Summer Starts And So Does The HVAC Trouble

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SETEC_Astronomy
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Summer Starts And So Does The HVAC Trouble

Post by SETEC_Astronomy » Sat Apr 10, 2010 7:06 pm

My AC stopped working the other day and I'm trying to track down the reason. Everything was working fine for about a week until the compressor stopped. When the thermostat calls for AC the compressor makes a loud hum for a few seconds and stops, waits a minute and hums again. My first thought was bad motor, jam, seize, start/run capacitor etc... Out of two caps that are easily accessible one measures spot on and the other a dead short. Seems like the culprit, right? Now to the part that confuses me, the good cap only costs $7.14 to replace from an online retailer. The bad cap has a price of $69! Why the difference? The capacity? The cheap(er) cap has three terminals (fan - common - hem) and a value of 5uF on one side and 50uF on the other. The bad capacitor is marked with the same voltage rating (330VAC) but a value of 176-216 MFD. The cap was made by Philips and is part number 63a0601. Why the major change in cost? I'm sure there is a good reason but I don't know it. Duty cycle? Three times+ the capacitance? What's the reason (if there is one) to not find a cheaper cap with the same specs, 330VAC 176MFD?

I don't know the cause of the failure, I can only guess age has played a role. It's either the cause or a symptom but the oil looks to have leaked out. I welcome your thoughts.

Edit: Removed images to conserve bandwidth

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Bob Scott
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Re: Summer Starts And So Does The HVAC Trouble

Post by Bob Scott » Sat Apr 10, 2010 7:59 pm

Hi SETEC, You did measure a short with the cap disconnected from the circuit, right?

That seems high! Shop around at electric motor rewinding shops. (Do they still exist?) Just ask for pricing on a 330VAC ~200uF Motor Starting cap. You don't need the Motor Running cap.
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Re: Summer Starts And So Does The HVAC Trouble

Post by SETEC_Astronomy » Sat Apr 10, 2010 8:23 pm

Bob Scott wrote:Hi SETEC, You did measure a short with the cap disconnected from the circuit, right?
That's correct. I even clipped off the bleeder resistor to verify my reading. The resistor measures around 6Meg. The cap still measures as a short (.006 ohms)


Bob Scott wrote:That seems high! Shop around at electric motor rewinding shops. (Do they still exist?) Just ask for pricing on a 330VAC ~200uF Motor Starting cap. You don't need the Motor Running cap.
Sounded high to me too. Thanks for the suggestion, I'll make some calls. And yes I mistyped in my original post, it says starting on the label, nice catch though.

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Re: Summer Starts And So Does The HVAC Trouble

Post by Robert Reed » Sat Apr 10, 2010 8:45 pm

Have seen a lot of them on E-Bay with your needed specs, most under $12 and some with free shipping. All touted as being new stock. However there was one or two way up there at $279 :shock I guess if you have a tight space where it fits in, you may have to be looking at those specs to.
BTW, the bleeders are extra and are listed as 15K-2 Watt.
I always thought oil filled caps cured themselves after an arc over - must have been very catastrophic. May want to also look at the start relay while you're at it.

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Re: Summer Starts And So Does The HVAC Trouble

Post by SETEC_Astronomy » Sat Apr 10, 2010 9:17 pm

Thanks for the suggestion Robert. I was a little concerned about the quality of caps on ebay but I guess they can't be much worse then the one that was there. As long as they don't burst into flames it's worth trying for those prices.

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Re: Summer Starts And So Does The HVAC Trouble

Post by Lenp » Sat Apr 10, 2010 11:39 pm

SETEC

Run away from that price!
Here is Grainger's link and the same page has bleeder resistors too!
http://www.grainger.com/Grainger/wwg/vi ... CatPage=85
If you don't have an account PM me! Most are under $25
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Re: Summer Starts And So Does The HVAC Trouble

Post by Bob Scott » Sun Apr 11, 2010 10:23 am

Replace the bleeder resistor if it really reads 6 Meg. The color bands are hard to see, but it looks like it is supposed to be ~15K.

The time constant for discharging the cap (t=RC) is 6,000,000 X .000200 = 1,200 seconds using 6 megs. I think 20 minutes for the cap to discharge through a 6 meg resistor might be too long.
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Re: Summer Starts And So Does The HVAC Trouble

Post by SETEC_Astronomy » Sun Apr 11, 2010 6:02 pm

Bob Scott wrote:Replace the bleeder resistor if it really reads 6 Meg. The color bands are hard to see, but it looks like it is supposed to be ~15K.

The time constant for discharging the cap (t=RC) is 6,000,000 X .000200 = 1,200 seconds using 6 megs. I think 20 minutes for the cap to discharge through a 6 meg resistor might be too long.
It does read six meg. The color code is brown-green-orange, as you said it's supposed to be 15K. That line on the far end appears to be a burn (not a tolerance band) and when I touched it with a probe it crumbled away. I'm going to my local electronics shop tomorrow for a new one.

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Re: Summer Starts And So Does The HVAC Trouble

Post by dacflyer » Mon Apr 12, 2010 4:56 am

i found out the hard way that there is a big difference between a start cap and a run cap.
make sure you get the right one..
i was told before the reason for the price jump, but cannot remember why or which cap it was.
i think the start cap is designed more to take the surge than a run cap.

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Re: Summer Starts And So Does The HVAC Trouble

Post by SETEC_Astronomy » Mon Apr 12, 2010 4:13 pm

Well, I got the new resistor and capacitor installed. Now the compressor runs for those few seconds and then turns off. It's not looking good.... :???:

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Re: Summer Starts And So Does The HVAC Trouble

Post by Lenp » Mon Apr 12, 2010 4:31 pm

Does the compressor actually start or does it just hum? Can you do an ammeter check during this time with a clamp on meter? It may be a shorted compressor winding. Take winding resistance readings with wires disconnected, and to ground as well. Post the results, along with the HP(BTU) rating of the compressor.

From another site...
http://forum.doityourself.com/archive/i ... 39978.html
You need a good digital meter and use the lowest range. Your lowest reading is between the "C" and "R" terminals, a few ohms (give or take) is about right. Your next reading between "C" and "S" should be a little bit higher than the previous. Your highest reading will be between the "S" and "R" terminals and will be the total of the first two readings. Keep in mind that all these readings should show very low resistance, but not zero. You should never have any measurable resistance (should be infinite) between any terminal and the compressor housing. Use the higher resistance range on your meter to detect grounded windings.
This method is good for detecting grounded or open windings and windings that are suspect (resistance too high). However, each make, model and size compressor will yield somewhat different values.


Hope this helps!

Len
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Re: Summer Starts And So Does The HVAC Trouble

Post by SETEC_Astronomy » Mon Apr 12, 2010 4:36 pm

Before I replaced the start capacitor it only hummed, it now runs/starts for a few seconds and shuts off. I don't have a hefty enough ammeter for a direct test and don't own a clamp meter. I could however try to whip up an adapter for my DMM with the help of online schematics. I'll test the windings, relays, etc... after dinner and post results with the BTU rating.

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Re: Summer Starts And So Does The HVAC Trouble

Post by dacflyer » Tue Apr 13, 2010 12:55 pm

if the unit is low on freon its possible that it has a low pressure shut down switch.
some have a low pressure shut down switch and a hi pressure shutdown switch..
does the contactor click off when the compressor stops? or does it stay activated.?
if contactor drops out, it may well be a low pressure sw.
if the contactor stay active, and compressor shuts off after a bit, then its possible that its going into overload shutdown,. i hope you did not accidently connect the start and run terminals backwards.. make sure all connections are correct if you took anything loose.
if its still doing the same, might be a bad compressor :(
how old is the unit ?

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Re: Summer Starts And So Does The HVAC Trouble

Post by SETEC_Astronomy » Tue Apr 13, 2010 5:08 pm

I was a bit discouraged after taking the winding measurements but I re-read this
Lenp wrote: Your lowest reading is between the "C" and "R" terminals, a few ohms (give or take) is about right. Your next reading between "C" and "S" should be a little bit higher than the previous. Your highest reading will be between the "S" and "R" terminals and will be the total of the first two readings. Keep in mind that all these readings should show very low resistance, but not zero.
My readings are as follows
Run - Common = 1 ohm
Start - Common = 1.6 ohms
Start - Run = 2.3 ohms

From all terminals to ground there is a resistance of 1.7 Mohms. I had re-connected the wires by this test and didn't want to risk taking them off again. The spade lugs bent quite a bit in the removal process.
dacflyer wrote: does the contactor click off when the compressor stops? or does it stay activated.?
It stays activated.
dacflyer wrote:i hope you did not accidently connect the start and run terminals backwards


I was very careful not to. I took pictures before I did anything and made sure I re-installed everything properly.
dacflyer wrote: how old is the unit ?
It's about 15 years old.


When I started the compressor with the cover off I noticed the contactor sparked badly on one side and started giving off smoke. I powered everything down and got a closer look and one side of the contacts had welded together. I know they weren't welded the other day because it's something I had looked at. It looks like oil from the capacitor dripped into the contactor relay. It could be two separate issues, one cause by the other. Failed capacitor, problem one, now fixed. Oil preventing the contactor from making a good connection, problem two. Likely? Possible?

How risky would it be to bypass the contactor and attempt to start the compressor? If it works I can go buy the part and be done. I'd rather not buy an nonreturnable part if it won't help.

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Re: Summer Starts And So Does The HVAC Trouble

Post by Robert Reed » Tue Apr 13, 2010 5:58 pm

Well you still are backed up by the service entry box breaker. However these still take a longer time to react than you would prefer. Maybe a two man operation-one at the service panel with his hand ready to trip the breaker, and the other at the compressor ready to scream "Trip Now" if necessary.

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