capacitors advice please

[josh1e]

Hi,

i've got a machine mart tiger 15/55 compressor. its 3HP & uses a 40uf capacitor to prevent a huge load being drawn as it starts up.

the compressor was used for several hours on & off for spraying & cut out. every time i try to start it, it blows the 13a fuse. (the induction motor starts to spin)

I've spoken to there tech support & he says check / replace the capacitor, the most likely fault.

the markings are partly rubbed off so i cannot see much info on the cap, can someone please let me know if they have any experience with this model / similar, also

what would happen if i replaced a cap rated @ 250v for one @ 450v?
what would happen if i replaced a cap rated @ 40uf for one @ 130uf?
what would happen if i replaced a cap rated @ 40uf 250v for one 130uf 450v?

seen quite a few on ebay but dont know if it would be safe to use a higher rated one, or what the effects / consequences would be.

thanks in advance,

 

[plugwash]

how exactly is the capacitor connected?

 

[josh1e]

it has flyleads which dissapear into the motor casing, i'll try to strip it a bit & take a look...

 

[ricicle]

I would definately not use a 250V capacitor on AC mains voltage as the peak voltage is in the region of 325V.
You need the 450V rated one, but as for is capacitance you will need to speak to the manufacturers.

 

[TicklyT]

The capacitor isn't there to reduce the surge on start-up, it's there to shift the phase of one of the motor windings compared with the mains supply.

The value of the capacitor is selected by the manufacturer to correlate with the inductance of the motor windings to generate a second phase. that's what makes the motor work.

Single phase motors are not inherently self starting. the magnetic field only oscillates, it doesn't rotate as it would with a three phase motor. They need either a capacitor fed winding or some other trickery with the magnetic flux around the stator to to create a rotating magnetic field.

A 3 HP motor will draw about 10 Amps anyway, and the starting surge will be considerably more, so I'm not surprised it gives a 13 Amp fuse a hard time!

Starting a motor is never easy on the supply. Most of the innards of a motor appear as almost a dead short across the supply at the moment they are switched on until the magnetic field develops around the stator. once the armature starts moving, it's contribution to the magnetic field limits the current further. It may only be for only for a short time, but it always happens.

Does the manufacturer approve connecting a 3 HP motor to the supply via a 13 Amp plug?

 

[josh1e]

Hi,

The original manual states to connect it to a 13a supply via a normal plug. even has a pretty wiring diagram for the colours

however an addendum to the manual which there tech guy sent me, does state that it needs wiring directly into a 30a supply & to disregard the page in the original manual about using a plug.

now, before you jump all over me for still using a plug, i bought it 2ndhand. it had a plug fitted. i had no manual until 2days ago. I had been successfully using it with a plug for 3-4 weeks. It was running fine until we did several hours of spraygun work.

I did consider connecting it to a 30a supply directly, although a quick conversation with there tech support bloke suggested that if it had been running fine on a 13a supply, then that "might" not be the problem & he suggested to have the capacitor checked / replaced if possible.

I did explain to him that i had removed the air outlet pipe from the motor to the tank, so the motor had no resistance when starting, it also ruled out any faults with the tank, pressure guages, safety devices etc.

 

[ericmark]

May be the original 13 amp fuse was not really a 13 amp fuse? I have seen people drill the ends and fit thicker cable. Not a good idea but it would confuse anyone changing fuse as they would as you have think it did work on 13 amp why not now?

 

[conny]

The cappy is probably the problem. It must be exchanged for the equivilant which is probably 380/440v 40uf, using a higher rated one will damage the motor windings.
If you are going to wire direct I suggest doing it through a 20A mcb, the starting current will be roughly 17A for a brief period till the motor gets up to speed. BTW, what speed is it? 2900? Ideally should be around this for a compressor. Alternatively find a local electric motor repairer near you in the Yellow pages and pop along to see if they will have a quick look at it for you. Should only cost a couple of quid to look and they may even do it for free.
Lets know how you get on.

 

[josh1e]

hi

thanks for the replies.

I've bought another cap (400v / 40uf) from a local rewind company, there is an earth which my previous one didn't have, but doubt that matters much. the original cap had no markings, but the new one has PAT / BREV on it.

does anyone know the correct way to connect it? I have a flylead coming out of the motor housing which gives me brown / blue AC leads with crimped spade connectors to attach to the cap.

i presume there +/- like normal, just can't seem to find PAt / BREV on google...

thanks

 

[ricicle]

It should be a non-polarised capacitor for use on AC so which way round you get the wires will not matter.

 

[josh1e]

yaaay, it lives !!!

new cap (400v 40uf) & it roared back into life !!!

I think the motor maybe running slightly faster, but that might be bad memory... or just a really old foobar cap that was dying slowly...

I've also removed the motor from the tank, fully cleaned the outside casing, heatsinks, drained & replaced the oil, etc etc etc. so hopefully it will keep running properly a while longer.

i'll keep an eye on it while i use it over the next few days. so far really chuffed. the cap was £10 so can't complain. now back to spraying the house ......

oh, it's still on a standard 13a fused plug - woohoo, hopefully i wont have to run a new circuit just for it (fingers crossed)

thanks for everyone's help

 

[conny]

Whenever a single phase motor goes wrong always suspect the cappy first. Glad we could help you.

 

[ban-all-sheds]

The cappy is probably the problem. It must be exchanged for the equivilant which is probably 380/440v 40uf, using a higher rated one will damage the motor windings.

It's the 40µF which is important - any voltage higher then the original will do just fine.

Nothing to do with this topic, but for the benefit of future searchers who might stumble here when looking for information on capacitors, this site is excellent:

http://my.execpc.com/~endlr/

 

[conny]

Hi BAS, when I mentioned a higher rated cappy I was referring to the 40uf not the voltage. Sorry, slightly misleading there.

 

[josh1e]

Hi guys,

thanks for the link to the site about caps. will make interesting reading, I don't know much about them (yet) but am happy i've got mine sorted

My compressor was a 3HP induction motor, The earth which the replacement cap had (original one didnt) i connected to the side of the motor casing where the ac supply earth is connected to. (the earth is from the mains on/off switch & combined pressure safety switch).

I also tested continuity from the motor casing & earth bond point to the plug & at various points through to check i put it all back together properly.

hope this helps someone else too, sounds like a common problem

thanks again