Apologies, been enjoying some homebrew tonight and it's making me ramble on about this idea ... any and all input is welcome on this project ....
Thanks for the info. I was hoping maybe I could avoid a transformer in this application because of extra cost....
SCIM = squirrel cage induction motor
Basically ... this is my idea 'peak shaving'
SCIM powered by natural gas engine running in parallel with the utility.
Yes I know the utility probably has to give permission for this kind of setup.
So it would basically, to the capacity of the induction generator, keep my power draw off the utility at zero. Thus I would pay only the 'connection' charge to the utility and very little actual kwh charge. But it allows me to have higher peak draws, thus consuming my net off the utility, without needing a high kVA generator. Also if my little contraption dies or blows up my utility supply is intact, and my many freezers full of meat are safe.
SCIM 240V/120V connected through a contactor and a breaker to my utility panel.
CT's on the L1 and L2 coming in from the utility
CT's on the L1 and L2 coming from the SCIM
Control system monitoring utility L1 and L2 for reverse power
Control system monitoring SCIM for overload power and reverse power
Small engine modified to run on natural gas driving the SCIM
Servo of some type controlling throttle on small engine
Small engine belt driving the SCIM, geared down if needed so engine can run slower and more quietly.
Have a 25HP Kohler twin cylinder pressure lube very nice engine that might work for this application, it's not natural gas though, I've heard you need a natural gas engine with hardened valve seats for this application.
Servo to control engine throttle
Control system monitors utility L1 and L2 for reverse power
And bumps up the engine throttle slowly as needed until reverse power for L1 or L2 is
zero.
If utility L1 and L2 are both reading reverse power, bump throttle down slowly until either L1 or L2 is not reading reverse power.
If both L1 and L2 read zero power or both read reverse power, bump throttle down. If RPM drops below threshold (less than 60 Hz), open the contactor and shutdown. This is in case of utility outage or main breaker trip.
Two shutdowns would be engine rpm above or below setpoints; that would be a fairly tight range depicting 60Hz output from the SCIM but allowing for slip.
I figure my 'average' draw is one kwh. I don't know how this is distributed between my L1 and L2. I have no 240vac loads.
Startup I'm not sure about, I think I would probably engage the contactor, and let the SCIM start the natural gas engine. Then let the control system take over and ramp up the engine throttle until it is generating such that either L1 or L2 is reading zero reverse power the utility. I can't close the contactor with the SCIM out of phase with the utility I don't think; I've heard of snapping driveshafts this way.
Drawbacks - if set up to control the throttle so that either L1 or L2 is not reverse power to the utility, it's basically 'giving' some power off L1 or L2 to the utility.
How do these new digital power meters work. Do they take the absolute value of the power? For some reason I suspect they do. I'm sure it will not 'credit' me for power given to the utility. What I would at least hope is it wouldn't charge me for power fed back to the utility.
Hmmmmm the more I look at this plan the more I think it would be much easier to simply get a prime power natural gas powered generator of sufficient size ...... and an ATS .... I don't know just dreaming and getting ideas I guess.
Value of project would be maybe $50/month so $600/year. So I think I could get payback within 1-3 years on this concept ... though the labor of coming up with the control system for this would probably not make it worth it except that it would be fun though ...
Hm payback on having a prime power natural gas generator ... that would be around $3000 minimum, no ATS, value $80/month $960/yr, a bit over three years simple payback ... would probably be worth it even considering maintenance costs, provided the unit can run continuously for at least 5 years or so without requiring major repairs or overhaul ...
OR wait, that's not right at all for the prime power payback, the payback on prime power generator is still only $50/month or $600/yr, because I would still want to maintain utility supply as a backup if the generator goes down .. so that would be a simple payback of 5 years .. hm that's a bit tougher to swallow .. and makes it probably not worth doing the prime power generator ... the 'peak shaving' might still be viable though ...
