What is a motor capacitor? - Page 1
Forum section: electronics / beginners

author: Faringdon
author_rank: Super Contributor
timestamp: January 19, 2022, 05:32:30 am
content: Hi,What is a "motor capacitor". The ones i speak of are metalised polypropylene and rated for mains useage.Why/How are they different from other types of film capacitors?Is it literally a capacitor which is supposed to just sit across line and neutral of the mains?....upstream of a motor? What kind of motor does it sit upstream of?Wiki tells of it,But does not allure to its comparison of ESL and ESR when compared to other types of film capacitors...and why it has to be for motors?....eg why not for SMPS?..Eg why not at the output of a Boost PFC SMPS?

author: Whales
author_rank: Super Contributor
timestamp: January 19, 2022, 06:25:46 am
content: A more precise name might be "single phase induction motor capacitor". These are used to create a second phase in single phase motors OR to lower overall voltage (speed) on a single phase motor (often only practical on fans). There are two subtypes:"motor run" capacitors can handle full load permanently (ie directly wired across A and N). These are a bit like X class capacitors in some ways."motor start" capacitors can only handle full load temporarily, but are cheaper and smaller.Packages are typically not PCB mount, they're traditionally a hand-replaceable part with connectors. They often last a shorter life than the equipment they are attached to.EDIT: Found my old topicOnline shopping: how do I find 'run capacitors' for induction motors?, it might be of use.

author: Whales
author_rank: Super Contributor
timestamp: January 19, 2022, 06:35:22 am
content: QuoteSingle phase motors are a really big market. The make the parts to "work with motors" and leave it at that, they don't always spec other numbers for the parts because their primary customer base doesn't care for using them at other frequencies, voltages, DC loads, etc.

author: rpiloverbd
author_rank: Regular Contributor
timestamp: January 19, 2022, 08:21:22 am
content: You may find this video helpful too:

author: Faringdon
author_rank: Super Contributor
timestamp: January 19, 2022, 08:51:24 am
content: ..Thanks, so its a pretty rubbish cap with high ESL and ESR?....shoudlnt be anywhere near a SMPS?

author: Whales
author_rank: Super Contributor
timestamp: January 19, 2022, 09:05:00 am
content: Quote from: Faringdon on January 19, 2022, 08:51:24 am"high" is a relative term. They would suit some SMPS and not others (topology, position, switching freq, etc). There is no blanket answer.If the caps you are looking at are missing specs that you think are important (for your application) then your options are to guess, contact the manufacturer or try other parts instead. I don't recommend guessing unless this is for fun or as an experiment.

author: Gyro
author_rank: Super Contributor
timestamp: January 19, 2022, 10:05:12 am
content: Treez, this is the third thread you have posted to try to find some back-door 'justification' for using bog standard 50/60Hz motor run capacitors in your 3.6kW half-bridge SMPS...https://www.eevblog.com/forum/renewable-energy/ripple-current-rating-of-film-capacitor-in-half-bridge-smps/https://www.eevblog.com/forum/beginners/what-is-the-ripple-current-rating-of-this-film-capacitor/As you have been clearly told in the other two threads, bog standard motor run capacitors are not designed to, and will not withstand, the dV/dt, dI/dt and operating current that will result in your intended application. Their metallisation is simply too thin. The better ones will expand lengthways and break the protective termination weak points. The cheaper ones will simply split and burn!Stop wasting the time of unsuspecting people with your hidden agenda!

author: Zero999
author_rank: Super Contributor
timestamp: January 19, 2022, 11:21:06 am
content: This isn't so you can avoid using electrolytic capacitors in an LED driver is it?Motor start capacitors are often non-polarised electrolytic, because they're cheaper and take up less space. As mentioned above, they're not designed for continuous operation.Motor run capacitors are typically metalised film, which is more bulky, but more robust. As mentioned above, they're designed for continuous operation.

author: Simon
author_rank: Unknown
timestamp: January 19, 2022, 07:03:21 pm
content: unfortunately the merge functionality of this forum is crap!

author: Gyro
author_rank: Super Contributor
timestamp: January 19, 2022, 07:42:05 pm
content: How about the purge functionality?

author: Simon
author_rank: Unknown
timestamp: January 19, 2022, 07:58:03 pm
content: Quote from: Gyro on January 19, 2022, 07:42:05 pmDon't tempt me!