Compressor Capacitor Help

[dannyp]

I ran the generator on a low rpm and the voltage was below 200 volts. If I let the air out quite a bit then the compressor wouls start fine on 200 volts.

just a heads up here but under revving a genorator is one of the best ways to kill them.

lower voltage but lower frequency is the key as your dealing with induction motors they are funny beasts. off the top of my head if you half the voltage and half the frequency the motor will still make the same torque ( in theory ) what really messes it up is a large proportion of genorators are the brush type with AVR, that dose everything it can to maintain output voltage ( not much it can do about frequency ) buy putting more power through the rotor. all sorts of funny stuff can start happing but the rotor usally ends up getting over loaded by the avr and at lower rpm the cooling fan isn't so effective thankfully most of the time its the avr that goes pop first and thats the cheepest bit ( usally )

 

[normspanners]

I couldn't find the right pastile size for my valve but I'll always make something instead of buying it anyway. Right, so when it was left overnight, it struggled. You released some air and it worked. Doesn't that point to air leaking back and pressurising the cylinder? I used to run a compressor on a generator and the compressor didn't like starting when it was fairly full because I ran the generator on a low rpm and the voltage was below 200 volts. If I let the air out quite a bit then the compressor wouls start fine on 200 volts. I can't see it being electrical or has my senility completely kicked in?

the off loader leaks to free air ie atmosphere that is the hiss you hear when it stops so its not likely to be holding air in the pipe but the head pipe can fill quicker if the pastile is leaking back but you would notice an extended hissssss usually.
running a compressor on 200 V and 45ish htz why ??? do you like killing them ? you must do, no-one with any mechanical sympathy would contemplate doing that !
I am suprised it started at all and its a wonder you didnt kill the generator also.
As a 3hp compressor is one of the hardest things to start that you will come across at home---- cold or with any pressure against it, Now put that on an extension lead or dodgy wiring on up to single 2.5 mm, I a can quite easily see it being an electrical problem as a 3HP motor is like a big void you are trying to fill with power /current inrush without it having to do any work !!! now put cold or thick oil into the equation and you have issues, but if you dont like the advise----- crack on.

edit just read dannys post and he is spot on with that .

 

[Alan Reynolds]

just a heads up here but under revving a genorator is one of the best ways to kill them.

lower voltage but lower frequency is the key as your dealing with induction motors they are funny beasts. off the top of my head if you half the voltage and half the frequency the motor will still make the same torque ( in theory ) what really messes it up is a large proportion of genorators are the brush type with AVR, that dose everything it can to maintain output voltage ( not much it can do about frequency ) buy putting more power through the rotor. all sorts of funny stuff can start happing but the rotor usally ends up getting over loaded by the avr and at lower rpm the cooling fan isn't so effective thankfully most of the time its the avr that goes pop first and thats the cheepest bit ( usally )

Been running like that for over 12 years now as it saves fuel. Must have a good set of electrics Incidentally, It's not me who's compressor won't start, all my 3 gennys and 4 compressors work fine and I could repair any of them if needed.The one that I run on low revs has to run all day with a very light load, chargers etc, except when the compressor is needed. Been an electronic engineer for 45 years so nowt scares me. I'm more interested in getting his compressor sorted before he spends money on parts he may not need. I usually find it's the pastile dirty/perished or the spring has gone weak.

 

[e5c0rt.mk1]

The capacitor turned up this morning, so i removed both the old ones and fitted the new one in place of them.

Switched the compressor on and it wouldn't start at all, the motor just turned really slow, worse than its ever been.

So i connected the 5uf capacitor to the new one and the compressor fired up perfect.

So dannyp i think your right about it being xx.5uf.

i expect that one cap is XX.5 uf i have never seen a 5 uf capacitor on a 3 hp motor yet.

I think its cured for now, as i've been using it all day and its hasn't been any trouble.

Thanks again everyone for your help

 

[carbon]

A motor capacitor won’t be xx.5uf as they usually are in multiples of 5uf with a typical tolerance of + - 5%. 35, 40, 50 and 65 are some typical values for single cap motors. Some small motors have 4uf and 6uf but are physically significantly smaller.

 

[normspanners]

I also cant see it being ** .5 I could see it being 65 ish including the 40 you have bought as the very smallest that will just start it and would expect it to be in the 150- 300 uf range, the motor plate can have the cap ratings but yours doesn't unfortunately. Even thought it is starting now I would still check what is correct it obviously has been messed with.
I did one the other day 3hp and the start cap was 500uf to give you an idea I couldnt find a 500uf immediately, so I put two 200 together and it started ok for weeks till i managed to locate a 500 so cant see a 5uf making that much difference to it starting or not, UNLESS (please read to the bottom re no cent switches ) I think the 5 has some numbers missing the start capacitor can have a 10 % tolerance or slightly more on larger ranges as I have proved. It is the run cap that has to be accurately replaced.
as has been said 35 is way too small imo for a start cap for a3hp compressor and will poss damage the motor if used against high start load consistently,
You can get special hard start capacitors for difficult starting motors like compressors or those on reduced voltages difficult situations etc this might help https://inspectapedia.com/electric/Motor_Capacitor_Selection.php
start caps have a life expectancy and are prone to deteriorating rapidly as they age.
this also might help https://www.ien.com/article/understanding-selecting-capacitors/171510
http://www.elcomp.net/conis.pdfand this is the conis /mkp pdf for your 5uf ? cap

as can be read there some motors use a special capacitor for motors with out centrifical switches

APPLICATION MKP-TLS capacitors are used for starting of single-phase asynchronous motors or three-phase motors in a single-phase net. MKP-TLS consists of two elements - MKP selfhealing capacitor with metallized polypropylene for dielectric and an electronic device TLS. The electronic device turns off the starting capacitor after attainment of 75 up to 100% of the nominal revolutions of the motor but not later than Tmax. The constructive features of the motors (quality of steel, scheme of coils winding, number of motor poles and supply voltage frequency) don’t influence on the operation of MKP-TLS. A centrifugal breaker is not necessary when a capacitor with a starting device is used.

Now if your motor definately hasnt got a centifugal switch then it is critical that the correct start capacitor is fitted!

 

[e5c0rt.mk1]

It 100% hasnt got a centrifugal switch.

This is a picture of the wires that go into the motor.

 

[carbon]

Nearly all the motors I work with don’t have centrifugal switch’s just one permanent capacitor. But they are only 1420rpm motors don’t see many 2810 rpm unless they are small fan motors. I do have a capacitor with three wires for fitting to a motor that doesn’t have a centrifugal switch to aid starting in difficult situations