Using 216uF 330V motor start/run caps as 20A capacitive current limit. - Page 1
Forum section: electronics / general

author: Psi
author_rank: Super Contributor
timestamp: March 09, 2020, 10:03:00 pm
content: As the subjects says, can you use a 216uF 330V motor start/run caps as 20A capacitive current limit at mains voltage?https://www.digikey.com/product-detail/en/cornell-dubilier-electronics-cde/PSU21630A/338-4153-ND/1551642The capacitive reactance of 216uF at 60hz is correct for 19.5A. (12.28 ohms)My question is more about how the capacitor will handle this task and if there are any dangers from it overheating etc..(I'm fully aware of the dangerous working with mains voltage on things that are not mains isolated.)

author: wraper
author_rank: Unknown
timestamp: March 09, 2020, 10:14:51 pm
content: It's crappy current limit. When you will connect such capacitor to the mains, inrush current can easily exceed your calculated current many times and blow fuses. And char switch or connector you used to close the circuit.

author: Psi
author_rank: Super Contributor
timestamp: March 09, 2020, 10:30:37 pm
content: yep, makes sense.For this application it doesn't matter too much if inrush exceeds the current limit for a sec.Also i can bring it up slowly with a variac so the inrush isn't a problem.

author: Zero999
author_rank: Super Contributor
timestamp: March 09, 2020, 10:34:55 pm
content: I doubt it will be able to pass that current for long, without overheating. No mention of the maximum ripple current or ESR is mentioned in the data sheet, but a power factor of 10% maximum, implies it will dissipate hundreds of Watts, even if it's less than that.

author: Psi
author_rank: Super Contributor
timestamp: March 09, 2020, 10:44:51 pm
content: Quote from: Zero999 on March 09, 2020, 10:34:55 pmyea, these are the sort of questions i'm interested to know about.

author: wraper
author_rank: Unknown
timestamp: March 09, 2020, 10:55:17 pm
content: Quote from: Zero999 on March 09, 2020, 10:34:55 pmYeah, it's simply bipolar electrolytic cap intended for very short operation time. Often you can find a few minutes rating printed on them. They go bad very fast when not disconnected.

author: Zero999
author_rank: Super Contributor
timestamp: March 09, 2020, 10:58:20 pm
content: Quote from: Psi on March 09, 2020, 10:44:51 pmWork it out. Assume a power factor of 8%, which is better than the 10% worst case. Calculate real power, from apparent power. Note the size of the capacitor can and imagine how hot it'll get. I haven't done any calculations but it's obvious to me, the capacitor will get very hot, very quickly if it's just connected straight across the mains!

author: wraper
author_rank: Unknown
timestamp: March 09, 2020, 11:01:14 pm
content: It doesn't have ripple current rating but don't expect it being able to survive more than a few amps for any prolonged time.

author: alpher
author_rank: Frequent Contributor
timestamp: March 09, 2020, 11:12:41 pm
content: Start/run doesn't exist, either run capacitor designed to "run" continuously or "start" cap designed to be connected for < 1 min (usually a couple of sec).Run caps top at ~50 micro F, so 216uF has to be a start cap and as such most likely will smoke.

author: duak
author_rank: Super Contributor
timestamp: March 10, 2020, 03:49:37 am
content: What sort of load will it be used with? If an offline power supply with a rectifier input, it will probably limit at a lower current than expected. If it's an inductive load, like a transformer, there's a danger of it forming a series resonant circuit, saturating the transformer and causing more current to flow than expected. I recall someone writing this up as a trick to make the transformer act as a voltage regulator.The film caps used in HVAC motors often have a lead that disconnects itself from the wound foil plates if the cap overheats and swells up. I don't know if electrolytic caps do the same. I suspect they do because the ones I've looked at seemed to be open circuit rather than a partial value.

author: WattsThat
author_rank: Frequent Contributor
timestamp: March 10, 2020, 04:15:48 am
content: Motor start caps will not handle anything more than a few starts an hour and when you attempt to repurpose them, be prepared for them to blow their guts all over the place with amazing force.BTW, DigiKey is NOT the place to buy such caps, the MRO supply houses online are a fraction of the price of DK and the shipping can be cheaper too. For example:https://www.supplyhouse.com/Motor-Start-Capacitors-19703000

author: Psi
author_rank: Super Contributor
timestamp: March 10, 2020, 10:43:02 am
content: Cool. Thanks guys.That answers all my questions.