Both caps blew! Why?

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My compressor wouldn't start this morning. So I figured it might be a
bad starting cap. But both caps had blown. All the guts right out of
the ends. Could it be that one cap blew first and then the other cap
failed because of the extra work it had to do?

Re: Both caps blew! Why? says...
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Could be.

Did you have any lightning due to stormes in the ares from the last time  
you used it ?

Re: Both caps blew! Why?
On Wed, 18 Jul 2018 14:19:18 -0400, Ralph Mowery

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No lightning. It worked fine yesterday and this morning wouldn't
start. The compressor is outside  so I wouldn't be able to hear when
the caps blew. So I don't know if they both blew at the same time or
if one blew and then the other. The caps are 115 volt caps and the
motor is 220 volts so each cap must be connected to one hot and the
neutral. Which makes me think that the motor may have been running on
just one cap for at least a little while. Maybe just yesterday. I
don't know how old the caps are but I have had the compressor for 4
years and I bought it used.

Re: Both caps blew! Why?
Are you sure they're start caps ? Some 3 phase motors can be run with a cap
 connected the right way, those are run caps. What I have seen is they are  
usually rated double the input voltage, like if it is 220 the caps are 440.

Per your other thread, if they were 115 volts and in series you had like 20
0 uF, not ridiculous. High, but plausible. Even then the voltage rating is  
not high enough. If in parallel then you had like 1,000 uF which is very hi
gh, is this like a 5 HP or something ? In any case a 115 V cap will not tak
e the voltage.

Re: Both caps blew! Why?
On Wed, 18 Jul 2018 16:39:47 -0700 (PDT), wrote:

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In my other post I corrected myself. The caps were wired in parallel.
So the total capacitance is around 1000 MFD. Just for fun I tried a
200 MFD cap and it would not start the motor. The motor would just
barely start rotating but I could tell it wasn't gonna come up to
speed before something got too hot. What happens if the capacitance is
too high? I think the phase will just get shifted too much which may
cause hard starting. On the other hand I seem to remember that the
starting cap doesn't just help with phase shift it also provides extra
energy for starting so maybe extra capacitance isn't all that bad. I
don't remember but would like to know.

Re: Both caps blew! Why?
A start cap is only in circuit during startup, it is switched out usually b
y centrifugal switch once a certain RPM is reached. Such a motor usually ha
s more starting torque, but the cap is not right for efficient running.  

A run cap is in the circuit all the time, and its value is important to eff
iciency and even running. If you take a run cap out of the circuit the moto
r will slow and slow and eventually stall under the load of the backpressur
e of the Freon. If you remove a start cap it can run forever but not start  

Only larger systems use a start cap these days, and even 40 years ago. Got  
to remember this is 2018 and 40 years ago was 1978. Maybe more did back the
n but probably not unless it is like a 5 ton unit or bigger. (5 tons is 60,
000 BTU) Is it that big ?  

To find out, disconnect the cap while it is running. Use the one hand rule  
and insulated pliers, usually they have spade lugs and are not hard to pull
 off. Another way is that there will be no voltage across a start cap once  
it starts, but a run cap always has voltage across it. And put your meter o
n the 1,000 volt AC range.  

But first you need the cap. Price them and see what kind of deal you can ge
t on like a "set". Couple of 100s, 200s and 50s. Total that gives you 650.  
If the original even really is 1,000 that should at least started unless it
 has head pressure. (when the Freon is high pressure at the outlet and low  
pressure at the inlet)  

I don't know the color comes for the wires, someone who does might but they
 won't work for free. You get into those things and you might have differen
t voltages and maybe even speeds. Furnace blower motors have a gang of wire
s, and they are nowhere near a compressor motor.  

Now, if you find it to be a run capacitor, somehow figure out how to measur
e current draw. Clampon ammeters are not expensive anymore, get the elcheap
o Chinese kind. You have all these caps, and the optimum one will be the on
e that results in the lowest current drain. If with all of them in circuit  
the draw is lowest, go get like a 750. Then fine tune that. Keep throwing m
ore and ore on there until the draw starts to go up. Once it gets past its  
peak back off. Well not peak but minimum. When you find the lowest current  
draw you have found the optimum cap value.  

I don't now where you are but I am in the US and the last time I got motor  
caps it was from WW Grainger. If you have ANY company name, use it, the pri
ce changes. Five caps might be $ 20 each, that's a Cnote. If you get them c
heaper it matters. Just remember they should be 440 VAC rated. Whoever put  
those 115 VAC caps in there should be flipping burgers.  

Need to know if it is a run or start cap. If it is a start cap forget all I
 wrote. But I really think it is a run cap.

Re: Both caps blew! Why?
On Wed, 18 Jul 2018 19:52:29 -0700 (PDT), wrote:

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Actually the proper caps are 115 volt caps. Seems weird on a 220 volt
motor but if you look at a wiring diagram for a dual voltage, 110/220
volt capacitor start motor you will see why. The caps only ever see
110 volts. It turns out that the capacitance is so high because the
motor is meant for starting high torque loads, like an air compressor,
which is what it doing. Since Grainger paired this compressor and
motor I guess they knew what they were doing.

Re: Both caps blew! Why? says...
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As the voltage is usually over 120 volts ( at least here at my house I  
monitor the voltage and it seems to run from 119 to 124) much of the  
time seems the 115 volt capacitor is slightly low now.  Most of the  
places I see recommend replacing the capacitors on the 240 volt motors  
with some around 400 volts, but those are the run capacitors.

Re: Both caps blew! Why?
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AHA ! I don't know why but I thought you meant a compressor for a central a
ir condensing unit. Some people call that the "compressor".  

So, I stand corrected.  

In a condensing unit it won't start when there's backpressure, it'll kick o
ut on overload a few times until the pressure bleeds off. In fact some newe
r electronic thermostats have a delay so it won't even try to start right a
way say if it just turned off and you reset the temperature, it will wait a
 few minutes. An AIR compressor now, well I can see my mistake. If you have
 it set to 100 PSI it will start with over 90 PSI loading it so a cap run t
ype motor won't cut it.  

Glad you got it figured out.

Re: Both caps blew! Why?
On Fri, 20 Jul 2018 12:14:07 -0700 (PDT), wrote:

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The compressor has an unloader so that there is no pressure in the
cylinders when it first starts spinning. But of course as you said as
soon as it delivers air into the receiver it is pushing air into 90
PSI. I am still a little amazed by the large value caps though.

Re: Both caps blew! Why?
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Eric "

Me too. Just now many HP is that thing ?  

Re: Both caps blew! Why?
On Fri, 20 Jul 2018 14:57:23 -0700 (PDT), wrote:

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Three HP.

Re: Both caps blew! Why?
On Wednesday, July 18, 2018 at 10:21:22 AM UTC-5, wrote:
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The caps blew because they are decades old, in a high stress application. O
nce the first one blew the second was way overstressed. AC rated caps are t
enuous beasts to begin with. My cousin in the HVAC business says compressor
 caps should be replaced every 5 years....

Re: Both caps blew! Why? says...
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I don't doubt that 5 year number for them.  I had one to go out after  
about that ammount of time on my heat pump.  A repair man replaced it  
and it took him about 15 minuits to find the problem and replace it for  
a cost of about $ 300.  I bought one off ebay for less than $ 20.  After  
about 5 years the capacitor went bad , so I replaced it myself and  
ordered another one for about $ 20 to keep as a spare.

  He also told me of a relay that often goes bad, so I bought one of  
those as a spare for about $ 20.  Sofar it has not needed replacing.

If replacing the capacitor , try to stay within the capacitance  
range,but you can go to a higher voltage rating.  Get one rated for 440  
volts if possiable.  Also look at the temperature rating.  While many  
are around 85 deg C see if you can find one rated for about 105 deg C.  
Those usually last longer.

Re: Both caps blew! Why?
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I had to replace ours a few years ago and the electric bill went down.  

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