Capacitor gurus

Limited-Time

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Looking to replace a capacitor in a room air conditioner. The unit runs on 110 vac, but the suspect capacitor is labeled 250 vac 10uF unit. Is the 250 the rated output?
 

GA_Boater

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The Motor start capacitor?

Really it's a margin for error in case the voltage exceeds what the max voltage could be when the motor starts. Use the same spec cap for replacement.
 

tpenfield

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The 250 vac (Volts Alternating Current) is the capacitor's voltage rating.
 

gm280

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Limited-Time, the capacitor is actually 10uf. And that stands for Micro Farad. It is a term used for a measurement of capacitance valves. The 250VAC is the maximum voltage it can work with without problems and it is an AC voltage as well. AC stands for Alternating Current, as in typical AC house voltage/current. So when replacing such a starting capacitor buy the same spec part for that usage. No, your air conditioner doesn't work at 250VAC, but the part can work with that high of a AC voltage. It is typical for manufactures to use such capacitors to be higher then the AC voltage that runs the compressor. Starting voltage can rise to assist in starting the compressor and then drop down after the motor is running. Hence, starting capacitor. Just a little background info.
 

Limited-Time

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Thanks guys. I under stood the AC and the 10uF rating. What I did not get was the 250 volt rating. But now I understand that's a max voltage rating. Thanks again.
 

thumpar

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Check with mouser or digikey. I prefer mouser for small orders because digikey adds an extra fee if it is too small. Are you sure it is bad? Most of the time they will bulge at the end or even leak when they go bad.

On a side note caps can be fun. In electronics class we used to take 5v dc caps and hook them up to the voltage generator backwards and way over voltage. They sound just like a firecracker. I wouldn't recommend overdriving with a cap as big yours (it would be hard to find enough to do it though).
 

Tnstratofam

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Check with local HVAC supply houses like Fergusen Enterprises, CC Dickson, Modern Supply or Grainger like Scott mentioned. Most will sell an electric component over the counter to a homeowner.

You can take the capacitor in question and have it tested at most of these places to make sure it is bad.
 
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bigdee

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When I replace capacitors I usually use one with a higher voltage rating. Most 240 HVAC units use capacitors with a working voltage of 370 and a voltage spike can often exceed this and kill capacitor so I replace with 440 volt caps. Since yours is on a 120 volt unit 250 volt is probably OK but I would use a 370 volt if possible.....a loose neutral could cause a spike above 250!
 

gm280

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Some DVM (Digital Volt Meters) have a capacitance scale on them and can easily read the capacitor. If you don't have one yourself, maybe a neighbor does. I have a couple Fluke meters that do have such capabilities. I would verify that yours is bad before buying a new one. They are not very expensive, but the capacitor may not be the problem either. :noidea:
 

Limited-Time

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Thanks again for all the responses. Here's the back story. I recently retrieved a large window air conditioner that has been sitting idle for several years. When I put it in place and powered it up the blower fan came on directly. However when the compressor triggers to cycle it does not, it causes the blower fan all but stop. It seems as if the current supply is being drained. So I figured the start cap may be faulty. After checking on line there are 2 caps in the system. One for the compressor motor (large) and one for the blower motor (smaller) the larger of the two was still available from Admiral. The second which I knew I did not need is NLA so I was looking for alternatives when the original topic arose. Thanks again.
 

gm280

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Thanks again for all the responses. Here's the back story. I recently retrieved a large window air conditioner that has been sitting idle for several years. When I put it in place and powered it up the blower fan came on directly. However when the compressor triggers to cycle it does not, it causes the blower fan all but stop. It seems as if the current supply is being drained. So I figured the start cap may be faulty. After checking on line there are 2 caps in the system. One for the compressor motor (large) and one for the blower motor (smaller) the larger of the two was still available from Admiral. The second which I knew I did not need is NLA so I was looking for alternatives when the original topic arose. Thanks again.

LT, so the fan did come on and run okay initially. So is the fan running all the time and when the compressor kicks on, the fan motor stops or comes close to stopping? That doesn't sound like the fan starting capacitor is bad. But it does point to maybe some corroded terminals. If the fan comes on without the compressor running, then the fan starting capacitor is doing its job. The fact that it nearly stops when the compressor kicks in points to something draining the available current and the fan suffers from that. So unplug the air conditioner and start checking connections. Could be some iffy wire nut connections or screw connections need cleaned and you don't need any starting capacitors at all. JMHO!
 

dwco5051

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The 10 uF cap on the fan motor is a run capacitor as most smaller AC fan motors are PSC (Permanent Split Capacitor) motors. Sounds like a problem with the compressor. Could be a number of things such as frozen compressor, open start windings, bad start relay, etc. First thing I would do is check resistance of windings of the motor. If that checked out ok then grab a few of jumper wires and a good start cap and by connecting the right connectors on the compressor and holding a jumper against the start winding pin check the compressor to see if it would run at all. Not for the feint of heart but the quickest way to test rather than throw parts at it. What was the rating on compressor capacitor? Some of the smaller units do use PSC compressor motors and can be revived by the addition of a hard start kit.
 

Limited-Time

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LT, so the fan did come on and run okay initially. Yes So is the fan running all the time and when the compressor try's to kick on, the fan motor stops or comes close to stopping? Correct That doesn't sound like the fan starting capacitor is bad. Agreed But it does point to maybe some corroded terminals. If the fan comes on without the compressor running, then the fan starting capacitor is doing its job.Again agreed, I was looking to see if a replacement was available(should it be required in the future) because the original part was NLA on several online parts websites. The fact that it nearly stops when the compressor kicks in points to something draining the available current and the fan suffers from that.Yes, and the compressor motor never actually starts, that's why I think the compressors start cap is bad. That part is still available online. So unplug the air conditioner and start checking connections. Could be some iffy wire nut connections or screw connections need cleaned and you don't need any starting capacitors at all. JMHO!

GM, thanks for the response, see above in red.
 

Limited-Time

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The 10 uF cap on the fan motor is a run capacitor as most smaller AC fan motors are PSC (Permanent Split Capacitor) motors. Sounds like a problem with the compressor. Could be a number of things such as frozen compressor, open start windings, bad start relay, etc. First thing I would do is check resistance of windings of the motor. If that checked out ok then grab a few of jumper wires and a good start cap and by connecting the right connectors on the compressor and holding a jumper against the start winding pin check the compressor to see if it would run at all. Not for the feint of heart but the quickest way to test rather than throw parts at it. What was the rating on compressor capacitor? Some of the smaller units do use PSC compressor motors and can be revived by the addition of a hard start kit.

dwco, thanks for the input. I'm waiting for the replacement compressor motor cap. Once here I will replace it and see what happens. If its still no go after that I'll refer to your and gm's post and dig a little deeper, thanks again.
 

bigdee

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When I put it in place and powered it up the blower fan came on directly. However when the compressor triggers to cycle it does not, it causes the blower fan all but stop. It seems as if the current supply is being drained.

Compressor is drawing LRA (locked rotor amps) you should hear the klixon click after a few seconds which breaks the circuit to compressor. Capacitor being bad can cause this OR compressor may be bad. Also check the 3 terminals on compressor for burnt connections.
 

gm280

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GM, thanks for the response, see above in red.

LT, I think I misunderstood what was wrong after reading your reply to my response. I thought, wrongly it seems, that your fan motor was the issue. Now I know it is the compressor. I think you are on the right track with the starting capacitor now. Sorry for the long previous post. I should read more and type less. :sorry:
 

bigdee

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Just to clear something up....window units (and most residential units) do not have start capacitors on compressor! They have a run capacitor but if it goes bad it will give symptoms like you stated.
 

Limited-Time

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The rest of the story.....so I replaced the compressor cap, same sheet.😱😱 the blower/condensor fan starts gets up to speed and stalls out. This time when its stalled out I push it in the direction it turning and it's rubbing on something. So I rotate it backwards and it frees up and repeates the stalling out. I do this several times then turn the unit off. Closer inspection of the axial fan revieals a crack in one of the blades. The crack was causing the blade to flair out and rub on the shroud. It was flairing out in a manner that held it aginst the shroud until it was turned off and could flex back. After finding the crack I pushed on the blade and it broke off. I powered the unit up and it ran fine except for a vibration from the unbalanced fan. The fan is NLA😢. So it's going to run with a vibration until it cools off enough here and I can pull the unit apart and find a crossover/replacement axial fan. Thanks again for all the responces👍👍👍
 
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hrdwrkingacguy

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Not sure if it was mentioned or not but the 250v rating is the back emf the motor will put out. A 230v motor will put out 330ish volts on the start winding due to resistances and acting like a generator...you can use a higher number, but don't go lower. Typically we use 330v caps but they can be 440v if necessary. But don't replace a 440v with a 330v. It won't last long. :eek:
 
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