Grid tie Lister generator

[cold comfort farm]

We are currently putting together a small CHP installation 1.5kw Electricity which we would like to grid tie.
Basically we have a 1.5kw Lister generator run on waste veg oil and later would add solar or wind. Our preference is to use the generated 240v for an isolated area away from the grid.  I would also like to feed excess power from the generator to the house via a grid tie inverter.   (not a normal inverter)
I was thinking that rather than spin a PMG alternator we could convert the output from the generator DC using a bridge rectifier.  I know there are losses and is not the most efficient way of doing it but all this is there apart from the rectifier and inverter and is free to use.  Due to the loads from the house it would be impractical to isolate the house from the grid and just use the generator.  My loads are 150w day, 300-800w evening and peak at 7kw (kettle and Oven)
 
I cannot find a rectifier bigger than 48V input what would be the output of a typical inverter.  I also intend to trickle charge a small bank of batteries (which we have) with spare electricity and a 240v charger so we can use the power stored in the batteries through the inverter when everything has been switched off or its gone dark or no wind.

Has anyone done this already as most of the inverter people are ignoring my emails. I think they think I am barmy.

Thanks in advance.

CCF.

[carlb23]

You should easily be able to find bridge rectifiers in excess of 600 Vac and 50amps. You will probably need to use capacitors on the DC side to clean up the ripple so the inverter likes it better.  I have not tried this yet since i only use my lister for backup power generation but my gid tie inverters for my 10kw solar array run anywhere from 150 VDC to 500VDC on the input side. 

Carl

[guyd]

You will need to generate three phase - single phase is too 'lumpy' when rectified and the GTI will kick up a fuss.

Look for a miniwind generator perm magnet type - I have three of them (was four - one blew up - my fault). They go for 395 quid. They are rated at 1500 watts @ 500 volts (500rpm). If you want a two groove 132mm SPA pulley for it - let me know - Ive modified one to fit it....

[carlb23]

If you are going to belt another generator to the engine you can get by with a large DC motor and feed the inverter directly from the motor.  3 to 5hp shunt wound DC motors are pretty plentiful around here.  I guess another option is to spin a three phase AC induction motor above nameplate and use it for a generator feeding cleaner AC to the rectifier.

 

[LowGear]

Moment of Education - Please.

Aren't DC generators really just alternators with built-in mechanical or electronic rectifiers?

Doesn't the induction motor system require a strong source voltage and frequency to sync with?

I'd use my real name but I get tired of being so ignorant.

LowGear

[guyd]

sort of (the DC question) but not really.

Take an induction motor, flash it (see elsewhere on here), stick some big capacitors on it in delta formation. Spin above plate speed, generate leccy.

In fact - you can spin it at any speed you like - you just need to alter the size of the caps. A 5kw motor, running at 300rpm needs 150uF per leg to work (and it does - I have one) - but generates 49 volts phase to phase. Normally you just need about 40uF - which conveniently is the same as a motor run cap.....



If my big DC motor idea works well - Ill have for sale a 3 phase 5kw motor and capacitors, foot mounted. In sunny somerset...

[ronmar]

As I recall induction motors are also pretty efficient as generators.  A 3 phase induction motor using capacitors for excitation to set the voltage output, with the output rectified might be a pretty clean and efficient DC source...  It might also be pretty cost effective as 3 phase induction motors are pretty common animals.

[cold comfort farm]

I looked at the induction generators and didnot want to go down that route for the reasons that when I am not here to babysit the generator needs to be run by 3 other people.  I need a press button here and you have power solution..  I think spinning a second wind generator is going to be the way to go.

thanks people.

[ronmar]

Well if you are using an inverter, once the capacitance for the induction motor was decided on that gave a voltage output(that would be rectified) within the acceptable range of the inverter, I believe a rectified 3 phase induction motor cranking out DC would be as turnkey as any other vegoil powered listeroid generator being rectified into DC to feed the same inverter.

[mobile_bob]

this induction generator rectified to dc and charging batteries is something i for one have
been very interested in seeing done.

the only question i have is how to taper the charge, i can think of a couple of ways of doing it
but none so simple as just using common bridge rectifiers

anyone have any idea's how to do this simply?

would slowing the rpm a touch taper back the output voltage or would you drop out of positive slip and
quit generating?

i dunno, but wanna know



bob g

[guyd]

Why would you need to slow the engine? If there is no load -there is no current - the IMAG just makes 240 volts (or whatever) but no current. You cant 'underload' it.

The built in governor will throttle back to keep the speed the same, thus the voltage the same. Why not use the 'vaugly 50z / 240v' to run a battery charger?