Whats up with Run Capacitors on Goodman A/C units ???

[hvacfella]

Lately, i have been measuring voltage on the top of combo Run Capacitors that far exceed the voltage rating stamped on the Run Capacitor furnished with the Goodman A/C unit.

IE: Todays measurement :
Goodman 2 ton model CK24 unit . Rating of combo run capacitor : 370 v. Voltage as measured on top between common and Herm : 389 v. Unit was operating correctly and incoming voltage entering and leaving the Contactor was 245 v.

Over the last few weeks, ive measured even greater variances on Goodman A/C Combo Run Capacitors between common and Herm as well as common and fan .How come they didnt put in 440 v. rated Combo Run Capacitors ?? Ive also had quite a few Combo Run Capacitors blow over the last few years on Goodman ... most likely due to the higher voltage imposed on them , than what the rating is.

Anyone else running into this ?

Dave

[bb]

Anyone else running into this ? Yes

I usually go ahead and grab a 35/5 440 off of the truck.

[hvacfella]

How can a Manufacturer not pick this up in their R&D ??

RoboTec..if youre still with Goodman, whats your take on this problem ???

[hal parker]

I too have replaced many,many run caps on goodman condensers. I always repcace with 440 .

[hvacfella]

Thats what i do also.

[resolutetech]

if a capacitor can be thought of as a balloon and voltage a pressure. (electrical pressure that is) how can what you guys see occuring happen. the balloon ( capaacitor in this case) can't have higher pressure than the thing pumping it up. (the line voltage in this case)

[beenthere]

The voltage across the cap is from the start winding, not the line voltage.

[Swampfox]

The caps blowing up is a GE problem, they do this on every manufacturers unit in which they are installed, not just Goodman, the compressor manufacturer specs the run caps anyway

[hvacfella]

'The caps blowing up is a GE problem, they do this on every manufacturers unit in which they are installed, not just Goodman, the compressor manufacturer specs the run caps anywa
'

ME: Not just GE caps either ; have found other brand caps where the voltage on them are considerably above the rating. Youd think these residental Compressor Mfg'rs would just go with all 440 v. rated caps, and be done with it.

[stephy]

Oh, beenthere...
You could take most of "us" back to school..
This is very much understood...
Thanks
Peace,
Stephen

[resolutetech]

ok the voltage across the start winding is the voltage across the start capacitor.
and guess what the voltage across the start winding is the line voltage.

[beenthere]

Not quite, but close.

All motors are generators, and all generators are motors.

The voltage from the start winding is an EMB, with is higher then the line voltage.

[hvacfella]

Correct. A motor is like a generator in that it generates higher voltage known as BEMF or Back Electromotive Force ; This increased generated voltage comes thru the start winding and thru the wire leading to the Run Capacitor terminal . The other terminal on the RUn Capacitor is for the Run winding on the compressor (or fan motor...as the case may be). The rating of a Run Capacitor , such as 370 volts...must not be exceeded during operation. Its common to find Goodman A/C units all the time where the imposed BEMF voltage is higher than 370 volts. I think the highest ive ever found was over 400 volts .

[junkhound]

Never ceases to amaze me how many hvac techs have a total misunderstanding of ac power, and how incorrect opinions are foisted off as wisdom, must be a gross error in some hvac text somewhere? .

FWIW:
1. good news - everyone is correct in that the caps should be rated at 440 Vac for good reliability.
2. bad news - not a single poster seems to understand ac power flow. The 389 or 400 V across the cap IS NOT generated by the start or run winding. The voltage across the cap plus the voltage across the run winding is exactly equal to the line voltage at EVERY specific point in time. When you hook up your digital voltmeter, you are probably reading 1.11 time the average voltage ( any cheap meter, even a Fluke 77 reads 1.1 times average vs true rms). If you hook up a scope (e.g. Fluke 123 or equivalent) across the cap and run winding, you would see that at times the volts of the sine wave across the cap is negative and across the winding is positive and both are higher peaks and rms than line voltage - read up about leading and lagging currents in capacitors and inductors before blathering about back emf, etc. and about resonance and voltage amplification thru resonance, etc. etc.


[Edited by junkhound on 06-30-2005 at 12:23 AM]

[rasmussen6880]

I found one today on a no a/c call that had blown apart like one of those magic worms you light a capsul and it grows in to an ash worm. Happy forth of july...

[mark beiser]

Junkhound, do yourself a favor. Before blasting people here over giving incorrect information, try researching some about how AC motors work, because you are dead wrong.

Back EMF is real, and the other posters are correct. In an induction motor with a run and start winding, there is an induced voltage potential between the run and start windings that exeeds the input voltage to the motor.
This is the same voltage that energizes the coil of a potential relay for a start kit, and is the voltage the capacitor must have a higher rating than.

http://cougar.slvhs.slv.k12.ca.us/~p...s/backemf.html

http://www.answers.com/topic/back-emf

http://www.newton.dep.anl.gov/askasc...0/phy00351.htm

http://www.chilternweb.co.uk/physics/emag/backe.html

http://www.4qd.co.uk/fea/pmm.html

http://www.madsci.org/posts/archives...5941.Eg.r.html



[Edited by mark beiser on 06-30-2005 at 01:16 AM]

[molabearair]

The voltage you are reading is generated accross the start winding called back emf

[Edited by molabearair on 06-30-2005 at 01:58 AM]

[beenthere]

You posted some nice links Mark.

Hopefully others go and read them.

[junkhound]

how AC motors work......original comment holds.

1. never said there was no such thing as back emf, but that is not what causes capacitor voltage higher than line voltage. A cap and inductor of any type (not even a motor) can give higher voltages across the cap than line voltage.....

2. interesting how a bunch of internet links can be re-interpreted to suit preconceived notions.

Hook up a 4 channel scope to a motor and them come back and make comments.

[mark beiser]

Junkhound, the links were just random Google stuff. Your origonal comment is still wrong.
I have plenty of hard copy text here, I'll see if I can find something to link that is more relevent to PSC motors when I get home tonight.