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My Cousin Vinny (Run capacitors and motor direction)

CDRIVE

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I received a phone call from my cousin Vinny today. It seems he has a weird problem with his home's air handler. The Cap Run SPH motor inexplicably runs backwards at times! My question is this... Have any of you guys ever experienced this? Would a shorted or very leaky run cap cause this? I would think that it would just struggle to start, run poorly, if at all ... and if it ran it would run hot. What say you?

Chris
 
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CDRIVE

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This is one of those times that the title choice may have been less than descriptive. This is something I bitch about often, but I couldn't help myself! Perhaps "Induction Motor Problem" would have been more apropos.

Hey guys, don't let me down here. My cousin Vinny is a fellow ElectronHead and I told him that we have a great forum at EP. Let's show him a some of that cerebral overload that we're noted for.

Chris
 

(*steve*)

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Google suggests that a sick run capacitor can allow the motor to start backwards.

But I'm no expert on these motors.
 

shrtrnd

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I don't know the circuit or the motor off the top of my head.
My question would be the physical situation concerning the load on the shaft.
*steve*'s Google search answer sounds like the culprit, but as a secondary consideration if that doesn't pan-out:
Just before the motor starts, is there any type of imbalance on the shaft load that would
cause it to turn in the opposite direction for the second or two when the motor is
energizing? The opening of a damper that would create a back-draft that would start
the load on the shaft turning backwards for an initial motor start-up?
A bent pulley, that could allow the weight of the load to start turning backwards at motor
start-up from a neutral position?
The issue sounds electrical, maybe the motor-type has a construction feature that
could allow this to happen if an internal (or external control) component failed.
I'm just offering the physical idea, because it sounds like your cousin might already
be aware of specific electrical issues, and the physical doesn't come to mind.
 

CDRIVE

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Hi Steve,

I considered Google but I figured that hits would return typical reversible induction motor wiring instead of the anomaly I'm looking for. I guess I should have tried.. You did and it paid off! Got a link? In the interim I'll give it a shot.

Thanks,
Chris
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Got a link?

Yeah, I try the "dumb as a brick" approach first. Even if I think I'm way too clever and know it won't work. Heh, it often does :D

The exact Google search was for "run capacitor motor backwards" (no quotes)
 

CDRIVE

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This is the kind of stuff I love. Check out this statement... "But a faulty capacitor does not cause the fan to run backwards. the running backwards is a symptom of having a faulty capacitor." It's statements like that that make me appreciate alcohol even more! Or maybe the alcohol is my problem?? :D

I found another hit where the responder commented that, and I quote,... " A weak capacitor could cause that". Presumably he's describing a cap that hasn't been exercising! :p

Chris

Edit: Wait! This might be a rare moment of lucidity! Come to think of it the first quote makes sense if you consider that a faulty cap would leave the motor vulnerable to start in either direction.
 
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(*steve*)

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Come to think of it the first quote makes sense if you consider that a faulty cap would leave the motor vulnerable to start in either direction.

Yes, that's how I interpreted it when I read it (perhaps not the first time...)
 

CDRIVE

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Well I guess this thread was really a desire for conformation of what I already surmised. I told Vinny from the get go that I couldn't imagine anything other than the cap causing this. I'm still surprised that he says it doesn't run hot though.

When the run cap on my air handler shorted I discovered the problem because it didn't sound quite right to me. In other words it didn't seem to be running at the RPM that my ears had grown accustomed to. I never did check for temperature rise but that was because an Ohmmeter check on my run cap confirmed a short, so I simply replaced it.

Thanks,
Chris
 

CDRIVE

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OK, this thread has been resolved. My cousin replaced the cap after finding this description of a PSC Motor described in the last paragraph.

One last note here. On my lathe & machinist groups cap start motor questions pop up all the time. These are vintage machine groups so the owners are typically not the original purchaser. More than once we've had members suspicious that the previous owner or owners had wired the motor incorrectly. This is because they expected the start winding to be heavier gauge than the run winding. They incorrectly deduced that the cap and centrifugal switch were inadvertently wired to the run stator. This of course is not true. The start windings are many more windings of much lighter gauge wire, which makes the start winding far more resistive (real power dissipation) than the run windings. The run windings are more reactive (apparent power) and present a much lower resistance to the mains than the start windings.

Chris
 

Y2KEDDIE

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It's been my understanding that the rotation direction is determined by the angular hase difference between the two fields created by the two windings. The windings create two separate fields and the one field tries to catch up with the other causing rotation of the shaft. With a shorted or open capacitor the second winding would not produce the angular shift, which typically stalls the motor. I can't see how it could possibly run backward.

I've may be wrong, I've seen some strange and humbling things.
 

CDRIVE

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I will admit that I don't have as much experience with cap run motors as I do with cap start versions. On those the motor direction is governed by the start winding. That said, my cousin replaced the cap and all is well. I would call that confirmable results.

Chris
 

CDRIVE

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Hey, I just noticed that the mods edited the title to be more descriptive.

Thanks, that's much better!
Chris
 
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