Why are motor capacitors (AC) so much larger than DC caps for same value?

Thread Starter

Hamlet

Joined Jun 10, 2015
312
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?
 

Marley

Joined Apr 4, 2016
348
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.
 

Papabravo

Joined Feb 24, 2006
13,728
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
 
Last edited:

schmitt trigger

Joined Jul 12, 2010
242
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.
 

Thread Starter

Hamlet

Joined Jun 10, 2015
312
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...
 

Papabravo

Joined Feb 24, 2006
13,728
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.
 

Thread Starter

Hamlet

Joined Jun 10, 2015
312
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..)
 

MaxHeadRoom

Joined Jul 18, 2013
20,700
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.
 

Thread Starter

Hamlet

Joined Jun 10, 2015
312
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?
 
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