Motor Capacitors and the power of Utube

[acheld]

Whoever said amp building wasn't practical?  Not true.

Last week, we wake up with a our second floor AC temp sensor way out of range.  It works, but I have the HVAC guy come out, and of course he says:  sure I can fix it, but your compressor pump is drawing max current, and your condenser coils are severely corroded.  And yeah, I looked and sure enough it looked bad, and yes when I put the clamp over the wires, it was just over the limit.   

The downstairs AC was working fine, but also had ugly corrosion involving the condenser coils.   

My family is not in the best of health, so I pull the trigger and say the magic word:  replace.   

They had the upper floor unit in stock, but the lower floor unit (which was working) would not be in stock here in MD until mid July at the earliest due to supply chain issues.  So, I had the guys come to install the upper floor unit today.

Last night, while practicing with our local combo over JamKazaam, I noticed warm air being circulated by the lower unit -- the one that had been working, and that can't be replaced until last week of July.  Oh f***.   Interestingly, the inside blower was fine, the compressor was making noise like it was trying to work, but the outdoor fan is not spinning.  My Navy linguistic skills were exercised at that point.

Fast forward 12 hours.   This morning, while the installers were removing the upper unit -- recalling this was the unit which was working fine, but with an outside temp sensor issue -- I spent some time online.  Sure enough, the symptoms were there -- compressor is running, but outside fan is not.   Yep, you can kind of start it by spinning the fan, but there is no torque. 

Who knew a cap can would fail in this way -- I am sure a bunch of you do know, but I did not. 

So, while the installers watch me with great, but unspoken skepticism.  I opened up the unit they had removed and took out the motor/comprssor cap can -- after all, that unit was working fine before it was removed.   It was dirty, but looked ok.

Now the installers are intrigued, but staying away. And I mean backing away like you don't want anything to do with what is going on.  I disconnected the power on the lower (now non-working) compressor/fan unit, opened it up and found a bulging cap can.   

Hey, I know what that means!   So, after discharging the caps, I removed it and reinstalled the cap from the working unit -- thankfully the two units shared the same spec cap can.

Reconnected power, and there you go.  Working AC!    This took five minutes to remove the working cap can, and another 5 min installing it in the now non-working AC unit.

What a stroke of luck that this thing failed just when the old working (but decrepit) compressor was being replaced.  And I am grateful that someone took the time to do a video on what the symptoms of a motor capacitor failure are. 

[Willabe]

Yep, AC went out last summer, fan not running. Opened up the outside unit, found the fan cap. Went to the AC supply house, new cap ~$10. Put it in, AC still running fine. 

   

[Ed_Chambley]

Any hobby I have ever gotten obsessed with, which would be everything.  I am not the poster child for moderation.  I have the knowledge translate into other areas.  Sort to builds upon itself.  Provided someone hasn't tried to fix it before I do, I can usually figure things out.


I ain't never figured people out tho.  I wouldn't be a people if you paid me!

[PRR]

> kind of start it by spinning the fan, but there is no torque. Who knew a cap can would fail in this way --

If you stick your finger in the fan enough times, you can learn some. A simple single-phase AC motor does not know which way to turn at start-up, thus it never starts, just moans and twitches. A 2-phase motor starts fine, but where do you get 2-phase power? A reactance will do, but how big? how good? Well, especially on a fan, you do not need real solid 2-phase because a fan is nearly no-load until it comes up to speed. In the old days, we built a choke into the motor to delay the phase, but costly and inefficient. Caps got better mid-century, and we have cap-start motors (a fan may use the one cap for both start and run).

Another tidbit you may know: when you take these caps out, short the terminals before you put fingers on them. If all is well, the motor winding discharges the charge, but if all was well you would not be messing with it. There's a fair chance the cap is now a dead-short and can't hold charge, but you can't be sure yet. So stick your screwdriver across the tabs.

[dude]

So stick your screwdriver across the tabs.

I've been shocked, after draining the can with a screwdriver, caps can have have memories, right. I with an air conditioner and the like, after shorting the can, have a alligator clip with a resistor to keep that memory in check. I have no idea if the clip is needed but after getting quite a jolt with a filter cap once, I don't take chances. The voltage was 160dc, luckily my other hand wasn't on the chassis.

[shooter]

I ain't never figured people out tho.  I wouldn't be a people if you paid me!

nope, me either.  I used to have panic attacks at malls, I cured that by waiting long enough for malls to fail 

[acheld]

The tech I watched on YouTube actually made the comment that the correct method of discharging these caps was to -- as we do so in our amps - discharge across a large resistor to ground.   As he says that in the video, he sticks his screwdriver into the cap leads and wiggles it around -- and then says, "but in the field we just do this."  I had to laugh. 

Me, I discharged the good cap after removing it and then double checked with my cheapo DVM.

I really had no knowledge of electric motors until recently, but have started to nibble around the edges of the motors being designed for cars.  The EE is amazing.  And yeah, PRR, I often wondered how the motor knew which way to turn . . . 

[PRR]

> which way to turn . . . 

I forgot to point out: this is similar to starting a 1-cylinder steam engine or a 1-leg bicyclist. Once it gets up to speed it runs OK. But it can't get started without an added hint.