Capacitor replacement advice

Question

Hi, 
 I am trying to source a replacement capacitor but am a little unsure what will be correct as a replacement. Usually with capacitors I would always swap like for like to remove any possible issues. Unfortunately I can't seem to source the faulty one which is a facon 21uf 330v -0%+20%. I've tried several suppliers but seems they don't want to say what would work as a replacement, you'd think if they're supplying them they'd be able to advise the best option when swapping. 
 Ive read online that the voltage is not an issue so long as you go higher than the one you are swapping i.e 450v instead of 330v. Obviously the uf rating has to be the same, or as close being slightly over I believe? 
 My main issue is with regards the tolerances and whether they need to be withing a certain parameter when swapping. I've seen one from a supplier which is 21-25uf 330v but doesn't say what tolerance is. Would this be a good swap or would I be better going for a universal one 22uf 450v +-5%? 
 Size is not an issue as it is from a refrigeration unit so plenty of space to fit it in and it is running the compressor along with another 25uf capacitor. 
 I've attached an image of the one I'm looking to change which is faulty. 
 Ay advice would be greatly appreciated and also if anyone knows anywhere that has some good information on this kind of thing for future, that would also be great. 
 Thanks.

mapj1 · Accepted Answer

OK, the Facom 04P series are AC electrolytic devices - that is to say rather like two electrolytic caps back to back. As such they are not rated for continuous duty, but they are very small compared to a conventional (polyester or polypropylene ) of the same capacitance. They have a loss factor of nearly 10% so can get very hot if mains is applied for too long, and are intended for starter circuuits with a pull in time lasting a few seconds, with plenty of cool down in between. ( a Facom data sheet that does not mention any of the nos on that model I presume they have withdrawn it.) 
 So generally, a higher voltage rated cap will last longer, and be bigger, but if you can afford the space go for it. 
 Start caps like this must be used in a circuit where they are de-energised after a few seconds. Run caps can take continuous duty, so if it fits, you can fit a run cap in place of a start but not vice versa. 
 A run capacitor has a more generous rating and size and price to match 
 exampl e (cornell dubilier are first tier makers so their data is good. so is the cost.) Note how life depends on temperature and voltage - not all makers are this honest, but the physics is similar. 
 
 A start cap is smaller, lower temp rated and shorter lived ( example ) note the dreadful power factor and the temperature rating compared to the above. But they are smaller and usually cheaper. 
 Values of caps for starter uses are usually OK erring up a bit, but not so much down, so for you not less than 21uF, but OK for 22-27. But in practice, most real motors will take quite a bit more, so a nominal 27-30 probably also OK. 
 Get a 25uf 20% or finer tolerance and you will be fine. 
 (a bit more cap = more starter torque is normally OK, too little and it may sit there and buzz unhappily without rotating.) 
 If you get stuck mouser have stock of most values from the common makers and can get it to you quite fast. Beware of their delivery charges though. 
 Mike.

Andy Millar · Answer

Craig Wheater said: I've tried several suppliers but seems they don't want to say what would work as a replacement, you'd think if they're supplying them they'd be able to advise the best option when swapping. 
 No I wouldn't think that at all - they're experts in stock control and pricing, they're not engineers. The capacitor manufacturer themselves may have application experts - or they may well design them to specs passed to them from equipment suppliers. Even if they do have such staff in house that's going to be several layers of people away from the sales end, and they're unlikely to see it as worth passing a request for one capacitor through the chain - the time spent passing the message on alone would quickly wipe out the profit from the sale. 
 I'm very wary of taking advice from a sales / distribution company on alternative replacements, they may know what they are talking about, or they may be just trying to make a sale (to be fair, most will be honest and say that they can't guarantee that replacements will work). Of course if it's a company that is also experienced in carrying out repair work, and guarantees its own repairs, that's different, they might well know what works as an alternative. 
 Fortunately we have Mike here with a fantastic answer!!!!

Capacitor replacement advice

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