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somized
06-28-2005, 10:09 PM
my fellow tech said it makes a difference to compressor operation, which voltage run cap you use. i say that voltage stated on the run cap is it`s dielectric limits, and the microfarads in new, should be same as old, at usual 208v to 240v conditions.

Mr Bill
06-28-2005, 10:13 PM
Originally posted by somized
my fellow tech said it makes a difference to compressor operation, which voltage run cap you use.

Well I have to agree with that statement. :D I would rather use the correct mfd than volts in an emergency.

wormy
06-28-2005, 10:17 PM
Use Same MFD
Use Equal to or higher than Voltage rating.
(i.e. its ok to replace a 370v with a 440v)

tecman
06-28-2005, 10:18 PM
You are right. The voltage is just a maximum operating voltage and has nothing to do with the function as a capacitor. Operating within its voltage limits, any capacitor with the same MFD value will perform the same.

paul

NormChris
06-28-2005, 10:20 PM
The voltage rating on a capacitor is the voltage limit of the capacitor. Exceeding the capacitor's rated voltage may cause the dielectric to break down and the capacitor to short out.

That said, you need to realize that a motor is an inductive device. When you the supply voltage is applied to the motor run winding, the voltage across the start windings will be increased to a higher voltage value. A motor acts like a transformer with the run winding acting as the primary and the start winding acting as the secondary.

This is why the capacitor has a much higher voltage rating than the supply voltage.

Toolpusher
06-28-2005, 10:21 PM
What problems would result in using a 35 mfd, when a unit had a 30 mfd ?

Just wondering, They always say stay within a certain percentage

tinknocker service tech
06-28-2005, 10:25 PM
sopositly you can go up or down 5% but i always put in the same size mfd but only use 440v caps
less to carrier on the truck
5mfd and 7.5 mfd 370v and 440 v on truck

Mr Bill
06-28-2005, 10:34 PM
Originally posted by tinknocker service tech
sopositly you can go up or down 5%

I always thought 20% but heck what do I know. :D

tinknocker service tech
06-28-2005, 10:39 PM
mr bill it may be 20% i was tought so long ago 5% and have always gone by that

bb
06-28-2005, 10:40 PM
I thought it was 10% over and 5% under.

Mr Bill
06-28-2005, 10:43 PM
Originally posted by tinknocker service tech
mr bill it may be 20% i was tought so long ago 5% and have always gone by that

No, your probably right for some reason that just came to mind.

karsthuntr
06-28-2005, 10:43 PM
Had a blown up cap today at 5pm. It was a 40/3 370V cap and I didn't have one on the truck. So I took 2 10uf, and 4 5uf, and linked them with a 3uf so I could get them a/c until I could get the right cap. It was ugly and I had to strap it under the electrical box but it works. :D

Toolpusher
06-28-2005, 10:51 PM
I'll bet that looked like a time bomb strap to the undercarrage.

No one has said what will happen if the higher mfd is used and not changed.

Mr Bill
06-28-2005, 10:59 PM
If you need a 30/440 can you cut a 60/440 in half? :D

tinknocker service tech
06-28-2005, 11:03 PM
tried it once problem was i could not get the smoke back in heheheh

karsthuntr
06-28-2005, 11:14 PM
Originally posted by mrbillpro
If you need a 30/440 can you cut a 60/440 in half? :D

2 60uf in series will give you 30uf. (if I remember that correctly, I've never had to use it).

NormChris
06-28-2005, 11:17 PM
Originally posted by karsthuntr

Originally posted by mrbillpro
If you need a 30/440 can you cut a 60/440 in half? :D

2 60uf in series will give you 30uf. (if I remember that correctly, I've never had to use it).

Yup, two 60 uf caps in series give you 30 uf because the dielectrics (insulations) add up and effectively push the plates farther apart thus lowering the capacitance.

This also causes the voltage ratings of the two capacitors to add up thus increasing the maximum voltage rating of the two in series.

Norm

bornriding
06-28-2005, 11:24 PM
A 35 in place of a 30 will cause slightly higher starting and running currents, slightly more torque. No problem in a temporary situation. Might shorten life a little if left in, but not sure that it would even do that.

mark beiser
06-28-2005, 11:26 PM
Get one of those Turbo 200 capacitors on your truck and save it for those really oddball dual run caps. It can be used to replace a single run cap too.
They are pricy, so you have to charge more for them, but it will save you a trip to the supply house.
I just wish the Turbo 200 would go up to 80µf.

robnjr
06-28-2005, 11:37 PM
Originally posted by Toolpusher
I'll bet that looked like a time bomb strap to the undercarrage.

No one has said what will happen if the higher mfd is used and not changed.

The motor or compressor will draw more current than it normally would, and its life will be shortened.....the higher the cap value, the worse the effect....too high (several times the desired) of a value, and you'll be emulating a hard start cap that doesn't switch out and stays in circuit....after all that's all a hard start really is: a temporary boost in the run capacitance, right?

BTW, a **very good** quality cap has at least a 5% variation in its stated capacitance rating (the standard quality smaller caps, like the type used on circuit boards, can vary by 20%, the "good" quality 10%). A 30-40 deg temperature change can add a few more percent...age can also cause variation in capacitance as the dielectric dries out.....bottom line: you really don't know what it is, unless you measure it!!!!

[Edited by robnjr on 06-28-2005 at 11:40 PM]