How is a motor run capacitor sized? - Page 1
Forum section: electronics / projects

author: aroby
author_rank: Regular Contributor
timestamp: April 25, 2024, 06:18:39 pm
content: 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!

author: Benta
author_rank: Super Contributor
timestamp: April 25, 2024, 09:56:56 pm
content: 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.

author: johansen
author_rank: Super Contributor
timestamp: April 25, 2024, 10:06:33 pm
content: 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.

author: Benta
author_rank: Super Contributor
timestamp: April 25, 2024, 10:21:14 pm
content: Quote from: johansen on April 25, 2024, 10:06:33 pm

author: bdunham7
author_rank: Super Contributor
timestamp: April 25, 2024, 10:26:37 pm
content: Quote from: Benta on April 25, 2024, 10:21:14 pmThat 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

author: johansen
author_rank: Super Contributor
timestamp: April 25, 2024, 11:33:12 pm
content: Size and cost constraints.A 35uf 63vdc 40vac rated film cap is 10$

author: aroby
author_rank: Regular Contributor
timestamp: April 26, 2024, 12:19:29 am
content: 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".

author: aroby
author_rank: Regular Contributor
timestamp: April 26, 2024, 12:21:47 am
content: 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.

author: Gyro
author_rank: Super Contributor
timestamp: April 26, 2024, 08:56:11 am
content: Quote from: bdunham7 on April 25, 2024, 10:26:37 pmYes, 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.

author: Zero999
author_rank: Super Contributor
timestamp: April 26, 2024, 09:31:57 am
content: 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?

author: armandine2
author_rank: Frequent Contributor
timestamp: April 26, 2024, 11:35:56 am
content: Quote from: aroby on April 25, 2024, 06:18:39 pm.... 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

author: aroby
author_rank: Regular Contributor
timestamp: April 26, 2024, 09:25:52 pm
content: Quote from: Zero999 on April 26, 2024, 09:31:57 amA 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!

author: bdunham7
author_rank: Super Contributor
timestamp: April 26, 2024, 09:27:36 pm
content: What is the exact size? Especially diameter and lead spacing.

author: aroby
author_rank: Regular Contributor
timestamp: April 27, 2024, 01:47:51 am
content: Quote from: bdunham7 on April 26, 2024, 09:27:36 pm12mm diameter, 38mm long

author: bdunham7
author_rank: Super Contributor
timestamp: April 27, 2024, 05:05:08 am
content: 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/115457681769OTOH, if you can get the exact replacement for $30, that might be the way to go.

author: aroby
author_rank: Regular Contributor
timestamp: April 27, 2024, 02:58:57 pm
content: Quote from: bdunham7 on April 27, 2024, 05:05:08 amInteresting, 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.

author: Zero999
author_rank: Super Contributor
timestamp: April 27, 2024, 08:38:47 pm
content: Quote from: aroby on April 26, 2024, 09:25:52 pmIf 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.

author: johansen
author_rank: Super Contributor
timestamp: April 27, 2024, 10:26:17 pm
content: Quote from: bdunham7 on April 27, 2024, 05:05:08 amyou can get a film cap which would last forever for that price.

author: aroby
author_rank: Regular Contributor
timestamp: April 27, 2024, 10:53:03 pm
content: Quote from: Zero999 on April 27, 2024, 08:38:47 pmI'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?