones I see are 4 spade, I only want 2 spade connector on the cap

Is there a wheel/wheels on it? If so it might be worth giving that go without them on

 

[brightspark]

I know what you mean, first thing I tried, it doesn't pick up, just stays humming.
I will order a new cap, all the ones I see are 4 spade, I only want 2 spade connector on the cap

theres no centrifugal switch in them so the starting wingdings through the cap is connected all the time to alter the phase angle top make it run .spinning them off is ok on a centrifugal switch motor even go backwards

 

[Agroshield]

...it doesn't pick up, just stays humming.

Has it just forgotten the words?

Cut off the excess spades with a pair of side cutters.

 

[Gragson]

Got a new capacitor, tested it, just slightly higher uF than the old one, but well within spec and % range.
Motor still just hummed. Removed motor, uTubed some motor tests, windings are open circuit can't see any obvious damage but for a 2nd hand £40 grinder not worth going further. I will strip this one for parts / wheels etc and get another.

Not all bad as learned how to test a cap and single phase motor windings also making sure no continuity from motor to housing

 

[hermetic]

There are centrifugal switches in some bench grinders, I have two kamasa grinders with them, and a makita without, strip it down and have a look, very unusual foir a winding to go open circuit without magic smoke appearing!
Phil

 

[Gragson]

Definitely no centrifugal switch, it's fully stripped.
Close look at the windings and I can see a breaks in the wires at one point, looks like physical damage not electrical breakdown, no idea how that happened.

 

[hermetic]

Buggr! If there are only a couple, can you solder and sleeve them?, broken cooling fan blade loose inside the motor? Pics?
Phil

 

[Gragson]

Buggr! If there are only a couple, can you solder and sleeve them?, broken cooling fan blade loose inside the motor? Pics?
Phil

I've stripped it and removed parts to keep, was about to scrap the rest, now you have me thinking micro surgery! Double bugger!
I will post some pics, something has hit the windings for sure. Here we go...shot to nowt tho

 

[Gragson]

Aerosol straw for scale.
The dink is pretty clear, sorry the pics are not.
One strand I've carefully picked out - severed, its matching strand I have not yet found.
One at least broken and possibly another, but need to get it under high light and magnified.



I'm not sure of method to fix that, damn fiddly is not a strong point but I will shelve it until I get the courage to have a go.
Advice welcome - or just bin it?

 

[Other Daniel]

Yup... Bin it

 

[Seadog]

Go for it. You've nothing to lose.

 

[DAPPH]

Gently ease the damage wires up with a big safety pin ,carefully scrape the varnish off damaged wire ends with a scalpel , touch a blob of electronics solder on the break then re varnish a couple of times with a small pot of high temp varnish ( £ 7 ish on eBay ).
One of the x 5 , 100mm dia magnifying frames by Blackhawk or a decent web cam should allow you to see the break better .

Come to that use a photo app on your phone and get a 20 to 100 sized magnification ( Thank Fizzy for that idea ..... he's always doing it )