Author Topic: Running GTI from generator.  (Read 2180 times)

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Offline DJ

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Running GTI from generator.
« on: August 20, 2018, 04:52:42 am »

Mate and I are interested in using our old lister engines running on used veg to help power the house and offset the the bills by backfeeding which we can do through our meters. Probably still more exy than just using the grid but gives a a chance to run our toys and convince the other halves they are useful.

The idea some time back was to use the 240V gennys we have and rectify the power with some caps for smoothing and feed that back into the GTI which would go to the house or add to the export on the meter.

It's been a while since I had the idea  and I have forgotten more than I knew now and it wasn't a lot to start with.  What i can remember and what adds up to logic is it's not as easy as this.  I can't remember why but would certainly appreciate the knowledgeable out there reminding me why or what one would need to do this on a reliable basis.  Mate and I are not very electronic savvy but far better with straight electrics.

The base idea was feed the power from the genny into an over sized rectifier, probably 3 Phase 100A. He has a single phase Genny, I have 3 phase.
From there we would add some caps, (the amount of I have no idea but probably go 100UF) and then supply that power to the GTI which should be around half it's DC input rating.

We have no clue what sort of ripple the GTI ( electronic type no torroid) would take or if it matters. 
I seem to remember there are issues with doing this but can't remember what. Something about feedback and harmonics etc

My revised idea which I know works is just to drive an induction motor and hook that up to the mains and use that as a genny which I think will be a lot simpler, cheaper, less hassle and more effective.

Mate is keen to use his new 5Kw Gen head so I'd like to know from the boffins if it is viable  and if so what would need to be done to make it reliable long term without frying the GTI?


Offline oztules

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Re: Running GTI from generator.
« Reply #1 on: August 20, 2018, 09:43:18 am »
The GTI and the caps will work... but there is a catch.

If your using a alternator that uses a capacitor to drive the exciter field, then you will probably have it go out of control voltage wise pretty quick, and blow the input caps from over voltage.... not pretty at all.

If it is a automatic voltage controlled unit, then there is a good chance of success. a few hundred uf will liven things up, and stop the hunting you would experience with no caps.

Best is to use the induction motor.... no surprises, and will work for ever...... there will be iron loss, but no rectification losses and converter losses. This technique has been successful for a very long time, and is pretty idiot proof.  I would think all grid tied windmills used this technique up until the turn of the century anyway.

It is not essential, but useful to have a choke across a relay, so that when you connect the motor to the grid, you will have a softer connection via the choke, then immediately switch it out ( short the choke out with the relay).

If you use a three phase 4 pole motor, you can use the third phase of the motor to close the otherwise N/O contacts of a 240vac coil driven relay. ( or 110v or whatever voltage your operating with on the mains.) to short the choke automagically I expect.

I recently used one of those inverter generator units that had a dead inverter in it. Repurposed the HV ( 600vac) winding by dropping a few coils ( they are under the flywheel) and then rectifying the resulting 3 phase 300vac, and directing it into a galvanically isolated GTI. This worked by driving the GTI into the house grid ( on an island in the ocean) so as to drive the house inverter backwards into the battery bank 100 yards away. It was down at the shearing shed... connected to the house up the hill. This was one of my inverters, so was happy running backwards into the bank with a few Kw of juice to charge the battery on dull days, without the noise next to the house.

.........oztules
Flinders Island...... Australia

Offline DJ

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Re: Running GTI from generator.
« Reply #2 on: August 21, 2018, 02:01:39 am »

Thanks very much for that Oz! Great info.

As the genny my mate ( and I ) have are the  ST(c) type, I suspect they will use a cap on the exciter.  Is there a way to prevent the problems you warn of with those or should they just be converted to an AVR?

have passed the info onto my friend and will await your advice re the energiser caps.

Thanks again.  It's terrifically helpful to get this expert insight and advise that is not available on the net and few people have knowledge and insight on it.

Offline oztules

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Re: Running GTI from generator.
« Reply #3 on: August 21, 2018, 09:20:01 am »
The capacitor in the genny is used to provide the excitation to the field exciter coil.
 
The field coil then excites the armature/rotor, which has coils on it that respond to this field. The rotor  coils have a diode across their output, and so makes the rotor current DC........so the rotor becomes a big bar magnet spinning inside the stator.

The power windings in the stator ( separate from the exciter ones) then feed the 240vac output, and it all self regulates..... good times....

But...If we decrease the cap value, the voltage on the output drops, if we increase the capacitance, the output voltage rises ( frequency stays the same from the governor). So by adding even series capacitance on the output, we alter the relationship in the tank circuit ( resonant circuit) of the exciter /capacitor, and the voltage will skyrocket with the addition of the "softening " caps we try to add. This will blow the GTI filter electrolytics  to hell and back... within a few seconds probably.... so no we can't use that technique with that style of genny.

If you have nice little genny, direct rectification will work, but the mppt of the GTI will cause it to hunt, as the mppt tries to fuzzy logic the input. As it gyrates around the best settings it can find, it necessarily tests the waters all the time... continuously, so changes the loading all the time.

If the genny has stamina, it may not change rpm too much, and won't cause much concern at all, if it is a bit soft, it may be like the tail wagging the dog sort of thing. The series caps gave the mppt a higher impedance source, so it could see the voltage sag, as it tried too hard, and come back as it eased up a bit. This gives the mppt something to work with...... but the cap excited genny........ no.

Direct connection via diodes will probably not cause the high voltage problem, as the GTI filter caps are after the diodes... but no idea how the hunting will effect the outcome. The fact that you can use a inverter welder on your genny, is a fair indication that the filter caps after the diodes won't cause a voltage problem, as the inverter welder has diodes, then caps..... essentially the same thing as the GTI front end with diode bridge ahead of it.

I may do the induction motor thing soon as well for another site here with different circumstances... we'll see. But it will need a control system to allow for a novice to use it, and just let the genny run out of fuel..... and unless you want the inverter to drive the petrol motor after it has run out of fuel.... this will be a problem for them..... ie only allow the motor to connect at 50hz or more, never less.... ie 3000rpm or 1500rpm motor dependent.

I'm not an expert in any way.... just good at blowing things up.. then having to fix them :-[


............oztules
Flinders Island...... Australia

Offline lighthunter

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Re: Running GTI from generator.
« Reply #4 on: April 13, 2019, 06:45:26 am »
Hi all! We recently got an ice storm that tops all ive ever seen. Folks everywhere without power. So, my solar generation has become a necessity rather than a fun option. It normally shuts off when sun goes down and back on when light is promising. The battery is a 510AH 48v but i only cycle it occasionally for health. The iced over panels and dense overcast combined with 1/2hp sump pump that cycles a lot gave some challenge. Of course i wasnt completely prepared. At 4am when pwr went off my first thought was..... I will unplug a few appliances including sump pump and switch the transfer and inverter on and go to work. As long as "SHE'S" happy, all is well. I check the basement an hour later an theres 1 foot of water on the floor. That wont work so I plugged pump in. And i skip the thought of goin to work "now i have an excuse  :)" and clean the carb on my ancient 2kw genset. Turned out to be an adjustment but an hour later running like new.

Ive never connected it for a DC battery charger before so i tried an old transformer with several taps and a rectifier with filter caps. Never got more than 6 amps or so charge rate with large induction spark and volt sag when connecting it. The transformer type was at fault, big enough but somehow wasnt suited to the task. So i used the taps differently and boosted voltage from 120 to 240 ....at least before loaded. Then since panels were iced over 4" thick i disconnected one of the grid tie inputs and connected what i hoped was 270 filtered DC from genset to it. Viola, after the 20 sec countdown we were contributing to AC load with 930w of power. I wasnt completely happy with it but good enough for now.

Basement was ok again by this time and i spent a couple more hours with a plastic scoopshovel and got 2 of the 3 arrays free of ice. All went good from there, the 2nd day was overcast also but all 3 arrays brought in 2800w with snow reflection, way more than we needed, topped up battery.

All in all it wasnt really a difficult thing even though i have the equipment ive never had to depend on it. Theres a big difference!

So back to the genset charger, i notice at times the grid tie input is only pushing 5-600W and then it changes to 930w every so often and runs for 10 min then goes offline for 20 sec then 525-600. The genset is real happy with this as the loading goes from 1/4 to 1/2 and gets real good gas mileage for the 2 quart tank. (2 hours 15 min).  But of course im curious so i put volt meter on the gti input and its 160dc, (minimum is 150) so the gti is boosting the power from 150 all the way to 400 then makes 240ac and matches freq of the power jack 48v inverter.  And, the boost stage gets too hot and faults, retries at lower power runs there till it cools and back to 930.

Ok so i get it, bad configuration and poor choice of transformer, but keep in mind, its working real good and most of all the genny is happy with loading. But i have to have things right.... so i grab a 3kw toroid off the stack factory wound for 120/240, this will really work good.... connect it up plug it in grid tie thinks about it.... generator rpm starts dropping power goes up and grid tie searches... it did a horrible job of figuring out max power point. Settles at a place of 30hz genset way overloaded only putting out 13-1400w. If GTI had let rpm stay at 60hz it could have gotten 2kw but it didnt, it got greedy seeing the caps with xtra power and the mechanical inertia... So back to the volt spongy transformer that doesnt get hot just full of springy inductance somehow but genset is happy and its all the power i really need, in fact battery hasnt been below 70 percent yet so maybe i dont need genset.

Now i cant help thinking theres a better way to do this. A capacitor and the 3kw 120/240 toroid perhaps i can choose the power point i like instead, maybe program an arduino to select one of 2 capacitors depending on demand? I really havent messed with caps for this sort of thing.

 Thanks all who contribute to this forum!

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Offline lighthunter

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Re: Running GTI from generator.
« Reply #5 on: April 13, 2019, 06:49:00 am »
Utility lines
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Offline lighthunter

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Re: Running GTI from generator.
« Reply #6 on: April 13, 2019, 07:04:14 am »
genset diagram
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