For the electronics boys,

[Gibbo2286]

A what's this then
I've got this little garden tiller that won't run, checked all the wiring and all's fine down to the motor and the motor looks all good but switching it on just makes an attempt to start then nothing. the grey item is warm to touch, I'm assuming it's a kick start capacitor for the motor but there are no markings on it so just guesswork for me,
Anyone help I've got a box full of capacitors but nothing like that.

[Gibbo2286]

Photo in macro and it does in fact have numbers. If you can read them.

[thorter]

I would hazard a guess that it is a thermal trip, possibly 105C cut out temp. It would likely be a bimetal switch similar to a thermostat, though thermal fuses are also used. Measure with an ohm meter, and report. If sure it is a cutout, try shorting out and see if motor runs. If not confident, connect a filament lamp across, say halogen 250W or more if it is a mains motor.

Protection is a good idea on something that can easily be overloaded.

[Gibbo2286]

Thanks for the input, checked it out with a meter and it's 240v on both sides, shorting it out smokes the motor, I guess it's for the skip as it's not worth the price of fixing.

[thorter]

A pity, motor looks OK in photo, but may well have shorted turns, running slow and taking excessive current. Disconnected from mains, turn armature by hand and look for a burnt section. If so, it is almost certainly scrap.

[Gibbo2286]

No sign of damage to the armature or comm but possibly a field coil short, might have another look at it today.

[MattBLancs]

I hate the "throw away society" type implications, but look at the price of secondhand ones and likely best save your time. I got a second hand electric rotovater for £20, hardly used.

Could do resistance check on the brushes and turn the commutator slowly by hand, would find your shorted winding. Not a great deal you can do knowing the answer though.

(I can't help but take everything to bits see what is wrong either! )

Matt

[Gibbo2286]

I never throw anything useful away if it can be fixed but sometimes it's economical sense to do so.

[thorter]

Seems most likely it is shorted turns somewhere, and the excessive current has fried the cut-out. The other common failure of universal motors is a bad wire connection to the commutator , but that usually merely makes it run badly, without causing excessive current draw.

For shorted turns in the armature, the back motor generation causes a very large circulating current and corresponding load on the rest of things that are still working properly. Shorted field turns cause an unbalance in the magnetic field between the two sides, and so the back motor voltages are different between the two sides of the armature, and again a large spurious current flows round the armature coils.

As long as the armature is able to rotate freely, and if you have a variac or a transformer giving a low AC voltage (perhaps 20 to 40 volts) at an amp or two, you can check for armature shorted turns. Remove the brushes, and connect the brush holders together (but not to the armature!), and apply the low AC voltage via the mains lead, thus energising the field. If the armature jumps round to a fixed position and is difficult to move from there, then you have shorted turns in it. If it can be set in any position without resistance, suspect the field coils.

[Gibbo2286]

Thanks for the posts chaps, it's now in the bin.

[Paul-R]

The WEEE recycling bin I hope.

[Gibbo2286]

The WEEE recycling bin I hope.

Stripped down to its bits, plastic in the plastic box and metal in the scrapyard pile, screws in my 'glory box' (Might be useful one day)

[myglaren]

Should have got Linguoer in, she would have fettled it.

[Paul-R]

Flip, that's impressive.

I did worry when she started digging in to the windings and cutting them out though. I'll have to assume that they'd been tested and found to be no good as I would've tried to reuse them myself. I also like to hold solder further away from the soldering iron!