Humming Bandsaw

[John Rumm]

I was cutting out the teeth on a wooden cog the other day, and hence had the bandsaw running for 20 mins or so. All went ok. Came back to use it half an hour later, and it just hummed at me when I turned it on... So I checked that it was free to move, and tried again. Still no joy. Then I though, its almost like the motor cap has failed. So tried manually spinning the bottom wheel before applying power, and sure enough it span up. Then I had a look round the back:

I thought I don't recall dropping something grey and gooey on that! It was set solid and well stuck, so I had to break it off with pliers.

A closer look at the cap showed an interesting failure mode:

Turned out on closer inspection to be a run cap rather than a starting cap. eBay had something suitable, so I wired that in today and we are back up and running.

[Andrew Gabriel]

I had that with a lawn mower. It died very slowly - initially it would only start if you lifted it clear of the grass, and eventually it would only start if you gave the blade a spin by hand. A replacement cap from CPC fixed it. Same gunk was oozing out.

Now the bearing has gone. It works if I turn it upside down and squirt oil towards the bearing (which can't be seen because it's behind a fan blade). Judging from the noise it makes, I rather suspect the whole bearing is spinning around in the housing, but at 20+ years old, it doesn't owe me anything.

[Adrian]

What could _possibly_ go wrong with that...?

[Andrew Gabriel]

Well, quite.

Most electric mowers have universal motors, and that would take your fingers off if you tried this.

This mower has an induction motor with (at the time) a dead run capacitor, so it had no starting torque. Still far from "safe".

[Brian Gaff]

Presumably it is to apply enough phase shift to get it to go in the right direction. I had a very nasty smell in my room the other day. It was just as if a large resistor had been burning up. could not trace it and everything was still working. Last week I needed to pull out some of the hi fi and stuff to get at a connection, and the smell was re invigerated. After some nose use it was found to becoming out of a filtered mains distribution socket cluster. Removing the top showed that the filter, a three pronged can with some gunge stuffed up it was blackened and a hole existed in the gunge. It seems a capacitor inside had met its maker with a slow leak and then gone open circuit. It was apparently rated at 1000 volts according to a friend but of course they never say for how long, do they! Brian

[Brian Gaff]

You could lose ahand? Brian

[Brian Gaff]

Indeed an item on a tv program of bizarre injuries was a person who tried to help a powertool to start and it took several fingers off.

One needs to be respectful of this stuff. Anyoen remember the old electric clocks that suddenly decided to run backwards? I had one and after some head scratching I found out that the only reason it ran only one way was a little ratchet and spring that pushed it the correct way if it tried to go the other way. How crude is that? The sping and plastic arm were nowhere to be found of course, so I just manually started it the right way if it ever had to be unplugged or there was a power cut. Brian

[John Rumm]

Yup it provides and additional "phase" if you like, to create an addition rotating field in the motor stator.

Some induction motors just have a starting cap to provide additional torque and get them spinning in the right direction, that is switched out of circuit by a centrifugal switch once the motor gets up to speed. Some have a run cap that is always in circuit (and some have both).

Indeed, and with filters, quite often the damage to them caused by transients is cumulative...

[meow2222]

Last time I looked, mfrs generally do include MTTF info with caps. Whether that figure applies to use with frequent transients is another matter, but at least one can compare them to a fair degree.

NT