Also, if I put two dc caps in series, -++-, I now have the equivalent of a non-polar cap,
and even when halving the capacitance, this construction is still smaller than an AC
motor run capacitor?

 

DC capacitors are usually Electrolytic which gets a much larger capacitance in a small space. Basically the dielectric is an oxide coating on one of the plates which can be very thin. AC capacitors have a plastic film (often polypropylene) as the dielectric (the insulator between the plates). This cannot be made so thin so the capacitor takes up a larger volume.
I don't recommend connecting two electroylitic capacitors in opposition because they need a permanant DC bias to maintain the oxide coating.

 

Also, if I put two dc caps in series, -++-, I now have the equivalent of a non-polar cap,
and even when halving the capacitance, this construction is still smaller than an AC
motor run capacitor?

I don't think you can make a non-polarized capacitor from two polarized ones. I do think that doing this carries significant risks including severe injury.

Edit: Not entirely accurate. See post #5 below

 

Two words: Ripple current.

There are two types of AC induction motor capacitors: start and run.

The start capacitors are indeed non-polarized electrolytics, and are only intended for intermittent duty, for the few seconds it takes for a motor to start.

The run capacitors are as described by Marley, film or paper capacitors. They can withstand the high AC ripple currents of the motor continuously.

If you attempt to use a start capacitor in a run capacitor application, it will overheat and vent out.

 

Excerpt from a Cornell Dublier application guide about connecting two polarized caps back to back to get a non-polarized cap:

 

Excerpt from a Cornell Dublier application guide about connecting two polarized caps back to back to get a non-polarized cap:
View attachment 208363

I would interpret this as saying that even if you can do it, you do have an alternative and should probably use it.
Good to know.

 

As, indicated, even the bipolar electrolytic motor start versions, should only be in circuit for one or two seconds.
Max.

 

Two words: Ripple current.

There are two types of AC induction motor capacitors: start and run.

The start capacitors are indeed non-polarized electrolytics, and are only intended for intermittent duty, for the few seconds it takes for a motor to start.

The run capacitors are as described by Marley, film or paper capacitors. They can withstand the high AC ripple currents of the motor continuously.

If you attempt to use a start capacitor in a run capacitor application, it will overheat and vent out.

Here's a weird question: Do the run/film capacitors have lower "ESR" than a regular electrolytic?

I'm asking because I am doing some experiments with capacitive discharge, have no 'scope, and
am looking for caps that dump their charge rapidly. (600v/10uf-200uf range). I'm assuming a run
cap rated for 450v AC is good for peaks to 600v...

 

Here's a weird question: Do the run/film capacitors have lower "ESR" than a regular electrolytic?

I'm asking because I am doing some experiments with capacitive discharge, have no 'scope, and
am looking for caps that dump their charge rapidly. (600v/10uf-200uf range). I'm assuming a run
cap rated for 450v AC is good for peaks to 600v...

Internal ESR of a capacitor is generally speaking small. It is the resistance in the external circuit that determines the discharge rate. It is critical in SMPS, but not really in AC applications at power line frequencies. Maybe I'm missing something.

 

Internal ESR of a capacitor is generally speaking small. It is the resistance in the external circuit that determines the discharge rate. It is critical in SMPS, but not really in AC applications at power line frequencies. Maybe I'm missing something.

I'm concerned about discharge at a 1Hz rate. Are some caps better than others in this regard, or is the resitance of the circuit the determining factor? I'm dumping the charge into the primary of a transformer. (maybe a new thread, but it's really all I have for now..)

 

For motor run usage I keep a stock of CDE (Cornell Dubilier) Film Capacitor 370V Polypropylene (PP), Metallized Radial, they are motor run and power factor correction & AFC rated.
AFC:
"Run capacitors now have an AFC rating, identifying the available fault current, which is the short circuit interrupting capability of the pressure sensitive interrupter in amperes. Most capacitors are rated at 10,000 amps, but 5000 amps is adequate for motor application"
Max.

 

For motor run usage I keep a stock of CDE (Cornell Dubilier) Film Capacitor 370V Polypropylene (PP), Metallized Radial, they are motor run and power factor correction & AFC rated.
AFC:
"Run capacitors now have an AFC rating, identifying the available fault current, which is the short circuit interrupting capability of the pressure sensitive interrupter in amperes. Most capacitors are rated at 10,000 amps, but 5000 amps is adequate for motor application"
Max.

Are you saying these caps can dump 5000A before fault?

 

Are you saying these caps can dump 5000A before fault?

Not sure exactly what the spec means exactly, I didn't write it.
The ones I have with the marking are rated for 370vac.
Max.