Help with common mode choke choice. - Page 1
Forum section: electronics / repair

author: Robert Smith Eco Warrior
author_rank: Regular Contributor
timestamp: March 29, 2023, 11:23:37 am
content: Hi All,We live off grid and our power comes from solar panels that charge a battery bank at 48v DC. We then use inverters to take this 48v DC and convert it to 230v AC to drive our appliances and tools etc.I was cutting about 100 wooden blocks using my De-Walt saw about a month ago and this was stop - start - stop - start x 100 and this cooked the common mode choke on the 230v AC output of the inverter. It was working while I was using it but then wasn't next time I came to use it. On investigation the windings had arced through onto the core and across to the other winding and had formed a blackened molten copper ferrous core mess.I can't get any information about the original and the part number doesn't relate to anything I can find.I unpicked the choke and measured what was left of the core and counted the winding turns. (58 turns per side and 1.32mm wire, with enamel).I got a new core of the same dimensions and wire from RS although I didn't know the core material and no way to find out but wanted to have a go at rewinding the choke as that would cost me around £30 and a new inverter is around £1100.I couldn't physically wind the wire neatly enough to get all 58 turns on each side. I was winding it neatly but not as neatly as a machine could do it.I gave up after hours of trying and went back to searching the internet again.I had looked at the Coilcraft website before but this time came across chokes that I hadn't found the first time around.These would physically fit and have suitable current carrying capacity but I have never ventured into understanding inductors so am wanting some guidance.I can tell you all I know...The inverter is a 3000vA unit so if worked hard could provide a continuous 13 amps at 230v AC at 50Hz. It is also capable of running to twice this for very short durations for motor starting etc.The inverter takes the 48v DC and then, using a big bank of mosfets or similar, switches the power for varying times to create an averaged out 'true sine wave' with a 20kHz switching frequency. This 48v AC goes through two (I don't know why two) big toroidal transformers to bring it to 230v AC.It then goes through some current measuring circuitry, a 20 amp fuse and through the choke and its four accompanying capacitors before heading out to our household items.I have the four chokes highlighted below and wanting to choose the most appropriate one. They have arrived here now and the wire on the two 12 amp chokes is thinner than the original so as the original died due to overheating I am thinking my choice is maybe between the two 15 amp ones..... but I don't understand the induction figures so wondering which to use?Edit: Would it also be good to use the spare 15amp one in an external filter?

author: jonpaul
author_rank: Super Contributor
timestamp: March 30, 2023, 11:05:51 pm
content: the saw and,similar motor drive tools, a both inductive loads and have high starting and stall current.The resulting high voltage transients and currents can dégradé and ruin inverters unles transient protected and rated,for such services.I would carefully check the tool ratings, and inverters nameplate and specs.I suspect the CM choke failure is only a symptom of transients stress.Consider a gas or propane powered generator or get a tool that's gasoline engine, not electricJon

author: Kim Christensen
author_rank: Super Contributor
timestamp: March 30, 2023, 11:21:54 pm
content: QuoteNone of those new chokes have 58 turns per side... Though the core material will also determine the inductance, I'm guessing the original had more inductance than any of those you ordered so I'd choose the highest current and highest inductance choke and see how it goes. These chokes are for RFI suppression, so the worst that's going to happen by choosing one with the wrong inductance is a bit more RFI emitted by the inverter.

author: Robert Smith Eco Warrior
author_rank: Regular Contributor
timestamp: March 31, 2023, 09:09:06 am
content: Quote from: Kim Christensen on March 30, 2023, 11:21:54 pmThank you.I have asked this question in several places and someone has finally given me a useful answer.When I tried to rewind one myself on a new core I could get to about 48 turns on each side. As I did not know what the effect of going from 58 to 48 turns would be I thought it might be best to buy a ready made choke even if it has far less turns as there may be other factors going on that I don't know about.I shall do what you suggest as I want to get it back up and running.