If you’re willing to risk that solution, and want monitoring, a $10 Microcontroller
and ~$1.00 worth of ancillary resistors and diodes will get you monitoring. If you
want to get really fancy, you could mount it all to a custom designed PCB for
around $10 ($5/sq.in. for 3 copies of the PCB) from oshpark.com <http://oshpark.com/>.
If you want to get really fancy, you can change out the power cords for real
PCB mount IEC outlets and put the whole thing in sheet metal for ~$45 more,
yielding a total cost of <$100 + whatever you value your time at.
The software for the MC would be dirt simple and probably take less than an
hour to write and fully integrate into your monitoring system.
The time to design the PCB for the fully loaded version is probably a couple
of hours with Eagle (if you use an MC, relays, and outlets that have Eagle
Libraries for their parts). Takes 10 days+shipping from Oregon to get the
PCBs. For an extra $5, they’ll ship USPS Priority. One really nice thing about
Eagle and OSH Park is that you can do small stuff in the free version of Eagle
and you can submit the Eagle .BRD file directly to OSH without having to turn
it into gerber files. OSH gives you a very accurate preview of your boards
which is a nice final check before submitting the job for fabrication.
Note: I DO NOT RECOMMEND using this solution. It has a number f shortcomings.
1. It depends on some external force to make the decsion about starting
or stopping the generator.
2. It can lead to a really rough phase transition when switching.
If you’re just feeding a UPS and you can make sure that one side
is down well before you switch to the other side, this is probably OK.
If you’re feeding some sort of motor and there’s potential for a live
switch, this can be very hard on said motor and can lead to graphic
and spectacular failures of said motor as it attempts to change
it’s armature position instantaneously to match the phase of the
new power source.
3. It would almost certainly never pass UL, CSA, or any other certification.
4. It’s the kind of thing NEBS was developed to prevent.
5. The potential for combustion if the world ever violates your expectations
is not insignificant.
Now, with the same hardware, if you have the MC do some additional detecting and
control the switchover process, you’ll add a few failure modes into the system,
but you can make the whole thing a lot safer and actually more reliable. You can
at least build something that will not damage your equipment or catch fire.
The cost would be about the same (same hardware, after all), but you’d need to wire up
a few more pins (more traces on the PCB) to the MCU and you might need a couple more resistors