AC motor capacitors. - Page 1
Forum section: electronics / projects

author: davelectronic
author_rank: Frequent Contributor
timestamp: July 04, 2021, 01:08:00 pm
content: I'm not sure if this is a begginers question or a project question. So I've got an airbrush compressor now, with two refrigeration compressors. It was a "silent" but new DIY compressor, but it wasn't silent enough for my needs. It's been converted to run these two refrigeration compressors. Just so you can see what I'm describing the unit is below. It's all running very well producing 8 bar 120 psi, I regulate that down to around 2 bar for the airbrush use. It's a 24 litre receiver so the air lasts a while between refills, which are automatically done via a pressure switch relay and solanoid unloader valve. My issue is the start run capacitors, they are 72uf 240 volts rating. One of the capacitors seems to get hotter than the other. I've yet to get an exact temperature, but after initial first fill of air, one capacitor is about 40°C the other feels at least 50°C. There is a definite difference between both capacitors temperature. I will get an accurate measurement of temperature, both compressors after the initial fill with air feel the same temperature. So my question is, am I looking to replace both start run capacitors ? I've looked around for temperature ratings of the AC capacitors, but the labels don't show working temperature on these types of capacitors. All components for the running of both compressors are identical. Start relay capacitors etc. Any ideas of this issue appreciated, and thanks for reading.

author: Siwastaja
author_rank: Super Contributor
timestamp: July 04, 2021, 01:21:02 pm
content: 50 degC sounds normal. These capacitors have ESR and there is loss of power due to I^2 * ESR, causing heat dissipation in the capacitor. Use it. If magic smoke is released or it gets so hot it burns the surroundings, or there are problems with motor starting or running, only then suspect a broken capacitor.

author: JayArr
author_rank: Contributor
timestamp: July 04, 2021, 01:55:18 pm
content: 40 or 50 is OK, I wouldn't worry until they get up around 80 or 100.

author: davelectronic
author_rank: Frequent Contributor
timestamp: July 04, 2021, 06:14:34 pm
content: Thank you for your replies, yes I will go with the current capacitors that are wired up to the compressor. I just thought it odd with difference in temperature between the two capacitors. But I guess with tolerance and losses, there not going to be matched. I will put a temperature thermocouple on there just to see the real temperatures between both.

author: davelectronic
author_rank: Frequent Contributor
timestamp: July 05, 2021, 01:23:12 am
content: So i measured the temperature of both capacitors, the hottest was mid 50's C and the cooler was mid 40's C 10 degrees C seems quite a bit between them both. I have been looking round for replacements, just incase one fails. But finding an AC capacitor with 72uf at 240 volts is tricky, there are 400 volts plus capacitors, but i doubt they would work. Managed to find a start motor capacitor at RS its 80uf at 240 Volts, looks the same capacitors as the one's in use on the compressor.

author: davelectronic
author_rank: Frequent Contributor
timestamp: July 05, 2021, 01:45:54 pm
content: Don't know if someone could give me a tip on which capacitors would be best ? I've found two ratings of capacitor, and was going to purchase a pair just in case of a capacitor failure. But from the two I'm unsure which ratings are the better match. The lower 77uf rating capacitor is 330 volts AC, and the 130uf capacitor is 250 volts AC. I think the lower rating microfarad capacitor is the closest value, but the voltage is quite a bit higher for that capacitor. The other capacitor has the closest voltage value at 250 volts AC, bit the microfarad rating is 130uf which is a lot higher than the 72uf 240 volt AC capacitors that are currently fitted. Any help with this appreciated. Both value capacitors are new stock. Each is in the images below. Thank you if anyone could suggest which capacitor would be better for this application.

author: antimatter503
author_rank: Contributor
timestamp: July 05, 2021, 01:52:45 pm
content: AC motor capacitors come in 2 flavors. Start capacitors like you have pictured and also run capacitors (silver metal can usually) make sure you don't use a start cap as a run cap or it will explode.The voltage rating only needs to equal or be above what your running. I would suggest the one closest in value to the original. Voltage being above doesn't matter at all in this case, it just means better insulation.

author: davelectronic
author_rank: Frequent Contributor
timestamp: July 05, 2021, 08:37:55 pm
content: Thank you for replying, yes those two capacitors above are listed as start capacitors. So that was lucky I didn't buy them and release the magic smoke. It's tricky trying to find the 240 Volts and close to 72uf start run capacitor. The closest I can find here in the UK is 80uf 450 Volts AC it's a start run capacitor, and another that is 60uf at 450 Volts start run capacitor. I can't work out why 240 Volt at around 72uf start run capacitor is so hard to find. Think I will have to keep looking, 450 Volt rating for a 240 Volt application has to be to high for single phase 240 Volts use.

author: Zero999
author_rank: Super Contributor
timestamp: July 05, 2021, 09:12:24 pm
content: Quote from: davelectronic on July 05, 2021, 08:37:55 pmJust use the 80µF 450V capacitor. The higher voltage rating is no big deal and a slightly higher capacitance doesn't matter either.

author: davelectronic
author_rank: Frequent Contributor
timestamp: July 05, 2021, 11:16:17 pm
content: Yes i think it will be ok, just when i thought it was usable compressor. Hopefully that will be the final thing to sort out. Oh other than some trim to surround the two fans (cosmetic) although would direct the air flow better. Thank you for the help working this out.

author: larryqiann
author_rank: Newbie
timestamp: July 14, 2021, 03:37:45 am
content: I personally would not be as concerned with the temperature of the capacitors as I would be the compressors themselves - Often hermetic compressors are suction cooled, which means a small amount of refrigerant, or at least the lower temperature from the evaporated refrigerant keeps the dome cool. There usually isnt any thermal coupling between the inside of the compressor and the outer case, since its all shock mounted for noise on springs. It would be best to make sure the oil pool at the bottom of the case stays as cool as possible for the longevity of the compressors, since air has different characteristics than refrigerant which may already be putting more strain on the compressor.A central AC compressor running with no, or incorrect refrigerant in it can shut down on overload in only a few minutes.

author: amyk
author_rank: Super Contributor
timestamp: July 14, 2021, 04:28:59 am
content: You mention "start run capacitors", but what type of motor is in your compressors? RSIR (no capacitor at all) or CSIR (starting cap only) is common, and higher power ones might be CSR (two caps, one for start and another for run) but PSC (one cap and start winding is on all the time) being used in a hermetic compressor is unusual.