Single phase power from a 3 phase motor

[(unknown)]

I am going to set up a co-generation system using a 19 hp Kubota diesel and a 3 phase motor to net meter and heat the house in the winter with the cooling water from the engine. I have seen it done so I know it works. What I don't know is how to size the capacitors on the third leg to get the proper output voltage to match the grid and how to get the power factor up in the 95% range. I have some info that talks about using capacitors and a buck boost transformer to get the output correct and matched to the grid but there is no basis for the values to be used to get it right. My other basic question is how do I generate power with coils that are 120° apart when the wave form of the incoming power are 180° apart?

Thanks for the help

Dave

[kitno455]

why not use a larger single phase motor, or even better, a low-speed genhead, like one of the chinese ST units? you cant 'turn' the 120 degree 3 phase into split single without rectification and inversion that i know of. using something that already puts out single phase and can have its windings split, or is already a generator seems easier.

you are also going to have to deal with the fact that loss of grid power means that you are going to lose sync with the grid, and when it comes back on (out of phase) it will kill your stator windings, or snap the engine-gen coupler (or worse). not to mention the fact that during that period you are backfeeding into your neighbors house, you will be a danger to the lineman. oh, and if your diesel stops, the motor will keep turning, not what you want likely.

allan

[monte350c]

Sorry I don't have actual experience with net metering, but I do have a reference for you.

page 32 and following in the document at this link:

http://www.eece.ksu.edu/~gjohnson/wind6.pdf

It absolutely can be done with the correct engineering behind it.

You'll want to take measures to avoid energising the lines if the grid goes down.

Fun!

Ted.

[nothing to lose]

Well just basic testing here, but it seemed to work ok. Used more feul than power created so of course not doing it much yet.

Simply put, I connected an AC motor to run off the grid as normal, this syncs the motor to the grid. Start engine, this runs the AC motor faster than rated speed and it produces power instead of using power, this feeds the house or grid. AC motor can be used as starter for the gennie engine

If power goes off the motor will still be spinning on the engine but not producing any power, not sure what happens when the grid comes back on. My "GEUSS" is the grid and AC motor will sync back togther again since the motor is not then producing any power. That is a geuss because the grid did not go down durring testing and I did not think to try it by flipping off the main power then back on again

 HOWEVER, I would not be wanting to run the engine for nothing when the grid is down this way, SO I think since there is only power when the grid is live, it should be very easy to put a switch in the line to shut off the engine when the grid is down!!

Now what you have is an AC motor that runs off the grid as a starter for your Diesel engine and a controll switch to only have the engine running when there is grid power. So the AC motor starts the diesel engine which runs the motor faster than it's rated speed and produces power only while the grid is live. When the grid dies the AC motor also stops making power, the switch then shuts off the diesel engine. When the grid power comes back on the AC motor is still connected and starts the diesel again.

 Some what automated system, and when you don't want it running just flip off the breaker for it so the grid power is off.

 Just don't forget and end up working on the system turned on durring a grid power failure where it could start unexpectedly.

If you just set it up to run all the time with out a switch to shut off the engine durring a grid outage and wonder what happens when it comes back on again.

 SHould be easy enough to test, shut off the breaker to the motor, have load between breaker and motor. Load should go dead after you shut off the breaker, not stay on. Then turn breaker back on and watch for smoke or boom, if load comes on and everything runs fine then the motor should have re-synced to the grid and is a gennie again.

I would do this when I could shut off everything except the gennie system being tested!

 This way the metter should not be running at all. Meter shows power being used when you run the motor on AC power, also a load like a light bulb. Start engine and when AC motor over speeds the light should be working fine, and you should see the metter spin backwards if your making more than 100watts power the light bulb uses

When you flip off the breaker to the gennie taking it off the grid, the light bulb should go off if the AC motor is no longer producing power (it should not be, and the bulb should be out.)

 Then the big test, flip the breaker back on :O

The bulb should light and the meter spin backwards again if all goes well.

Some meters will not spin backwards though even though you are feeding power to the grid, so check that first. If you are running the gennie and a load but the meter is not trurning at all then yours may not go backwards. If so I would add larger loads like 500watt bulbs and see if the meter spins forwards, if not then your gennie is making power and your not using grid power for the load, but the meter is not running backwards.

Basically it take the grid to energize the motor so that it can generate power, no grid no power. This is not using any caps!! Using caps is if you want it to generate power without being connected to the grid, the caps energize the motor kinda like the grid would, except you will always be making power.

For grid tied gennie you do NOT want to feed any power to the grid if it goes down, that could hurt or kill workers not expecting it, so I would NOT use caps on the AC motor. If you need power when the grid is down, do somethng else for that power but DON'T be feeding the grid while doing it.

Properly built and installed, no-one should ever even know it's there unless you tell them or the meter reads less later than it did before

That's what I will be doing when all my stuff is completed someday (if ever), just holding the meter near zero best I can till I can get off grid completly.

 If the grid is up I can feed it, when down I can't, and in between no-one knows.

Primarily the house will use most of the power that's made anyway, but for things like 220 well pump I'll need the grid for while, so the well eats 220 from the grid, and the gennie gives it back.

[kitno455]

do not attempt to disconnect such a system from the grid and then reconnect. you are flat wrong about the motor no-longer generating if the grid power is removed. in fact, there is enough residual magnetism in the rotor to start up generation from a dead stop if no load is connected, and there is CERTAINLY enough magnetism if the gen is already running. it will stay self-excited, and it will continue to gen power when the grid goes away. but the frequency will diverge, and when grid power is restored, something is going to break.

i would definately investigate REAL grid-tie equipment. 230 volts coming together out of phase is no joke. dead lineman is also no joke.

allan

[nothing to lose]

 You may be correct and anything tried certainly needs to be well tested!

However what little testing I did so far and about everything I have read from alot of sources indicates the AC motor WITHOUT CAPS should shut down when the grid does. Anyone trying any such thing of course needs to do much testing on their own sytem to be certain that it does shut down! Various motors may work differently also perhaps. And other things to consider

One of the reasons I suggest installing some type of kill switch so the diesel engine can only run if the grid is working. No way an AC motor will be producing any power if it is not spinning. And if the object is to feed the grid, you don't want the engine burning feul for nothing when the grid is down either.

[nothing to lose]

"but the frequency will diverge, and when grid power is restored, something is going to break."

 I have not tried it, but would that breaking most likely be a fuse blowing or a breaker poping, or would it be a big boom at the main transformer or fried motor windings??

Not tried it, but my "thought" would be the safty devices like fuses or breakers would be the first to go.

[kitno455]

i personally have not tried to grid-connect a generator that was not already synced via  a controller. but i have heard stories fuses are far from instant, so they are no substitute for thinking ahead. most likely the generator for a brief instant will act just like it is shorted out, with a nearly unlimited supply (or suction) of electrons filling (or empyting) the stator windings. the gen could soak that up, and no problem. or it could attempt to reverse the rotation of the machine. that would likely cause coupler or belt failure.

oh, and you are likely correct about the lack of capacitors killing the gen, though i was/am concerned about whatever capacitance is in the attached wiring, in the power supplies of the neighbors computers, etc.

allan

[nothing to lose]

"oh, and you are likely correct about the lack of capacitors killing the gen, though i was/am concerned about whatever capacitance is in the attached wiring, in the power supplies of the neighbors computers, etc."

I see what you mean, but then again I think there will be a tremdous load on the grid if it goes down so I dought I would be getting any thing off it.

Think of all those airconditionares, fridges, tv's, washers, 240V dryers, electric stoves, etc.. If the grid goes down that's instant loss and I won't get any feed from anyone elses house. Also since my own would probably also be trying to run various items that should be a load to draw off any stray juice in the lines.

Still best to be carefull and test alot before just trusting anything.

I had not tried this but ask about it before and the answer was NO. Would running one motor synced to the grid and making power and a second motor synce and making power keep each other in sync and making power if the grid went down.

 The answer I got from people smarter than me was if the grid goes down both motors will stop producing power. Why I don't know? It would seam to me that both motors would see the power from the other motor and not know where it came from and just assume it was the grid and both would use each others power to stay in sync. Like I said, I never tried it myself, but I was told it would not happen because of something (I forget what) and the motors need the grid to keep the fields energized, something like that.

This is something I also plan to test at some time but have not yet. Get 2 motors running on the grid at the same time then start an engine to over speed them, each will be a genny in synce with the grid, and have a load on the line that connects them to the grid, then I kill the grid and see if the load dies or if the 2 motors power each other and the load still remaining gennies.

I finnaly found some tools I been looking for for about 2 weeks now. Things spread out to many places. Perhaps I can do some more gennie testing in this area. I have an idea for a way to run a gas engine cheaply and have a couple to play with. Running a gennie would be a good test for both ideas at once. See if I blow an engine with the fuel or motors with the grid

[(unknown)]

I appreciate all the input however there seems to be a lot of speculation and opinion rather than than fact. The system works, that is not the issue. I am well aware of the need for protection such as over under voltage and frequency relays as well as a reverse power relay. The capacitors on the third leg will have bleed off resistors so if the grid did go down the generator would stop producing power due to the lack of and even damping of excitation. The only time a 3 phase motor will run as a stand alone generator is when there are capacitors on all 3 legs. When the generator syops generating this would also shut down the prime mover so you do not get random reconnection of the load. An induction motor does not need the initial sync to the 60 cycles before connecting power, the only time that is a problem is if you are already generating 60 cycle power and you want to connect it in parallel to the grid, then you need to be very closely in sync. All you need to do is turn the motor at or close to the name plate RPM, apply power from the grid (If the rpm is low the motor will draw power from the grid or if you are over, you will start to produce power, you then set the prime mover to turn a 1740 rpm motor at about 1860 or so rpm and you are generating. I am just looking for info on sizing the capacitors for the correct voltage and power factor. The fellow that showed me his installation has now passed away and with him went a lot of valuable info.

Thanks

Dave