Need some help please with new diesel generator

[Hilltopgrange]

Hi all

       I have just purchased/stole a little Lister Petter ex M.O.D.(British Army) diesel generator from ebay see link

http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&ih=012&sspagename=STRK%3AMEWA%3AIT&viewi



tem=&item=220117710172&rd=1&rd=1

it is rated as 3.5 KVA and is 110v only (but I suspect this could be altered to 220v) My reason for the purchase is to use it as a standby 2kw charging source for times of low wind or high demand. I have an everlasting source of free veggie fuel oil so I just couldn't resist it; the engine is worth the price on its own!

So here is my objective, I wish to charge my 48v 510 ah @5hr battery bank at about 2kw. Auto start /shut down and charge control will be added later but for now I am the auto start and charge controller lol

So here is my problem and where I may need a little help please! The generator is 110v out and I want to step it down and rectify to charge my 48v bank

 if I take 3.5 KVA  220v - 110v step down transformer and supply it with 110v instead of 220v will it step down to 55v rms ?

Similar to this transformer

http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&ih=013&sspagename=STRK%3AMEWA%3AIT&viewi



tem=&item=230137418202&rd=1&rd=1

If I then rectify this 55v rms to DC the peak voltage is higher but for the life of me I can not remember the multiplication factor to work out how much higher, something tells me it is 1.4??? no doubt someone can put me straight!

So will it work?

 if not I would welcome any suggestions on how best to proceed

Many thanks in advance for those that take the time to reply.

Russell

[Flux]

This may not turn out as easy or as well as you had hoped. I haven't checked the links but I assume the alternator is single phase. This never leads to particularly good results. You are modest in only wanting 2 kW you may get it.

The 55v ac will come to 55 x 1.4 peak ( may not be desperately accurate with an alternator). You will need some form of ballast resistor between the rectifier and battery to limit current to a sensible level. How much you need depends on the alternator and the transformer. Conduction only takes place when the rectified voltage is above the battery voltage and consists of a series of high current pulses, both of which are easily limited by the alternator and transformer reactance. Because of the spiked waveform the rms winding losses in both will be high. There is an added loss from the ballast resistance.

Things work better with a large inductor between the rectifier and battery, this maintains conduction over a longer period. I don't think you will have enough volts at 55v to run it as a true choke output rectifier but if you could find an inductor with lots of taps then you may find a value where it will give the current you want without the ballast. This may improve things quite a lot, but at added cost, complexity and an added copper loss. You may be able to make the choke from a large microwave transformer, with a gapped core ( try 2m) and a multi tapped winding.

The end result will never be as good as using 3 phase but if it all comes cheap and you don't need to run it very often then good luck with it.

Flux

[Ungrounded Lightning Rod]

I notice You've got a 3.5KVAR genny and are contemplating using a 3.5KVAR transformer with it - but one rated for twice the input voltage to try for 2 KW output.

A transformer will work at lowered voltage but the VAR rating must be reduced in proportion, because the limit is heating and nearly all of the heating is from I-squared-R losses, i.e. proportional to current, not wattage.

Your transformer will have an effective rating of about 1.75 VAR when used at half-voltage, which means a 2 KW load will overheat it - and the genny will have enough output available to fry it nicely if you draw even more.

If you're going to use the transformer at half-voltage in this way you need one that's rated for 7KVAR.

(Also:  Don't go HIGHER than the rated voltage - at least at the rated frequency.  Go too far that way and you saturate the core, at which point the current climbs drastically and the magic smoke gets away.)

[Hilltopgrange]

Hi Flux

        As usual thanks for the reply. I think I need to wait until the generator arrives and I get a proper look at it, it is due tomorrow Thursday so fingers crossed.

        The weather forecast is for 20-25mph winds tomorrow so I hope to get some results on the 10 footer open stator mill you advised me about. This will be its first real test since raising it. The increased offset seems to have helped with tracking the wind but tomorrow should tell if it furls or not! we will soon see.

Thanks again

Russell

[Hilltopgrange]

Hi Ungrounded Lightning Rod, thanks for the reply.

        It seems this isn`t going to be a straight forward job, never is with me anyway lol. I have contacted the seller of the generator and apparently these units were designed as dual output 110v or 220v but the British Army had a particular job in mind for them and had the 220v side disabled. If this is the case I may run at 220 and try and get a transformer to suit or a 48v charger but they are like hens teeth in these parts! I do have a large transformer from the battery charger of a 72v forklift the charger was single phase supply at 220v and it had 3 of these large 4kva transformers but only 2 are usable. I did a quick test on one of them today by hooking it up to 220v and with its own giant rectifier I was getting 84.5v dc with no load. I know the battery will clamp the voltage down but I suspect this is still to high, I don`t want to do the battery any harm. I have added a couple of pics of the transformer and bits for clarity.

Many thanks again

Russell

[Ungrounded Lightning Rod]

(Change all occurrences of KVAR to KVA.  Sorry 'bout that.  B-(  )

[Ungrounded Lightning Rod]

I do have a large transformer from the battery charger of a 72v forklift the charger was single phase supply at 220v and it had 3 of these large 4kva transformers but only 2 are usable. I did a quick test on one of them today by hooking it up to 220v and with its own giant rectifier I was getting 84.5v dc with no load. I know the battery will clamp the voltage down but I suspect this is still to high, I don`t want to do the battery any harm.

I suspect you'd damage the rectifiers and/or transformer before the batteries.

Those transformers would be fine if you were running 72V of batteries.  But for 48V you'd need to tap their output winding at about 2/3 of the current output.

[Flux]

72v is high to run in the conventional way, you will need a fair bit of ballast resistance. You may be able to tap the winding or even regulate the alternator to lower than 220v.

If you decide to go with the inductor in the dc output then 72v should be close to what you want. You may need a bit of resistive ballast if inductance alone can't deal with it. Again reducing the 220v input voltage would be easier if the alternator is adjustable.

Don't worry about the batteries, they will not be the things that suffer as long as you don't run the charger beyond the gassing volts stage, they will clamp the voltage, you just need to keep the current within the limits of the individual parts ( transformer and rectifier).

Flux

[Hilltopgrange]

Thanks guys for your help, I will give it a go when the gen set arrives, its like waiting for a baby lol. Thanks again for the advice and I will let you know how it goes!

Russell