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TheDuke
08-18-2006, 11:23 AM
question i f I do not have a dual capacitor on the truck and the one I am replacing is 35/5 uf 440volts
I can replace it with one single 35 uf 370 volt and one 5 uf 370 volts correct

cep a/c
08-18-2006, 11:29 AM
Sure, if you know how to wire the fan and the compressor cap and ground. The primary diff between 370 and 440 is insulation factor. You are wiring to a 240 volt system. The 44 would likely last longer but the insulation factor is good in either the 370 or 440 case.

t527ed
08-18-2006, 11:31 AM
Originally posted by TheDuke
question i f I do not have a dual capacitor on the truck and the one I am replacing is 35/5 uf 440volts
I can replace it with one single 35 uf 370 volt and one 5 uf 370 volts correct

no, should be a 440 cap or caps.

keepitsimplestupid
08-18-2006, 11:40 AM
The higher the voltage rating the higher the reliability.
They should be rated at xx VAC Working Voltage. Furthermore, you may not know what the guy before you did. I would write the values of the original caps on the replaced one.

A good rule of thumb is to size the voltage rating 2x the rating of the motor.

The cap will short if the voltage rating is too low. This could result a motor failure.

Also parallel capacitors add with the same working voltage and series capacitors follow the formula 1/C = 1/C1 + 1/C2 + 1/Cn and their working voltages would add. So if you had a two 10 uf 200 VAC caps. you could get a 20 uf at 200 VAC or a 5 (fixed math error) uf cap at 400 VAC. The ESR (Effective Series Resistance) of the caps in series may or may not be lower. The ESR should be the low as possible.

[Edited by keepitsimplestupid on 08-18-2006 at 03:45 PM]

iheatncoolinnc
08-18-2006, 01:29 PM
Correct me if I am wrong. You can replace a 370 with a 440 and expect a longer life. You should not replace a 440 with a 370. (If that is what was originally designed for the equipment) We are eliminating our stock of 370VAC capacitors and paying a little more for the 440VAC to solve this problem. This was suggested to me by one of my suppliers this year. Also we are keeping one of those expensive multi capacitors on each truck. I think these are 370VAC though.

Wild Leg
08-18-2006, 02:09 PM
Originally posted by cep a/c
Sure, if you know how to wire the fan and the compressor cap and ground. The primary diff between 370 and 440 is insulation factor. You are wiring to a 240 volt system. The 44 would likely last longer but the insulation factor is good in either the 370 or 440 case.

Back EMF dictates the voltage rating on a run capacitor.

Supplied voltage has little to do with it.

Generally speaking, you should never use a capacitor of lower voltage rating than the one you are replacing.

If nothing else, it voids the UL rating of the equipment.

Measure the voltage across the capacitor, with the equipment running.
That will give you the back EMF of the motor, and an idea of whether a 370 volt will work in place of a 440 volt.

Steve Wiggins
08-18-2006, 03:00 PM
Originally posted by iheatncoolinnc
Correct me if I am wrong. You can replace a 370 with a 440 and expect a longer life. You should not replace a 440 with a 370. (If that is what was originally designed for the equipment) We are eliminating our stock of 370VAC capacitors and paying a little more for the 440VAC to solve this problem. This was suggested to me by one of my suppliers this year. Also we are keeping one of those expensive multi capacitors on each truck. I think these are 370VAC though.

You are wrong.

It is ok to replace a 440 with a 370 but I wouldn't replace a 370 with a 440 unless it was an emergency situation.

Sticking a larger than manuf required voltage is the similar to pulling out a 30 amp fuse and replacing with a 40 amp fuse.....you wouldn't do that now would you? Then why is it ok to do the same with a capacitor?

paul42
08-18-2006, 03:07 PM
replacing a 370 volt capacitor with a 440 volt capacitor is analagous to replacing the tank on an air compressor with one rated for a higher air pressure. Changing the tank doesn't change the air pressure, it just gives a higher safety factor.

The analogy with a fuse is completly wrong in this case. a 440 volt capacitor will not supply more current or more voltage. The capacitor is not a safety item designed to limit anything, the fuse is.

snewman24
08-18-2006, 03:07 PM
Originally posted by keepitsimplestupid

.....Also parallel capacitors add with the same working voltage and series capacitors follow the formula 1/C = 1/C1 + 1/C2 + 1/Cn and their working voltages would add. So if you had a two 10 uf 200 VAC caps. you could get a 20 uf at 200 VAC or a 10 uf cap at 400 VAC......

actually, the two 200 VAC 10uf caps in series would be the equivalent of one 400 VAC 5uf (not 10uf) capacitor.

keepitsimplestupid
08-18-2006, 03:46 PM
Snewman:

Fixed the math error in the original post. Thanks.

keepitsimplestupid
08-18-2006, 04:04 PM
Now, what ia a problem with fuses is replacing one rated at 250 V with one rated at 32 V. The 32 V glass variety was common in automotive years ago.

Voltage rating is like horsepower. You can have more than you need, but I wouldn't replace a cap rated at 10 V with rated at 10 killovolts because other problems come into play and the first is physical size.

The 10 KV cap needs a keeper when in storage and is going to have a lousy effective series resistance. The 10 KV cap builds up a charge just by sitting.

keepitsimplestupid
08-18-2006, 04:07 PM
UL rating is probably voided by not using any parts recommended by the manufacturer and could possibly be voided by dressing the wires differently.

Wild Leg
08-18-2006, 04:11 PM
Originally posted by Steve Wiggins

Originally posted by iheatncoolinnc
Correct me if I am wrong. You can replace a 370 with a 440 and expect a longer life. You should not replace a 440 with a 370. (If that is what was originally designed for the equipment) We are eliminating our stock of 370VAC capacitors and paying a little more for the 440VAC to solve this problem. This was suggested to me by one of my suppliers this year. Also we are keeping one of those expensive multi capacitors on each truck. I think these are 370VAC though.

You are wrong.

It is ok to replace a 440 with a 370 but I wouldn't replace a 370 with a 440 unless it was an emergency situation.

Sticking a larger than manuf required voltage is the similar to pulling out a 30 amp fuse and replacing with a 40 amp fuse.....you wouldn't do that now would you? Then why is it ok to do the same with a capacitor?

Steve, somebody has managed to post some bogus info under your name.

Using a 440 volt capacitor in place of a 370 volt capacitor is like using 600 volt wire for 240 volts.

Look it up.

gus99
08-18-2006, 06:13 PM
Originally posted by Wild Leg
Back EMF dictates the voltage rating on a run capacitor.

Supplied voltage has little to do with it.

Generally speaking, you should never use a capacitor of lower voltage rating than the one you are replacing.

If nothing else, it voids the UL rating of the equipment.

Measure the voltage across the capacitor, with the equipment running.
That will give you the back EMF of the motor, and an idea of whether a 370 volt will work in place of a 440 volt.
[/B]

That's not really a true EMF measurment. To do it right you need a lab storage oscilliscope. The EMF will be very high at startup and close to unity at running load. Unless your meter can take a snapshot of the first few milliseconds you really aren't seeing the full EMF.

Bottom line is don't replace a 440v cap with a 370 unit. Even if it's just over the weekend. It could voilently fail.

Gus99 - EE

jacob perkins
08-18-2006, 06:32 PM
Originally posted by gus99
. To do it right you need a lab storage oscilliscope.

Gus99 - EE

bwaldleg's got one of them things...sure he does.... and uses it all the time to check crapacitors.

TheDuke
08-18-2006, 08:06 PM
I thought if i took a a dual 35/5 or even 45/5 uf 440 volt and replaced it with two single caps that were 370v it would work never did it but.
I see lot of dual caps out there in the field that other tech's replaced the fan cap off a dual 440 volt with a 5 uf 370 volt cap and yet when i look at the dual cap it is a 440 but the fan cap was put in as a 370 seperate from the 440 cap for the fan, that is why I started this post.
thank's again for all the replays
we had a nice summer here did real well this summer starting to slow down now but, its ok I worked every weekend for the past 13 weeks to get through the slow month of september.

iheatncoolinnc
08-18-2006, 08:48 PM
So I guess next time in question I will tell the customer that I need to take the unit back to the shop with me for a small charge (Recover, New Dryer, Silver Solder, Oxygen/Aceylene, Nitrogen, Evacuation Charge, New Freon (Only The Best), and hook it up to my lab storage oscilliscope to see what kind of capacitor I should put on his unit. I do not know if the last person put the right capacitor on his unit and the model and serial number has faded away to nothing. I tried to check the compressor number but someone took a screwdriver and took the plate off.

Steve Wiggins
08-18-2006, 09:23 PM
Originally posted by Wild Leg

Originally posted by Steve Wiggins

Originally posted by iheatncoolinnc
Correct me if I am wrong. You can replace a 370 with a 440 and expect a longer life. You should not replace a 440 with a 370. (If that is what was originally designed for the equipment) We are eliminating our stock of 370VAC capacitors and paying a little more for the 440VAC to solve this problem. This was suggested to me by one of my suppliers this year. Also we are keeping one of those expensive multi capacitors on each truck. I think these are 370VAC though.

You are wrong.

It is ok to replace a 440 with a 370 but I wouldn't replace a 370 with a 440 unless it was an emergency situation.

Sticking a larger than manuf required voltage is the similar to pulling out a 30 amp fuse and replacing with a 40 amp fuse.....you wouldn't do that now would you? Then why is it ok to do the same with a capacitor?

Steve, somebody has managed to post some bogus info under your name.

Using a 440 volt capacitor in place of a 370 volt capacitor is like using 600 volt wire for 240 volts.

Look it up.

So you are telling me that if there is a weak link in the electrical system you want the 440v run cap to not be it? You would rather the compressor or fan motor be the weak link? Even though the manufacturer says to use a 370v you are going to override their decision?

My god man are you really thinking about what you are saying?

Mr. Leg like many others has fallen into the mesmerizing hypnotic trance of always believing old wives tales just because everyone else believes them. That is how religion got started you know.

Always listen to the WigMaster, for he will guide you in the right light.

Steve Wiggins
08-18-2006, 09:28 PM
You guys can play white glove tech all you want but real world contracting is improvising with what you have got to work with. Use whatever cap you have whether its 370 or 440 it really doesn't make that big of a difference.

icyflame
08-18-2006, 09:50 PM
[QUOTE]Originally posted by Steve Wiggins
[
You are wrong.

It is ok to replace a 440 with a 370 but I wouldn't replace a 370 with a 440 unless it was an emergency situation.

I'm wondering if you realize that you said this backwards!
You can change a 370v with a 440v but not a 440v with a 370v.

The mfd must be the same!

comfortdoc
08-18-2006, 10:18 PM
Originally posted by TheDuke
I thought if i took a a dual 35/5 or even 45/5 uf 440 volt and replaced it with two single caps that were 370v it would work never did it but.
I see lot of dual caps out there in the field that other tech's replaced the fan cap off a dual 440 volt with a 5 uf 370 volt cap and yet when i look at the dual cap it is a 440 but the fan cap was put in as a 370 seperate from the 440 cap for the fan, that is why I started this post.
thank's again for all the replays
we had a nice summer here did real well this summer starting to slow down now but, its ok I worked every weekend for the past 13 weeks to get through the slow month of september.

It's possible the fan motor was changed and the new capacitor that was installed for it is the one the replacement motor uses.

[Edited by comfortdoc on 08-18-2006 at 10:33 PM]

hvac hero
08-19-2006, 07:39 AM
Originally posted by icyflame
[QUOTE]Originally posted by Steve Wiggins
[
You are wrong.

It is ok to replace a 440 with a 370 but I wouldn't replace a 370 with a 440 unless it was an emergency situation.

I'm wondering if you realize that you said this backwards!
You can change a 370v with a 440v but not a 440v with a 370v.

The mfd must be the same!

Just to throw another opinion in this whole mix. I've heard the mfd can be off plus or minus 10%. lol

icyflame
08-19-2006, 12:03 PM
That 10% is in determining if it is good or bad. If you have a 35mfd you would replace it with a 35mfd.

davo
08-19-2006, 05:48 PM
I cannot believe some of the potentially dangerous and destructive advice given here.
It is ALWAYS alright to replace a 370vac rated capacitor with a 440vac rated capacitor. It is NEVER alright to replace a 440vac rated capacitor with a 370vac rated capacitor unless you can verify that the compressor and fan motor call for a 370vac capacitor. Even then a 440vac capacitor is a better choice. A capacitor can explode and start electrical fires in the control box if you use too small a vac rating. Motors and comressors are damaged much more by failed capacitors then anything to do with selecting a larger vac rating. In fact, There is no negative effect of using a larger vac rating except a slightly higher parts cost. It is not a "fuse or circuit breaker". The capacitor is not a "safety link" for the compressor or the fan motor. The safety link is something called a thermal overload built into the compressor (on some they use a external control)or fan motor (in the common winding, opens on heat, closes when it cools). As for the statement of "White Glove Technicians", I would gather the person who made this statement likes to replace a lot of capacitors.
The MFD rating must remain the same when replacing capacitors, no matter if you are putting two in parallel, have the proper rated one, or are going to try to series capacitors.
Many contractors are finding it more cost effective to stock only 440vac capacitors as it lowers their truck and shop inventory quite a bit, although the 440vac capacitors cost more.

[Edited by davo on 08-19-2006 at 06:12 PM]

Mr Bill
08-19-2006, 08:34 PM
Originally posted by Steve Wiggins
You guys can play white glove tech all you want but real world contracting is improvising with what you have got to work with. Use whatever cap you have whether its 370 or 440 it really doesn't make that big of a difference.

I never like to change the mfd but it is ok to go from a 370 to 440 matter fact at my supply house you can't hardly even find a 370 volt anymore.

keepitsimplestupid
08-19-2006, 08:35 PM
Excellent summary!!

icyflame
08-20-2006, 12:51 AM
Way to go DAVO...tell um like it is!!!:D

riderman
08-20-2006, 01:05 AM
Awsome post davo!

I cannot disagree. I understand a few things about caps myself...LOL

JBF
08-20-2006, 09:57 AM
Dave was right about never replacing the 440 V cap with a 370 volt cap. The 440 volt cap will have a better life expectancy although the effective micro-farad rating might be slightly lower with the higher voltage cap.

Putting two 440 volt caps in parallel will always give you the sum of the two caps and is OK.

Putting two 200 volt caps in series may give you unexpected results. Generally:

10 uF @ 200V in series with 10 uF @ 200V will give you 5 uF @ 400V in an ideal world. Unfortunately caps have a characteristic called leakage resistance that gets worse as they age. If one of the caps is older, different brand, or is exposed to more heat than its partner, the voltage across the leaky cap can easily exceed the 200 volt rating of the leaky cap. If you put balancing resistors across each cap equal to 10% of the caps' leakage resistance this problem can be averted. Watch the power dissipation in the balancing resistors.

I'd just use the right 440 Volt caps.

[Edited by JBF on 08-20-2006 at 02:30 PM]

blackwidow
08-20-2006, 11:44 PM
i too stock all 440 volt caps... cheaper to only stock them than to stock all mfd's in both....

to the poster about the seperate replacement cfm caps being 370 volt and the original dual being 440 volt... It is common knowledge to replace the cfm cap when changing the motor... they are cheaper than replacing the dual cap... Also every replacement c f motor that i sell only says like.... 1/4 hp... 1075 rpm.....1.9 amp....5 uf capacitor... none of the ge's, fasco's or ao smith motors actually say what volt for the cap......

Steve Wiggins
08-21-2006, 12:25 AM
Disagree all you want but I am agreeing with the manufacturers on this. Anyone got any proof to counter me? Or are you going to base your belief on an old wives tale?

comfortdoc
08-21-2006, 01:21 AM
Steve please read some of this. I do not disagree that an effort should be made to replace a capacitor with the exact rating, but there are accepted industry guidelines in place for selecting the replacement when an exact match is not readily available:
http://dnr.louisiana.gov/sec/execdiv/techasmt/ecep/hvac/d/d.htm

And from York Capacitor http://www.yorkcapacitor.com/support.html
Electrical values. The capacitance, measured in microfarads, needs to match the original. Most capacitors have this value marked on the case such as 5 mfd, 15 mfd, etc. For dual units (those with three terminals on top), both values are listed such as 25/10 mfd, 50/15 mfd. The second electrical consideration is that the voltage rating of the replacement be at least as high as that of the original. It is okay if the replacement has a higher voltage rating. Typical voltage ratings for capacitors found in appliances are 330 VAC and 370 VAC.

[Edited by comfortdoc on 08-21-2006 at 01:25 AM]

lbjheat
08-21-2006, 07:44 AM