Sciencemadness Discussion Board

Capacitor replacement/equivalency

Panache - 24-11-2011 at 07:47

Could one confidently use a 240V capacitor to replace a 110V one. The capacitors are on this piece of kit

http://www.chillercity.com/OPMANUAL/cc.pdf
It's the cc-100
Which I recently bought quite cheaply however it didn't function as designed, when I opened it I found both electrolytic capacitors leaking with cracked cases, otherwise all was pristine.

Problem is in a 240V country 110V gear is hideously expensive because of lack of demand. I got quoted some four hundred dollars at the refrigeration supplier, and that's for one, I need two.

The capacitors run at 240 uF at 110V, 50/60 Hz, each compressor runs on just the single capacitor, I am assuming they are motor start capacitors.
Any input would be appreciated.

watson.fawkes - 24-11-2011 at 10:43

Quote: Originally posted by Panache  
Could one confidently use a 240V capacitor to replace a 110V one.
Yes. The voltage ratings are the maximum rated voltage. Any lower voltage applied is OK.

Panache - 24-11-2011 at 11:11

Quote: Originally posted by watson.fawkes  
Quote: Originally posted by Panache  
Could one confidently use a 240V capacitor to replace a 110V one.
Yes. The voltage ratings are the maximum rated voltage. Any lower voltage applied is OK.

Fantastic! Thnx so much.

Mr. Wizard - 24-11-2011 at 15:03

Don't mix the AC rated ones with the polarized DC ones. Usually the higher voltage ones are slightly bigger than the lower voltage rated ones. Are you using it to start or run an electric motor?

Panache - 26-11-2011 at 03:50

I'm assuming they're motor run capacitors for the two danfoss compressors in the unit. Do refrigeration compressors use run or start caps, given the large capacitance it's likely they are run.

Panache - 5-12-2011 at 12:16

Well that didn't work guess i need to check the gas pressures now, just as a side note if the capacitor is cracked and has salty residue oozing from the cracks its likely compromised, correct?
When checking the oressure do i run the nit with gauge on or do i just bunge on the gauge with unit cold?
The gauge ports run directly off the compressors.

Mr. Wizard - 5-12-2011 at 22:37

Yes, a cracked case with leaking salty stuff is a sure sign of failure. The electrolyte is there to help form the microscopically thin layer of Al2O3 which forms the dielectric layer in most common capacitors. Often they are designed to fail with a plug or a weak spot so they won't explode when overloaded.

What did you do? Is the old cap cracked, or did the new one fail? Don't be too quick to open up the gas system, as it may then be beyond repair, unless you have the coolant and tools to reestablish the correct pressures. Lack of coolant will not cause the capacitors to fail, so checking the gas pressure won't tell you anything. It is unlikely there is too much pressure, as it used to work , right?

What model of danfoss compressor is it? Maybe we can locate a schematic of the motor, and a method to test it.

froot - 5-12-2011 at 23:01

Did the compressor motor run after replacing the cap? If it did then the compressor is not pressurising or the refrigerant has leaked out. If not then measure for open circuit between the 2 motor coil connections. Usually a blown motor cap indicates that too much current went through it because of a short in the motor, or excess voltage applied.

Panache - 5-12-2011 at 23:32

Thankyou most kindly for your help and interest thusfar. Upon startup it worked fine, the light for 'on' came on and then immediately came on the first stage, which ran for about five before the second stage also kicked in. These both ran for around thirty minutes before i shut down the unit. The immersion probe had not cooled at all, it is specced to reach -100C in thirty minutes.
I must also add that this identical performance was witnessed before i changed over the caps.
Something else i found in the unit between the two compressors is an oil/lubricant helical seperator.
The two refrigerants are R134a and R23.
There is a selfindicatingdryer inline that reads dry. I will read the specs off the compressors in a couple when back with them. The network of refrigeration lines between the two compressors and various components is reasonably complex but very sound, really the thing appears as new.
Cheers again.
I have a commercial CPS digital gauge and a gas reclamation unit, if thats of any help, but no expertise, should've paid more attention with my old man when spending summers working with him doing HVAC and refrigeration. I can duct tape up flex in a tiny 50C ceiling cavity lying on upside down though, and i clean a mean filter.

Edit-there is the distinct 'hissing'at the probe end of refrigerant expanding, it just doesn't get cold

[Edited on 6-12-2011 by Panache]

Mr. Wizard - 6-12-2011 at 10:55

We await information about the compressor model, and current draw.
There was a notice in the instructions that stated the unit should NEVER be operated above -25C. Did you stick the probe into a cooling solution to get this required starting point? If you didn't then it would be impossible for the unit to work. The way I understand it, the unit has the capability to go to -100C , but not take itself from room temperature to that. You might have to get a freezing mixture to get to that -25C point, so the thing can work.

A clamp on ammeter would be a big help. Check the current draw on the two motors (compressors) and chart them during start up and while the unit is running in a -25C cooling mixture.

AndersHoveland - 6-12-2011 at 14:08

Quote: Originally posted by Panache  
Could one confidently use a 240V capacitor to replace a 110V one.

The capacitors run at 240 uF at 110V,


A capacitor will effectively have a lower capacitance if charged by a lower voltage than it is rated.

some basic information about capacitor formulas:
http://www.coilgun.info/theorycapacitors/capacitors2.htm

The basic formula is:
E = (CV²)(0.5)
where C is capacitance measured in Farads, V is voltage, and E is energy measured in Joules. (note that V is squared in the formula)

[Edited on 6-12-2011 by AndersHoveland]

Panache - 6-12-2011 at 19:07

Quote: Originally posted by Mr. Wizard  

There was a notice in the instructions that stated the unit should NEVER be operated above -25C. Did you stick the probe into a cooling solution to get this required starting point? If you didn't then it would be impossible for the unit to work. The way I understand it, the unit has the capability to go to -100C , but not take itself from room temperature to that. You might have to get a freezing mixture to get to that -25C point, so the thing can work.
.


WHOOPS!!
The photocopied manual i had was badly printed i read the never operate the probe above -25C as 25C.
Will go and see how it performs now, at least the caps have been replaced.
I'm so sorry, and thankyou so much, saved me a $200 call out!

Mr. Wizard - 6-12-2011 at 22:56

Quote: Originally posted by AndersHoveland  
Quote: Originally posted by Panache  
Could one confidently use a 240V capacitor to replace a 110V one.

The capacitors run at 240 uF at 110V,


A capacitor will effectively have a lower capacitance if charged by a lower voltage than it is rated.

some basic information about capacitor formulas:
http://www.coilgun.info/theorycapacitors/capacitors2.htm

The basic formula is:
E = (CV²)(0.5)
where C is capacitance measured in Farads, V is voltage, and E is energy measured in Joules. (note that V is squared in the formula)

[Edited on 6-12-2011 by AndersHoveland]


You are using the formula to get the energy (E) available from a capacitor, not the capacitance. The C in the formula is in Farads, and E is energy in Joules.
http://en.wikipedia.org/wiki/Capacitance

Actually the capacitance could go up as the voltage goes down, as the layer of aluminum oxide would be thinner at a lower voltage, but in actual usage it doesn't seem to matter.

The stored energy does go up with the square of the voltage, but the capacitance doesn't.

dann2 - 7-12-2011 at 17:17


It can be easy to mix up Faraday and Farad.
They are two different things.
Farad-------Used as the unit of capacitance
Faraday---------A unit of charge (number of electrons).

With a given capacitor the Faradays that will be stored in it
(electrons sitting 'in' the plates) will increase as you place
a larger and larger Voltage accross it.

Panache - 24-12-2011 at 04:55

Ok it still doesn't work, with the bath at -25, -35, etc etc, -75.

Not that i understood how it simply wouldn't work if the intial bath temp was at ambient, i struggle to find logic in this, but thats a digression.

So to recap, i'm going to get the compresser data, etc and we'll continue on as before.