A.C. motor capacitors vs oil-filled filter caps

[k4kyv]

Murphy's  Law struck yesterday, Friday evening at 6 PM just before the long weekend, and we have guests expected.  The upstairs central air conditioner conked out. I checked the outdoor part of the unit, and noticed the compressor had popped the circuit breaker.  When I turned on the breaker, I could hear the compressor running, but the fan was not moving.  I could manually give the fan blade a spin using a stick, and it would turn, but slowly. If I started it in  reverse, it would keep on spinning in reverse.  Within a  couple of minutes the compressor popped the breaker again.

I opened the unit and checked the motor capacitor with my digital capacitance meter, and sure enough, the reading was zilch. The  capacitor is a two-section item, with 33 mfd for the compressor and 3 mfd for the fan.  The  compressor cap was OK. So I looked in  the junkbox for a temporary substitute. All the a.c. caps were way too high, between 10 and 40 mfd, but I found a 3 mfd 4000 volt oil filled transmitting capacitor that checked good. I JSed it in place using clip leads, and sure enough the fan motor now works normally. The unit has been running perfectly for over 12 hours now, so I'd guess the filter cap will be satisfactory till I can locate a replacement when the stores open Tuesday.

The only thing that might be of concern is the a.c. current rating of the capacitor.  A HV filter cap is designed to withstand high voltage DC, and carry a relatively small ripple current. OTOH, a motor capacitor carries a heavier current @ 60~. After installing the capacitor, I measured the a.c. voltage across it, and it read over 300 volts.  The original capacitor is rated for only 330 volts working according to the info stamped on the side. Maybe I should look for something with higher voltage rating for the permanent replacement, but 330 WV seems to be standard amongst all the a.c. caps in my junk collection.

[WD5JKO]

Don,

  I had the same thing happen here a few years ago. They combine the capacitors to cut cost an to perpetuate the lie that you cannot use two separate capacitors. In my case the fan motor had bad bearings. Getting exact replaement parts meant dealing with Trane, and over 1 kilo-buck if I did it myself. Instead I went to Granger (one in Austin). I bought a universal fan motor, and two capacitors all for about a hundred bucks. Done, works great!!

Here is one replacement idea:
http://www.grainger.com/Grainger/DAYTON-Motor-Run-Capacitor-2MDV2?Pid=search

I would also replace the compressor cap at the same time..cheap insurance.
http://www.grainger.com/Grainger/DAYTON-Motor-Run-Capacitor-2GU17?Pid=search


Jim
WD5JKO

[KE6DF]

There are plenty of 440 vac capacitors out there. Probably they would be a good replacement with high voltage ratings.

Here is an example:

http://www.ebay.com/itm/McQuay-P-N-037516401-3-MFD-440V-Oval-Capacitor-/300378167344?pt=LH_DefaultDomain_0&hash=item45efef1830

There are lots of two section caps with one section of 40 uf and another of 3 uf or similar. I'm guessing the idea there is to use one section for the compressor and one for the fan.

[k4kyv]

Probably someone knows a lot more about capacitor motors than I do. How critical is the capacitance?  The original one is marked +/- 10%.

I cannot see any conceivable reason why two caps combined into one case would be in any way electrically different from two separate caps. The only problem might be making them fit into the space.

[WD5JKO]

I cannot see any conceivable reason why two caps combined into one case would be in any way electrically different from two separate caps. The only problem might be making them fit into the space.

   Yes space and vibration are concerns. Might want to get the optional rubber top in case the JS mounting comes free. Also going from 1 cap to two changes the wiring harness since you go from three terminals to four. The repairman is doubtful to try to save you any $, and instead replace 100% OEM.

Jim
WD5JKO

[KA2DZT]

Probably someone knows a lot more about capacitor motors than I do. How critical is the capacitance?  The original one is marked +/- 10%.

I cannot see any conceivable reason why two caps combined into one case would be in any way electrically different from two separate caps. The only problem might be making them fit into the space.

Don,

Don't use too large a run cap.  Use a 3mfd if that's what was in it.  The start windings are thinner wire and should only carry a smaller current.  Although I not sure what type of motor you have.  On a capacitor start motor the start cap is switched out by the mechanical switch and is only putting current on the start windings for short period of time.  Start caps are big.  The run cap is left on the start winding supplying a smaller current.  Your fan motor is probably some type of split phase motor which works differently.  My uncle was a motor re-builder, he would know.

I see no reason why you shouldn't use two separate caps.  You may just need to add a wire.  You know how to add a wire??  You just cut into the right wire and solder in the extra lead.

Fred

[W2PFY]

Industrial battery chargers of the ferro resonant type have caps in the transformer windings. Capacitors that were just slightly above the operating voltage always failed. When we installed caps with at least twice the voltage ratings of the circuit, they never failed. I would go with a higher voltage rating if you can find them.

[W1ATR]

Don. Yeah you have to stay with the same capacitance rating, but the higher voltage is fine. You can replace that hv job next week with a simple 3mf 370vac cap. At three mikes, that must be a pretty small condenser with a 1/6hp common PSC fan motor. I probably have 20 caps like that on my shelves and they cost about 3 dollars. put the fan wire on one side, and make a small jumper to go from the other side over to the common terminal on the old cap. I do it all the time if I don't have the exact dual section cap I need on hand on a call.

But, realistically, if the cap is old enough that the fan side opened, then the hermetic side is probably not far behind. Just replace the whole thing with another 33/3mmf 370vac dual section for 10 bucks or you can up it to a 33/3mmf 440vac for a few bucks more. No need for any hammy hambone stuff under the hood, and call it a day.

J

[k4kyv]

  Yes space and vibration are concerns. Might want to get the optional rubber top in case the JS mounting comes free. Also going from 1 cap to two changes the wiring harness since you go from three terminals to four. The repairman is doubtful to try to save you any $, and instead replace 100% OEM.

The cap has three terminals.  One  common (which goes to one of the 230v hots) and one terminal each which goes to a separate lead to each of the motors. No wiring harness. I don't think the fan motor has a separate starter winding and relay to switch it in. The capacitor is in the circuit all the time; the motor will not begin to spin or attain full speed without it. I can manually start it by moving the fan blades with a stick in either direction and the fan will keep on spinning in that direction, but not fast enough to cool the radiator, and within a couple of minutes the compressor kicks the circuit breaker.  With the JS capacitor, the motor works normally, and the A/C unit has been running for almost 24 hours now.

Even if I replace the dual section cap with separate units, I'll probably replace both, since the compressor section of the old cap might soon go as well.

[DMOD]

Probably someone knows a lot more about capacitor motors than I do. How critical is the capacitance?  The original one is marked +/- 10%.

I cannot see any conceivable reason why two caps combined into one case would be in any way electrically different from two separate caps.

I would say +_10% cap value is sufficient. After all you are only causing a low current phase change in the LC circuit to get the fan turning with low torque.

As some others have stated, a 440V unit might be better.

I had this happen to my AC unit in Kansas before I moved and I ran separate 440V caps to the Fan and Compressor, because it seemed the 3 ufd was suffereing from a heat stroke due to starting current for the compressor.

Phil

[WA1GFZ]

I remember when motor caps were oil filled.

[k4kyv]

I found a drop-in exact replacement this morning for $12.  35/3 mfd @ 370v. The new cap is ± 5% instead of ±10%. Took about 5 minutes to install; the only tool needed was a screwdriver.

A service call to a HVAC repair service would have no doubt cost $100-200.

[K5UJ]

A service call to a HVAC repair service would have no doubt cost $100-200.

if they honest.  otherwise...."Well Mr. Chestuh, yew gonna need a new compressuh and I don't think we can retrofit anythang...we might have to set yew up a whole new system..."

[Opcom]

A service call to a HVAC repair service would have no doubt cost $100-200.

if they honest.  otherwise...."Well Mr. Chestuh, yew gonna need a new compressuh and I don't think we can retrofit anythang...we might have to set yew up a whole new system..."

But don't worry, this month, we have a special for $3000 off the regular low price (the Dallas Texas guy with his commercial again). Who believes such claptrap? Wait until the winter and he will do it "no profit, just to keep his men busy". Time to find an updated wholesale price list. Merchants and servicemen run the straight path when you know their costs.

[k4kyv]

But there is a warning decal on the enclosure stating that the access panel is to be removed only by qualified service personnel.

At least it didn't say "no user serviceable parts inside".

[Opcom]

"no user serviceable parts inside" means "open it, they are hiding something from you".

[N5RLR]

But there is a warning decal on the enclosure stating that the access panel is to be removed only by qualified service personnel.

In my case, I own the bloody thing; this is all the "qualification" I need.  Plus, I know my way around a tool set, have done my own repairs and handled/shipped HVAC parts at a previous employer.

Ditto on replacing both start caps if/when one fails.  Tip:  Write the date of replacement on the new cap(s) with a permanent marker.  And fitting two in the wiring compartment where one resided previously, can be a challenge but doable with a little creativity.