Starter capacitor rating for garden shredder

[Mike Clarke]

The starter capacitor for my Alko shredder has released its smoke and lava.

It's marked as 25UF, 450V and the closest that CPC have are 20UF and

30UF. Would a 5UF difference be OK and if so is it best to go for higher or lower capacity?

[Bob Minchin]

Ebay

[Bob Minchin]

I think it will be the start and run capacitor in fact

[John Rumm]

With that rating it sounds more like its a run cap rather than a start cap. (do you here a centrifugal switch kick in shortly after startup?)

If its a run cap, then its best to replace with one of equal capacitance.

Ebay is usually a good source. Tis where I got one last year when my bandsaw run cap decided to let out the magic goo:

[Brian Gaff]

Personally, I've not seen much difference over a very wide range of values on motor capacitors, though maybe its more efficient at some values or its the power factor or something, but functionally, I suspect that you would see no real difference. Brian

[Brian Gaff]

I'd also go for a higher working voltage than the original, so nasty mains spikes don't upset it as well. Brian

[Mike Clarke]

I wasn't aware of the difference but I suppose it is a run cap. It certainly has something that kicks in and out on starting and stopping.

Oddly it did, sort of, run with the failed capacitor except that it frequently blew the 13A fuse on starting and sometimes ran in reverse,

Looks like exactly the same failure mode as mine.

[The Natural Philosopher]

Then its a starter cap.

That's because it is a starter cap not a run cap.

Its job is to make sure the motor starts the right way round.

[John Rumm]

The device ratings sound more typical for a run cap, however if you hear a click just before the motor reaches full speed, and a similar one as the motor spins down, then its more likely being used as a start cap even if the device itself is not a typical start capacitor.

(you can use a polypropylene run cap for starting if it has the right capacitance, but a bi-polor electrolytic as often used for start caps, would probably not last long in continuous operation as a run cap)

A start cap is really only switched in to create additional phase leading the main supply - it creates the torque necessary to get it spinning, and also sets the direction. Once the motor is running, there is enough slip to keep it spinning on the single phase. #

Without it, the motor may just sit there humming, unable to get turning. It will also likely be drawing high inrush while doing it, which can be significantly higher than the normal run current.

(normally with a failed start cap, if you spin the motor as you apply power, it may start ok)

Should add to that, for a start cap; equal or higher capacitance is ok so long as the voltage rating is high enough. (if yours is using a run cap as a start cap, then the voltage of the cap you have is probably higher than actually required)

[John Rumm]

It sounds a bit like the OEM already did that... you would not normally need a 450V device for a single phase motor start cap.

[John Rumm]

For a start cap, this is true.

For a run cap, the cap is providing an additional phase supply to a winding - so if significantly different from the original, you may find you get a more "lumpy" supply of power with more motor noise and vibration.