Burning points in pressure switch

Howdy. Hope I can find some help here.

I have a 12VDC solar system that runs a 240VAC deep well pump (12 volts from battery kicked up to 120 AC by an inverter and then to 240 with a transformer).

My problem is that the points on the pressure switch burn out every couple months. I have the pressure pump controlling the 120 line that goes to the transformer. I'm using just one wire on one side of the pump.

As the points burn, the resistance causes a huge extra demand on my batteries -- sometimes running from 130 amps DC (normal) to 400+ amps.

All the wiring seems OK, but I must be getting arcing at the points.

Thanks for any help.

Josey

Does the inverter have any shaping to the output (i.e., is it close to a square wave, or a sine wave)? Square waves through transformers aren't very efficient - it would draw much more current than you expect. The flat tops essentially saturate it. You might be better served with an inverter that can make 230. I know they make them, since we have one on a missile launcher we make (well, it has a 24vdc battery pack, though). Sine waves are much more efficient. Not sure if this would make enough difference, but think it would. It's been too long to remember how to calculate the max current draw. First thing I'd do is try a relay with a much higher load capacity. My unprofessional opinion.

Thanks, Jim:

It is a sine wave inverter -- about 30-50 steps per wave. I don't notice any distortion in the wave pattern when the loads rise. The pressure switch is rated at about 1.5 horse, which puts me right on the limit. This set up used to work fine, but something changed.

I put in a new pump and buffer before I saw the burnt contacts.

Where would I find higher capacity contacts? I live in a remote area, so shopping for an item like that would have to be done by internet.

Anyway, thanks.

Josey

Hi Josey,

You and Jim are way over my head. My ham radio studies didn't get that deep into circuit science. I understand the square wave somewhat and know about rectifiers etc. I know the pressure switch is going to burn if you have an AC pump, because of the inrush current drawn at start up. This amp draw can be as much as 5 times the running amps of the motor. That coupled with a transformer, which is pretty lossy to say the least can cause the problem you are having.

I could sell you the contactor if we were on a different site, but can't advertise here. They are not too pricey if you stay with the air conditioner units. They are called definate purpose contactors or relays. I would get at least a 30 amp to give the points a little more longevity. You may have to find a box to put it in or you may find one already in it's own box. Another keyword would be pumpstart relay. They sell them for sprinkler systems, and most of them will start a two horse motor. They have a 24 volt coil.

One more thought would be a soft start motor. We're getting on the pricey side of things, but just a thought.

bob...

www.grainger.com might have something you could use.

OK, I finally figured it out, and thought some of you might be interested.

I kept burning points on my pressure switch, but it would take weeks before the points got bad enough that they would overload my inverter (I have solar power only).

Replacing the points cured the problem -- for a while.

This last time, however, even replacing the points did not help. I checked everything -- inverter, generator, step-up transformer (final step in going from 12VDC to 240VAC), wiring, well pump, etc. The capacitor on the starter/buffer drive looked like it was leaking, so I replaced it. Didn't help.

Finally, I did continuity checks on the wires going from the cabin to the well, and found that the three-wire pump wires were, basically, bonded.

These were quality wires that came with the 240V deep-well pump, and I had run them through buried conduit. All I had at the time was 2-inch conduit.

When I pulled the wires out, they were shorted and burnt. I guess that my inverter/generator was able to overcome the shorts when they were small and when the points were in good condition. However, the shorts in the wires must have caused arcing in the pionts, burning them out prematurely.

So what caused this? The 2-inch conduit ran underground, but I had brought it up from underground and two 90-degree fittings to curl it down so no water could enter where the wires came out and into the capacitor drive.

A mouse or two had climbed in the pipe. I know this because I found a bunch of sunflower-seed hulls inside. They had chewed the wires and caused the short.

What threw me was the temporary solution when I replaced the points.

Josey

Why didn't we think of that?