Looking for a Programmable Auto-Transfer Switch

GreenPowerManiac

Hello everyone,

Am looking for an auto switch to go back and forth, Grid/Solar Array when a certain voltages are met. When input is less than 12.5v I'll need it to transfer back to the grid.

Is there a nice product out there that does this, and cheaply ?

GPM

Cariboocoot

Re: Looking for a Programmable Auto-Transfer Switch

Hi GPM; long time no post.

How do you define "cheap"?

If you haven't already got a charge controller or inverter with programmable gen-start probably your best bet is to use something like Morningstar's relay driver http://www.solar-electric.com/moredr.html and program it to actuate a relay when the Voltage goes low. Another choice would be this Voltage-controlled switch: http://www.solar-electric.com/volconswit.html

You'll have to come up with your own relay arrangement in either case; one that can handle the power needs. This might require two relay "stages" if you're switching 120 or 240 VAC with significant current. You might want to try for a self-latching relay to keep it from using power when on.

GreenPowerManiac

Re: Looking for a Programmable Auto-Transfer Switch

Thanks for the reply, Coot.

Maybe I'm missing something or haven't said it correctly yet. Right now I have a manual transfer switch on the load center and a relay switch along side of the box to turn the inverter on/off. Would like to set it up with an auto transfer switch controlled by this relay you describe ? Is that possible ?

Cariboocoot

Re: Looking for a Programmable Auto-Transfer Switch

The manual transfer switch (for AC loads) would be replaced by a relay actuated by the relay driver so that the changeover occurs automatically when Voltage falls below the minimum set point. This may require the driver to operate a small relay which in turn would operate a large relay, as it is inevitable that with an increase in contact rating comes an increase in coil power demands. I think the MS relay driver handles four separate relays at a maximum current of 750 mA each.

Between the initial control relay and the ability to handle up to four it would seem fairly easy to get one to handle turning off the inverter. Although it may be preferable to have the inverter turn off at 12.5 Volts and then not back on again until 14+ Volts is reached (indicating Absorb time has been achieved). This is going to depend on your preferences for operation and what kind of relays you can find that will meet the power needs.

In all likelihood you will have two small relays controlled by the relay driver: one for 'OFF' and one for 'ON'. These would trigger two separate self-latching relays with heavy contacts, one for the AC loads and one for the DC to the inverter (this may be lighter duty that can take the place of the on/off switch).

Don't ask me what relays you should get or where to find them; I live in the wrong country! We don't even have Radio Shack anymore!

GreenPowerManiac

Re: Looking for a Programmable Auto-Transfer Switch

Not sure how I can picture this setup. Does it go in place of the Transfer Switch on the Load Center ? Since my MorningStar controller is in the barn and the load center is in the house (150 feet away), would make it more difficult yet to run more wires, much less the voltage loss between the two. Already have a relay setup on the inverter to switch it on/off from the load center.

Cariboocoot

Re: Looking for a Programmable Auto-Transfer Switch

That would definitely be a problem: the DC 150 feet away from the AC transfer. But it is a problem that exists no matter how you try to do it. You want to switch the AC source based on the DC Voltage level ... you'll have to work out some way of overcoming the distance between the two.

I would keep all the DC equipment at the same location, and just run wires to operate the self-latching relay that switches the AC source. Much would depend on what equipment you can find to do the job.