design of a delayed-AC-soft-starter  - Page 1
Forum section: electronics / projects

author: Sonny_a67
author_rank: Newbie
timestamp: November 25, 2021, 08:50:36 am
content: Hello everyoneI have an AC unit for a camper which I want to run off an inverter. The inverter is plenty powerful enough for the normal current draw of the unit, however there is a spike as for most inductive loads which can go up to 20A for 150ms and that's enough to trip the fuse on the inverter. The problem is, this spike does not occur when the unit starts up but a bit later. At first, the unit turns on the fan for 20 to 30 seconds and only after that starts up the compressor. That's when the spike happens. Therefore, I can not apply normal zero-crossing or similar soft-starting schemes to get rid of that spike.I was wondering if anyone has an idea what else can be done to tackle this problem.Thank you so much in advance!

author: sam2243
author_rank: Newbie
timestamp: December 12, 2021, 05:26:43 pm
content: hi sonnymy understanding is that it is the starter winding that causes the problem , there are products available to deal with that but are fairly expensive .I do not have experience in this area (at your own risk) but if it were mine I might try ntc inrush current limiters on the starter winding bearing in mind that the starter winding is prob on a ptc current limiter to reduce current after unit has started .

author: NiHaoMike
author_rank: Super Contributor
timestamp: December 12, 2021, 09:24:23 pm
content: Some inverters support V/Hz scaling, Vector was one brand that offered it for cheap many years ago but they apparently ran into issues with patents.More or less, what you want is for the inverter to detect that it's reaching the limit and automatically scale back the voltage and frequency such that the ratio between the two stays the same.

author: alanapar
author_rank: Newbie
timestamp: December 12, 2021, 10:04:45 pm
content: Why not get inside the unit, and insert your soft start solution directly in line with the compressor?

author: BradC
author_rank: Super Contributor
timestamp: December 13, 2021, 05:34:27 am
content: Quote from: alanapar on December 12, 2021, 10:04:45 pmMy neighbours Panasonic unit does just that. It has a sequencing soft starter, which is basically a couple of relays and a big arse power resistor.

author: bdunham7
author_rank: Super Contributor
timestamp: December 13, 2021, 05:51:32 am
content: What type of motor is in the compressor and how large is it? There are solutions, but they aren't all that simple to implement properly. One issue is that if you use reduced starting power for the initial spin-up, you have to make sure the off cycle is long enough for the pressures to fully equalize. If you are going to use a power resistor, supplemental capacitor and a relay, you'll need to experiment quite a bit.https://eltwin-hyper.com/single_phase.html

author: james_s
author_rank: Super Contributor
timestamp: December 13, 2021, 06:30:00 am
content: I've never been inside an RV air conditioner but single phase domestic units that I've seen are always a PSC motor. Some of the modern stuff is inverter drive with BLDC or 3 phase induction motors but in that case this wouldn't be a problem in the first place.

author: Sonny_a67
author_rank: Newbie
timestamp: December 13, 2021, 03:00:11 pm
content: Thanks to everyone for your comments! I was trying to avoid having to open the unit as it was not so easy to install but it seems like I found a way to just open the hatch from the top so I will try to find the relay which is responsible for turning on the compressor and implement the soft-start circuit after that.