RPC: Steveco 90-66 startup relay keeps re-engaging

Hi guys,

Just have to start by saying thanks - this particular forum is where I got 99% of my info and inspiration when I went to build my 10hp RPC. The lathe it powers is a 7.5 HP Romi T17.

Everything is working pretty well, the idler motor starts on about 1000uf (10 x 100uf in parallel) and the voltage is nicely balanced using smaller run caps (65uf and 100uf). I'm using a Steveco 90-66 to switch in the starting caps, which is probably more than the contacts are rated at but it works fine. It starts quickly, runs smooth and cuts out at exactly the right moment.

However... After running a couple minutes, at 1-2 minute intervals I hear a clunk - the starting circuit re-engages for a split second, there's a spark from the Steveco as it immediately disengages, then it runs fine again for another couple minutes. I think the clunk is mainly the motor being suddenly slowed by the start caps, then as quickly speeding up again when they're gone.

What causes this? Is this something you have run into before? I can post the circuit specifics if needed, but it's a pretty standard design. I don't have bleed resistors on the start caps yet, if that makes a difference.

Thanks in advance!

For some reason, the voltage across the relay coil is dropping below the point at which the relay drops out and re-closes the normally closed contacts.

Double check the wiring of the start section. It sounds like something is not correct.

Thanks JST. I did re-check the wiring and found the problem. Kind of a dumb mistake, so I'm posting this for future reference.

I had hooked up the start caps to terminal 2 and the output to terminal 1. This is backwards, and on a normal relay wouldn't matter, contacts are contacts... and as noted it did start fine. But a potential relay compares the voltage on two terminals (2 and 5 in this case). Since one of them was measuring capacitor voltage, when it disconnected, over time the cap was bleeding off voltage, until it finally triggered the relay, got a "top up", then switched off again.

Working great now! Thanks for your help.

ky_metro said:

I'm using a Steveco 90-66 to switch in the starting caps, which is probably more than the contacts are rated at but it works fine.

Click to expand...

For how long? A secondary power relay....

About a second. If this one fails, I'll add a contactor.

rons said:

For how long? A secondary power relay....

Click to expand...

For how long will it work fine? Part of good work is to prevent problems especially when you can't see what circuits are doing. Add to that you know it will fail. If you can afford a mercury wetted relay like the Magnacraft WM60A-24D which I just got for $24.50. It has to be mounted in the vertical position because a small bulb inside is floating in a solution of mercury. When you energize the coil the bulb moves down from it's steady state floating position and contacts the poles.

Alright then. I ordered a 50A SSR that should do the job just fine. That should take all the load off the Steveco, and will be cheap/fast to replace if it ever fails.

A few years ago I built up a 7.5 HP RPC to make use of a new old stock 7.5 HP Baldor motor that I found at a second hand store. In doing some research for my parts, I read a thread here where Peter Haas had mentioned the fact that a Steveco 90-66 will only be able to control start caps for no more than about 5 HP. To get around that problem, he rerouted the wires in the Steveco so as to separate the coil from the points. Then, instead of running all of the start capacitor current through the Steveco, it just controlled a large contactor with a 120V coil which took all of the load.

I followed his suggestion and now the relay coil is wired between L2 and L3 on pins 4 and 5 which will cause it to open at close to the running voltage. The points (between pins 1 and 2) are only used to switch 120 V power from L2 to a three pole contactor. I use three start caps and each one has its own pole in the contactor.

That was a very good idea because the RPC has been in almost daily use for something like 6 years now with no smoke.

Sadly, the Steveco 90-66's are no longer available. I guess there are some other potential relays that can do the same job, but the Steveco's sure worked well.

Speaking of Peter Haas, has anyone heard from him lately? He helped me out quite a bit.