7 replies to this topic

[Phil-R]

Hi all, I've picked up a compressor off eBay, and amongst other things, it's missing the motor capacitor ( or maybe 2 capacitors?) I've contacted draper and searched all over the internet, but I can't seem to find out what value capacitor I need??? Help!!! 

 

 

If anyone on here has a 3HP / 2.2KW belt drive compressor that has a motor that looks like the one above, could you help me out and open the case up? I need to know...

 

1) How man capacitors

2) Their values (uF)

3)What values the resettable thermal fuse is (I'm guessing 15A)

 

Looking around the internet, there seem to be several brands that use identical 3HP motors on theirs, so doesn't have to be a draper, but has to be same power and similar speed (1200 - 1500 RPM)

 

 

[Shifty]

I have exactly the same one but with a blown compressor. £75 and you can have it.

[Phil-R]

Interesting. I'm hoping this one just needs a £10 capacitor to get it going, so I need to try that first. But maybe interested in yours after I've fixed this one (or if it turns out I need a new motor). Would you be able to take the cover off and let me know what cap I need? Pretty please :)

[Shifty]

Will do, I'll have to dig it out though

[DomCr250]

It's a single capacitor on that motor - I think it's the same motor as I purchased about 10 years ago to run a three phase compressor I converted.

I'll try and have a look at it tomorrow.

You can google induction motor start capacitor size chart (!) which may help!

[tiger99]

This is presumably the RUN capacitor, not the START capacitor, unless there is switching to remove it from circuit once the motor is running. The values would be very different.

I have just done some googling. Beware of the majority of well-intentioned web sites, which assume, without saying so, that they are for 117V 60 Hz. That makes for very different values!

I don't have a compressor right now but I do have a huge radial arm saw of very similar motor power. It's capacitor is 35uF 450V, and yours should be somewhere in that region. I am just giving you that info as a second check that the value obtained from a similar compressor is plausible. The exact value is quite important, and nay be a bit different.

Please ensure that a bleed resistor of 220kohms 1/2 watt is connected across the capacitor as otherwise you may find the hard way that in certain circumstances it can contain a lethal charge for a very long time after the motor is switched off. It is customary for safety reasons to put the resistor directly across the capacitor terminals.

The only other thing to watch out for is that the capacitor is rated for motor run applications. It may have been an oil - filled paper type but nowadays will be polyester. These are quite reliable and tend not to make a big bang if they do fail.

[Phil-R]

I had seen a few capacitor size charts and formulas, but the more I looked, the more conflicting the results became!! 

 

I've measured the resistances between all the wires going into the motor and sketched out a circuit with a single (run) capacitor that seems to make sense. After removing the V-belt and spinning it by hand, I can't hear a centrifugal switch, so I'm pretty sure it's just a single run cap now.

 

 

... I do have a huge radial arm saw of very similar motor power. It's capacitor is 35uF 450V, and yours should be somewhere in that region. I am just giving you that info as a second check...

 

Good idea. I've just looked at my pressure washer which is states 2.7KW ( 3.6HP!!) so a little more power and uses a single 40uF cap, which concurs with your findings. I also found an invoice for 30uF that I bought for a 2HP motor a few months back. It will be interesting to see what figures you guys come back with.

[Phil-R]

I still need to know the capacitor size and also the resettable fuse rating. Did anyone get a chance to take a look?