How to determine what the winding resistance should be on a refrigerator compres - Page 1
Forum section: electronics / repair

author: Yamin
author_rank: Frequent Contributor
timestamp: June 15, 2019, 08:03:53 pm
content: Hi guys,I've got a ice cream display which when the compressor turns on it trips the circuit breaker. I have ruled out loose connection etc. and it all points to the compressor.What I have found is that the run capacitor which should be 20 uF is 14 uF and the Start relay contacts have arched and corroded away. I am going to replace both of those. (I changed the capacitor and tried the relay after cleaning and sanding down the contacts, still the C.B kept tripping).Anyway going to my original questionI measured the coil resistance of the compressor and I get:Common ---- Run = 1.5 ohmsCommon ---- Start = 5.4 ohmsStart ----- Run = 6.9 ohmsDoes this look alright, 1.5 ohms seems low doesn't it? and I also checked whether the compressor was grounded and it wasn't.Any help would be greatly appreciated.

author: joseph nicholas
author_rank: Frequent Contributor
timestamp: June 15, 2019, 08:21:56 pm
content: Hi, You should be aware that compressors generally run hot when functioning normally. The usual failure mode for these things is the thing just wears out and starts drawing more and more current. I would check the current draw and compare it to the manufactures specifications as a place to start. Hope this helps.

author: Jeroen_Bezemer
author_rank: Contributor
timestamp: June 16, 2019, 08:50:18 am
content: Check for isolation breakdown with a megger of at least 500V against ground/chassisVerstuurd vanaf mijn ONEPLUS A6003 met Tapatalk

author: Dacke
author_rank: Regular Contributor
timestamp: June 16, 2019, 12:22:44 pm
content: There should be an LRA (lock rotor amps) number on the plate /sticker of the compressor, this is current draw at start up. It should reach close to this number when it starts then drop off very quickly to lower than RLA (run load amps). You'd need an amp clamp to check this. It's hard to tell anything by the resistance of the windings except that the order looks fine: the lowest resistance will generally be between common and run with highest from start to run.

author: bdunham7
author_rank: Super Contributor
timestamp: June 16, 2019, 04:10:02 pm
content: Quote from: Yamin on June 15, 2019, 08:03:53 pmWhat size is this compressor--in terms of HP, kW or volts/amps? And which 'circuit breaker' is tripping--the one in your electrical panel or the thermal cutout in the compressor?

author: amyk
author_rank: Super Contributor
timestamp: June 16, 2019, 06:36:14 pm
content: Due to inductance, DC resistance will look a lot lower than the rated running current implies.

author: Yamin
author_rank: Frequent Contributor
timestamp: June 18, 2019, 05:57:09 am
content: Hi guys, just to give an update. I had a look at the data sheet of the compressor (NJ2212GK) and the resistance rating for the start winding is 7.25 ohms, the rated resistance value for the run winding is 1.95 ohms. So the readings I get are not 'so' off but there are a bit off. Could that be an issue.I have replaced the starter relay and the run capacitor and still the breaker keeps tripping. The locked rotor value for the compressor is 36A, and the current MCB that was connected to it is rated at 10A. Just for the sake of it I tried a 32A MCB to see whether it was an issue with the MCB. Still it kept on tripping. I couldn't get enough time to even check the current draw.Thanks

author: amyk
author_rank: Super Contributor
timestamp: June 18, 2019, 12:07:25 pm
content: I would try this next:Quote from: Jeroen_Bezemer on June 16, 2019, 08:50:18 amDoes the compressor actually try to turn, or is it stuck? In both cases it'll make a humming sound, but if it's stuck it'll sound like a loud transformer.