Motor Capacitor Replacement ?s

Thanks Will. They're probably the exact same caps I'm gonna get from McMaster, but branded otherwise. I know about caps via DC (I've built circuits in the past) but wasn't too sure of them pertaining to AC motors. But, this is how we learn. I'll get mine on Friday. We shall see.

Believe it or not, there is no storage of AC voltage possible in a capacitor.
the capacitor stores and releases the voltage as a DC conductor only.
so when we use capacitors in an AC circuit, the caps charge is DC and we introduce DC power into the AC system.
 
Finally got the time to replace the caps. There's no schematic or circuit drawing on the cap or inside the cap covers so I can't tell which lead is which. I guess I'm used to DC polarized caps, guys. Are these run and start caps non-polarized (doesn't matter which lead goes to which cap pole)? Thanks guys.
 
AC, non polarized so it doesn't matter.
 
I replaced the motor on my compressor a while back and left the old one in the corner. Local scrap guys picked it up a month ago or so.

I should have pulled the capacitors off of it 1st, but didn't even think about it till now. Windings were all smoked on it so not a cap failure. Anyhow just a note to think about that if you are tossing a motor.
 
Oh yes, I learned that cap drain lesson long time ago. It was a ....shocking moment. :p You definitely do not forget those lessons quickly. BTW, I first thought the centrifug switch was the culprit. It still could be but I doubt it. I thought of going to 3ph VFD setup but it'll have to wait. I'll be happy to have the lathe running properly again.

I have worked at a Ford Dealership since the mid 70's. Back in those days we had car parts called, points and condenser. If you charge the condenser and lay it on the break room table, you are sure to hear some cussing in a short time.
Yeah, I got bit once.
What does, "can't win for losing", really mean anyway?
Can't help with the capacitor/centrif. though, I am as ignorant about electrical motors as the next guy.
 
I replaced the motor on my compressor a while back and left the old one in the corner. Local scrap guys picked it up a month ago or so.

I should have pulled the capacitors off of it 1st, but didn't even think about it till now. Windings were all smoked on it so not a cap failure. Anyhow just a note to think about that if you are tossing a motor.
Electrolytic capacitors do go bad over time. Make sure you test an old one properly before using it. A motor shop can test it quickly, and usually for free.
 
Generally speaking, electrolytic capacitors (which motor caps are) are polarized devices meaning that they have to be put in a circuit with a specific polarity. Motor starting caps are an exception. They are considered to be nonpolarized but technically, internally they're two polarized caps wired in parallel with one another but opposite in polarity. I bring this up just to point out the importance of choosing the right kind of capacitor. For instance, you may run across a 150uF cap rated at 250V and think you can use it on your motor since it has the right rating. This is not necessarily so unless the capacitor is made specifically for starting an AC motor.

One other thing....these caps, even though they're AC can still hold a charge of one polarity or the other. The amount of charge really depends upon when in the AC cycle the power was turned off. So be careful when swapping them out. The purpose of the starting cap really isn't to hold charge, but to cause a phase shift of the AC voltage that's being applied to the motor. One set of motor windings gets straight AC and the other set gets a phase shifted AC. This gets the magnetic field within the motor to start revolving around the rotor. The magnetic field then drags the rotor along with it do to magnetic forces acting on the rotor windings. When the cap goes bad, the magnetic field stops rotating and the rotor stalls. Thats the gist of it anyways....probably more than you wanted to know, LOL. Count yourself lucky....my students would have had to endure this for an entire semester. :oops:
 
Hi Tony- I believe the non-polarized caps you mentioned are actually two caps wired in series, you see these in loudspeaker crossovers and such; they have a plus sign on both ends. A 50 uF unit would consist of two 100 uF caps in series.
The ones for ac motor use are truly non-polar oil filled units, they are physically large given their relatively small capacitance values for this reason.
Mark
 
I think the confusion here is that yes, the cap is wired in series with the winding, but internally what you have are two caps wired in parallel but with opposing polarity. I've not actually taken one apart but I would imagine that there are diodes involved in order to protect the caps for the opposite half cycle that they'd be in the wrong polarity.
 
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