Bandsaw motor problems

[Heath Robinson]

Hi all.
Apologies for the long post, there's a fair bit to describe.
I've got a 3-wheel inca bandsaw, which is playing silly buggers.
There's been an odd and intermittent vibration and groaning of late, which I traced to a combination of a fairly stiff-to-turn bottom wheel, an old and lumpy drive belt, and secondarily these two causing the collet on the motor belt tensioner to come off, allowing the tensioner to come un-tensioned.
Now, while fixing these, I started with the tensioner and belt, and before I noticed how tight the wheel was, the motor stalled a couple of times.
The last time, it smelled a bit funny, and without the belt on would only run at half speed. Opening up the connection box, I found a wire had shorted, from the smaller capacitor (inside the connector box, maybe 2-1/2" by 1") to a motor terminal, severing the wire.
I soldered this up, put everything back together, and voilà, all runs as it should.
While lightly filing the back edge of the blade, the motor stalled or at least dropped to a slower speed and juddered for a second, before there were two loud pops, and the NVR switch clicked off.
The pops turn out to be the larger capacitor having split it's plastic case, and popped it's self.

I think I did the drive belt tensioner up a bit tight, in an effort to stop the belt bouncing at the bottom, as it seemed to bounce and rumble more than I liked. It's a cheapo one from a local car place, so I've ordered a new Gates notched belt for it.

So, anyone know how to work out what large capacitor I need? There's no markings on the popped one.
Also, anything I should check with the motor to ensure all is happy and healthy?

Thanks in advance
Jake

 

[CHJ]

Did you fix the stiff lower wheel bearing as a first action?

Sounds like most of your problems are because the motor is overloaded and is stalling causing very excessive current to flow.

If you can give the power rating of the motor then a rough guide to the capacitor range that should be suitable can be determined.

All assuming you have not damaged the insulation of the motor windings with the excessive current and heating.

 

[Heath Robinson]

Yeah, changing the bearings was first order of business. Bearingboys.com were fantastic as usual, ordered Friday afternoon, promised them by Tuesday, arrived Saturday morning!

I should have said, it's a 0.75kw single phase cap start/cap run motor.
I read one website (the only one that I've found that even hazarded a guess) that it should be in the area of 180uF. Which doesn't seem to be a size readily available in 240v flavour.

Thanks for the reply
Jake

 

[CHJ]

...I should have said, it's a 0.75kw single phase cap start/cap run motor.

Then you need a nominal 20 mF.* capacitor 400 volt working.

Working voltage of capacitors used on A/C power need to have a voltage rating at least 1.3 times the RMS power as an absolute minimum.


* Not super critical capacitors usually have a 6% or so +/- tolerance. and you would need test equipment to match it closer under load.

 

[Heath Robinson]

Stunning, cheers.
Any recommendations on where to get a decent capacitor, or is there little difference?

 

[CHJ]

Try this company http://www.ebay.co.uk/itm/20uF-Motor-Ru ... SA:GB:1123 there are several other companies similar which I have used.

Maplins are another source.

Just make sure it is Run Rated not just Start.