How is a motor run capacitor sized?

[aroby]

I am trying to understand the sizing of a run capacitor for the 2 motors in a valve actuator for a swimming pool valve.  Only one motor is active at a time, and the simple supply circuit (24VAC) is delivered by a separate control board sending power to the appropriate motor.  A Richey 35uF 150V NP capacitor sits across the motor.  I assume it is a run capacitor that delivers a smooth flow of power to the motor, which has some pretty decent torque.   

I need to replace the small board in the actuator and would like to replace the capacitor at the same time.  But these capacitors are very difficult to find and seem to cost USD30 or more.  So, I'm looking for alternatives, however I am not clear why this is 35uF and why it is 150V.  Can someone help me understand how these values are determined?

Thanks!

[Benta]

That it's 35 uF is printed on the motor. Period.
That it's 150 V is open to discussion.
It's a non-polarized cap, meaning either paper (no longer common) or poly(xxxxxx) insulation, and for both it's unlikely finding types below 150 V.
33 uF might be easier to find and would work just as well.

BTW, it's NOT a smoothing cap, but there to provide a 90 degrees phase shift for the second motor winding.

[johansen]

i have no idea why they need a 150 volt electrolytic non polarized cap on a 24vac motor.

consider replacing it with any standard 35uF motor run cap. (6$, but they are 1.5" diameter and 4 inches long)

or try and find a 150vac rated film capacitor (typical 10uf cap is 1" diameter and 2 inches long)

or you may find they used a 150v non polarized capacitor so that it would survive 1000 hours of operation at 24 to 35 volts ac at an amp or so of current.

if you only need a 35 volt rated capacitor then you may be able to find a smaller film capacitor, or even ceramic capacitors for less than a 6$ (huge) motor run cap.

[Benta]

electrolytic.

[bdunham7]

electrolytic.

That certainly looks like an NP electro to me.  They probably used a 150V one to get a high enough current rating.

https://www.cde.com/resources/catalogs/BPA.pdf

[johansen]

Size and cost constraints.

A 35uf 63vdc 40vac rated film cap is 10$

[aroby]

Yes, I didn't explain it well. It looks like two motors, but probably is just one with the secondary winding and with the phase shift you mention.   There are 4 wires to the motor, two "common" and two "hot", the capacitor is across the two "hots". 

[aroby]

Size is the issue, there's not much space in the housing.  I found it surprising that the capacitors are a) very hard to find and b) extremely expensive, but I guess the two go together.

[Gyro]

electrolytic.

That certainly looks like an NP electro to me.  They probably used a 150V one to get a high enough current rating.

https://www.cde.com/resources/catalogs/BPA.pdf

Yes, it's an electrolytic, there's no point in putting NP on the other types because they are inherently non polarized. You often see them in speaker crossovers.

[Zero999]

Why do you want to replace the capacitor?

It looks fine.

Are there any signs of physical defects, not evident in the pictures?

Have you tested its value and effective series resistance?

[armandine2]

I am trying to understand the sizing of a run capacitor

  I assume it is a run capacitor that delivers a smooth flow of power to the motor,

I am not clear why this is 35uF and why it is 150V.  Can someone help me understand how these values are determined?

Thanks!

.... not sure this is going to help - or is wholly or vaguely correct  - but here goes something to ponder:

single phase with no phase shift means you don't get a rotating flux - it just pulses and you have no torque on to the rotor.

introducing an auxiliary winding, which is phase shifted up to 90 degrees using a capacitor will get you the rotating flux.

the primary winding on a phasor diagram is heading northeast and with an LCR meter you could plot that - and determine the capacitor value to shift the phase to the auxiliary winding 90 degrees. The resultant ideally heading off to the East.

I got this half remembered from Slemon -
https://www.amazon.co.uk/Electric-Machines-Gordon-R-Slemon/dp/0201077302

[aroby]

Why do you want to replace the capacitor?

It looks fine.

A friend of mine also has one of these actuators and his board got eaten away, more than mine, with saltwater dripping on it.  I made a replacement board, but need the cap!

[bdunham7]

What is the exact size?  Especially diameter and lead spacing.

[aroby]

What is the exact size?  Especially diameter and lead spacing.

12mm diameter, 38mm long

[bdunham7]

I assume these motors run only very intermittently and only for a minute.  Do you know how much current they draw?  These might work OK.

https://www.ebay.com/itm/115457681769

OTOH, if you can get the exact replacement for $30, that might be the way to go.

[aroby]

I assume these motors run only very intermittently and only for a minute.  Do you know how much current they draw?  These might work OK.

https://www.ebay.com/itm/115457681769

OTOH, if you can get the exact replacement for $30, that might be the way to go.

Interesting, thanks.  I didn't think to look on eBay!  The motor does only run for maybe 30 seconds, and I believe the control board can supply up to .75A.

[Zero999]

Why do you want to replace the capacitor?

It looks fine.

A friend of mine also has one of these actuators and his board got eaten away, more than mine, with saltwater dripping on it.  I made a replacement board, but need the cap!

If you're making your own board, then presumably you can change the footprint, so long as it's not too big for the case.

Have you measured the voltage across the capacitor? Does it actually get anywhere near 106V, the RMS voltage with a peak of 150V? As mentioned above, the voltage rating is probably selected for the size.

I would use two 68µF low ESR capacitors connected back-to-back, to form a 34µF non-polarised capacitor. Choose as larger case size as you possibly can, given the space constraints and voltage rating, which presumably doesn't need to be above 50V, although check first to be sure.

[johansen]

OTOH, if you can get the exact replacement for $30, that might be the way to go.

you can get a film cap which would last forever for that price.

[aroby]

If you're making your own board, then presumably you can change the footprint, so long as it's not too big for the case.

Have you measured the voltage across the capacitor? Does it actually get anywhere near 106V, the RMS voltage with a peak of 150V? As mentioned above, the voltage rating is probably selected for the size.

I would use two 68µF low ESR capacitors connected back-to-back, to form a 34µF non-polarised capacitor. Choose as larger case size as you possibly can, given the space constraints and voltage rating, which presumably doesn't need to be above 50V, although check first to be sure.

I'll investigate that. Trouble is the actuator is 1,300 miles away!  The board is quite space challenged and has a DPDT switch on the underside that pokes through the case and secures the board.   When you say 2 68uF back to back, I assume that is two polarized caps joined on the -ves and with the +ves connected to the motor?