grid tie output of ups

[BillK]

Conceptually, my thought was to remove the batteries from the UPS and connect the battery leads to my PM generator.  This would give me 60Hz, 120V AC.  I would then like to connect the output of the UPS to the grid.  I would sync the UPS with the grid by building a circuit that would detect when grid power arives at zero volts going positive.  At this point the circuit would close the DC side of the UPS.  I haven't tested the UPS yet, but I am assuming that when DC is applied the output instantly goes positive.  The sync circuit would continually test for syncrinization and would make or break the DC side as appropriate.  Does it work?

[Ungrounded Lightning Rod]

Conceptually, my thought was to remove the batteries from the UPS and connect the battery leads to my PM generator.  This would give me 60Hz, 120V AC

Don't try this without a battery.  Connect your genny to the battery in the UPS, or remove the UPS battery and hook it to the big one that should be part of your PM generator setup.

The battery is not just a storage device.  It's also a high-capacity voltage filter circuit, removing the ripple from the generator.

I would then like to connect the output of the UPS to the grid.

Danger, Will Robinson!

I would sync the UPS with the grid by building a circuit that would detect when grid power arives at zero volts going positive.  At this point the circuit would close the DC side of the UPS.  I haven't tested the UPS yet, but I am assuming that when DC is applied the output instantly goes positive.

Nope.  It's an oscilator and needs to "wake up".

The sync circuit would continually test for syncrinization and would make or break the DC side as appropriate.  Does it work?

Nope.

If your UPS is not designed to backfeed the grid it won't sync and you'll fry it.  Even if it is you don't get it to sync by trying to turn on its power feed at the right instant.

Also:  Running a UPS without the battery attached is another way to fry it.

If you want to backfeed the grid, get an inverter with a "sell" switch.

And NEVER EVER backfeed the grid without a device specified for the purpose and UL approved.  If you do, the next time there's a power outage and a lineman is working on your transformer to restore it, he'll find the high end of the allegedly-dead pole-pig has 8,000 volts (Y) or 12,000 volts (delta) on its terminals.

Killing lineman is not an acceptable design bug in a homebrew project.

[BillK]

the power company protection would be built into the sync circuit.  if the DC side is open then there would be zero volts on the AC side.

I'm looking to do this on the cheap.  I know there are commercial products available, but that takes all the fun out of it.  Any ideas?

[Victor]

Only if blowing up inverters is your idea of fun!

[Ungrounded Lightning Rod]

The power company protection would be built into the sync circuit.  if the DC side is open then there would be zero volts on the AC side.

And once it is on (assuming it syncs rather than catches fire), if the line power fails the inverter will keep it hot.

Even with moderately complex detection gear, a motor running on the line side (even a small one) will keep the inverter happy - especially one that was originally designed to run standalone.  Detecting line failure is not necessarily simple, and not for a homebrewed project.

Also:  In addition to (and largely because of) the hazard to linemen, it's ILLEGAL to backfeed the power lines with a device whose design hasn't been inspected by labs such as UL and approved for such service.

Then there's the liability issues.  Does your insurance cover killing electrical workers?

I'm looking to do this on the cheap.  I know there are commercial products available, but that takes all the fun out of it.

Linemen's jobs are already hazardous enough, without frying them because you wanted to save a few bucks on your hobby.  Every one of them (who has been working for more than a few years) once knew somebody who was then "burned".

If you can't afford to buy one of the commercial ones, you obviously can't afford to build several of your prototypes to be tortured and set on fire, then hire Underwriter's Laboratory to test them to destruction and give you design feedback or approval.

If you were really rich and just in it for the fun, that would be the way to go.  (But the really rich usually find ways, when having fun, either to not spend a lot of money on it or to make it pay off.  That's why they're still rich.)

I'm sorry to rain on your parade.  But you should NOT attempt to backfeed the power grid with a homebrew inverter.

If you want to homebrew, doing the rest of the system (within electrical code requirements) is just fine.  But you'll need to buy any inverter you use to backfeed the grid.