A Big*** Can/Cap.. - Page 1
Forum section: electronics / general

author: BloodyCactus
author_rank: Frequent Contributor
timestamp: July 19, 2015, 09:33:36 pm
content: My AC died this morning, its a 3 ton unit made in 99 and ridden hard. Anyways, 'POP' aaaaaand no compressor. So HVAC man showed up with his grotty old fluke and they pulled this baby out, with a nice bulge on the top. Its... BIG! Apparently the compressor needs a bit of oomph to kickstart it.anyway, it was more than the cap, and now were replacing the heat pump, compressor and the bigass box inside the house. Getting a York vs our current cheapo noname the previous owners installed.Ive never ran into such a big capacitor before.. just thought I'd share.

author: marshallh
author_rank: Unknown
timestamp: July 19, 2015, 09:59:02 pm
content: the 3 terminal units are a combo start/run cap. And yes, they will ruin your day when they go bust.

author: TMM
author_rank: Frequent Contributor
timestamp: July 19, 2015, 11:51:01 pm
content: I've got a 30V, 30A psu here with this in it:8x 22,000uF 40V. Have to think twice when using that one to avoid a massive current dump.

author: EEVblog
author_rank: Unknown
timestamp: July 19, 2015, 11:56:40 pm
content: My lab aircon went bust too, same thing, the motor start cap.I posted photos on twitter somewhere...

author: houdini
author_rank: Regular Contributor
timestamp: July 20, 2015, 12:13:58 am
content: I have these from a project i build a while ago, I think the bank was .01F at 900v iirc.

author: richfiles
author_rank: Regular Contributor
timestamp: July 20, 2015, 01:25:54 am
content: REALLY Big asscapacitors!These big ones are oil filled film capacitors.The last picture is exactly what it says on the tin. A huge capacitor bank. At my old job, we had a similar machine, used for magnetizing rotor magnets for electric motors. Our capacitor bank took up TWO full depth 19 inch racks worth of space. A third rack had the impulse trigger unit and about a third of a rack of additional capacitors. We used to call it Big Blue. I was told, that before I came to work there, a fault caused it to vaporize one of the magnetizing fixture windings once. Sent the operator to the hospital for burns. No electrocution, and she recovered, thankfully... I think some of the larger gauge wire in the larger magnetizing coils was probably approaching a third to a half a cm thick. I don't know what size wire the average magnetizer coils were wound with, or what percentage they charged the bank to before firing. I'm guessing the bank probably got overcharged while using a smaller magnetizing fixture, and fired, with more energy than could be handled. I suspect that the fault could have been with the interconnect, as there are supposed to be interlock pins to set limits. The while fixtures would slide into a mount, and at the back, there were massive pins, that would engage the could with the machine. I think there were also smaller pins that set configurations. If the connector or the wiring behind it were damaged, I suppose anything is possible.

author: EEMarc
author_rank: Regular Contributor
timestamp: July 20, 2015, 02:25:17 pm
content: Single phase AC compressors often fail because of the start cap.I saw a demonstration with the start winding open. The motor is unable to start even under no load. Given a spin, the motor will start and run fine without a load. If the leads that go to the start cap are shorted, the motor puts out a small torque at low speed and is able to start if there isn't too much resistance. The start cap increases the low speed torque. The cap shifts the phase of the start winding causing the motor to act sort of like a 2 phase motor. That shift increases the torque at low speeds. Not as good as a 2 or 3 phase motor, but close enough.A few years ago, the same happened to the A/C at my work. The HVAC tech replaced the cap and a fuse that blew of 2 one for each line going to the AC compressor. A year later, the AC went out again. After some troubleshooting, the cap was good, but the fuse that wasn't blown had blown. I assume that it was damaged when the A/C motor blew the other fuse and eventually failed. My friend decided to use a piece of 18 gauge wire to bypass the fuse.At least there was the other fuse there in case of a locked motor condition. The wire actually worked for long enough for me to get a replacement fuse, there wasn't any noticeable insulation damage.Nice pictures of the PF correction caps.

author: SeanB
author_rank: Super Contributor
timestamp: July 20, 2015, 04:14:22 pm
content: 35/5uF capacitor, one side ( 35uF) is used for the compressor, while the other side (5uF) is for the outdoor unit fan. They have that failure mode which disconnects the terminals inside the unit safely. I do not like them much, as they are a pain to keep in stock ( a few dozen values and combinations) so I often just replace with 2 separate units.For a compressor running with frequent starts I add an external hard start kit, which will often allow the unit to start and run even if this capacitor has failed. Busy fixing one where the capacitor did not disconnect, but where it went high resistance on the spade connection and cooked itself, the wiring and the case, with a lot of smoke coming out.