Sticking Potential relay
Hey guys, after a lot of research I built an RPC to power my new lathe. Now I have got some experience with electrical, but little when it comes to capacitors other than hooking them up per prints.
My phase converter is a 5HP 3 phase motor using a magnetic contactor for starting, and a potential relay to cut out the start cap once it is up to speed. Originally I bought a Supco universal potential relay which worked fine at first. My RPC would not get up to speed before blowing my 30A TD fuse though so I unded up bumping up the start capacitor. I am now running a 430-516 UF start cap which gets my motor up to speed quickly and was working fine until my cheap potential relay contact stuck and caused the start cap to make a nice smoke show.
I thought, okay, I cheaped out on the start cap so I will just fork out the cash for the Steveco 90-66 and be done with it.
I hooked it all up with a new start cap and it fires right up, but the idler motor is making that grinding noise (the one it makes when there is way too much power on one leg right before it blows the start cap) and so I quickly switch of the RPC before blowing another start cap up.
Now, I am reading the specs of this 90-66 relay and it says it is a minimum pickup of 208 and max 239.. Does this mean that if my RPC is unbalanced and I am seeing around 260V on that leg that it would possibly not let the potential relay switch out the start cap?
I was planning on balancing this out (currently have two 60UF run caps, one from L1-T3 and one from L2-T3) but I need to be able to start it up and run it to get a good reading and then make adjustments..
While it was running I was seeing something like 238 (L1-L2) 245 (L1-L3) 264 (L2-L3).
I am sure everything else is right since it was up and running beautiful for a while. Had the machine going forward, reverse, all speeds, etc without a hint of a problem.
One other thing. I notice that the Steveco instructions show the cap before the No-1 terminal on the relay. I have mine wired up after No-2 and before the motor T3. Could wiring it this way cause the contacts to weld together before they have a chance to open? I would think it would be an equal load across the contacts being wired either way?
Picture of my converter with the wires shown.
Blue is T3 (generated power) input to the Steveco relay
Green is the feed to the start cap (the other wire on the start cap feeds T3 after the magnetic contactor)
Red is the L1 input to the Steveco relay.
Here is the print I drew up showing how the start cap is wired in. (since revised to the larger start cap, and the Steveco vs the APR-5 potential relay)
So what do you RPC masters have to say? Like I said, it worked fine for a while. Started and stopped the machine about 10 times, then let it sit for a few days, then the next start was no good and blew the cap. Then I took the APR-5 apart and found the stuck contact which I was able to free up with a screw driver, but I bought the Steveco to fix the problem and it seems to still be doing it.
Is there any chance I damaged my idler and the grinding I hear is not actually the start cap sticking, but a bad motor? It still fires right up, but just sounds bad
Thanks in advance guys!
I'm not going to comment on the relay issue, as I don't do capacitors in rotary converters
as a rule - but I DO have to say that is far and away the BEST darn schematic diagram
I've ever seen anyone draw for their converter.
You draft for a living, right?
Might want to update your schematic with the Stevco. As its drawn now, it looks wrong to me - The sensing coil needs to go across T3/L2.
Should look like this:
Rotary Phase Converter Designs and Plans
Agree, looks like the diagram is labeled incorrect. The output contact terminal on the potential relay should be labeled #1, #5 is one side of the potential relay coil and #2 is the other side of the potential relay coil and input contact. So I would have manufactured phase feeding #5, L1 phase feeding #2, and #1 feeding one side of the start cap, the other side of the start cap connects back to manufactured phase.
Thanks guys, I will take a look again and try that out..
I guess you could say that, lol.. I am a sales engineer, but I do draft on a daily basis. My previous job was in the electrical union and I had to take some blue print reading courses on electrical so I picked up a bit from that, and now I do all of our wiring for a gantry robot that we build at work so I guess I am just exposed to a lot of these kinds of prints.
It is much easier for me to put something down on a print and run it through my head a few times before I go and actually wire it up though.
Originally Posted by jim rozen
You guys are the best!
Though, that's what I get for wiring up the new potential relay before bed when I am tired..
You are right that my diagram was wrong, but I had the original potential relay wired correct as you guys stated.. I forgot that I never updated my drawing..
So, the problem was that I had every contact on the steveco wrong. #1 and #2 had to be swapped, and my manufactured leg I had on #4 by mistake which is just a jumper contact so that played a large role.
Lathe is up and making chips now!! (can hear the relay kicking out the start cap like it should, and no more grinding noise!)
I will update my schematic and post it for everyone when I get some free time. It is the least I could do to give back to this site for allowing me to even build one in the first place!