Question for wpowokal or Flux or anybody else

[wildbill hickup]

Well I got the MO trans all wired up, finally recieved rectifiers. wpowokal I took your suggestion and removed two turns on each leg Total of 15 turns per leg now(unfortunatly had to stay with romex, all I have at this point). Came up with the following AC readings (came close to what you said) 12.97 on one and 13.00 on the other. according to the info you gave me when rectified that should have come up with the following 16.95 and 17.00 VDC respectivly (ac/out X 1.4 - 1.2). Ok what I'm reading with the meter is 10.95 and 11.01 respectivly (no load). Now I know that this was discussed (I thought it was a comment in my diary in responce to what you said, but no)around the same time as my posting. Found it here under First test produced 4 volts.

sahlein commented:

Pardon me for interrupting here but, I have noticed something that I don't thoroughly understand.

A couple of times I see "rectified voltage" being mentioned as higher than

the output of the alternator.

I've always understood that the Output of a full-wave bridge is .9XVin.

In other words, since the computer won't type what I'm trying to write, the output of the bridge is 90% of the input voltage.

Am I missing something here, or is it just a matter of the terminology that you guys use??

Please guys, don't take this as an insult, it just sounds a bit odd to my ears.

I'm just a run-of-the-mill industrial electrician.

Joe S.

To which Flux responded:

Your 0.9 figure is correct for the mean voltage into a resistive load such as a lamp or a heater.

In this case we are talking about a rectified alternator charging a battery. Current will start to flow into the battery when the alternator voltage exceeds the battery volts ( neglect diode drop). This means that conduction will start when the peak ac voltage exceeds the battery volts. The peak is 1.4 x rms so roughly speaking current will start to flow at 8v when rectified and fed to a battery. the conduction will not be continuous with single phase, but with rectified 3 phase it will be once you have exceeded about 3% ripple.

A similar situation occurs with a rectifier feeding a capacitor, the no load dc voltage on the capacitor is the peak ac voltage.

My question is this, do the same calculations apply to what you said for my transformer? Bottom line is, will this charge my 12V batteries or is my low DC reading due to something else? Like to much resistance in the romex wire, or in the rectifier? Oh ya don't know if this will have a bearing but. rectifiers are 600V 25amp.

Wildbill

[wildbill hickup]

Woops that wasn't supposed to go into the hydro section sorry!!!!!!

[Flux]

I am not quite sure what you are doing, I presume your transformer is fed from the mains.

With those ac voltages you will get charging current when rectified into a battery.

Your rectified volts without a battery are about right, but dc readings into a meter with absolutely no load on the rectifier are sometimes a bit misleading. The snag is that if it is mains fed from a stiff supply you will not have a stable arrangement. With low batteries the current will be excessive and the charge will taper off too quickly for optimum charging. It is not quite the same as an alternator with limited current capability.

You will need a ballast resistor to limit the current to a safe value with low batteries.

I am not sure where you have 2 voltages are they single phase from the same transformer or have you 2 transformers rectifying a 3 phase supply.

If it is two windings single phase and you intend to parallel the rectifiers it may be easier to fit the ballast resistor on the primary side but it will need to be much higher in resistance, perhaps an electric fire element or something.

Flux

[wildbill hickup]

Flux thanks for your help,

I'll try and fill in some of the blanks.

My idea is this:

When running my gas generator I never seem to get the max out of it even though it's running a top speed. Seems like a waste of gas. I thought if I ran a lead to a battery charger (MO transformer rewound) I could charge a bank of bats at the same time the gen is powering items in the house. This stored energy could then be used when not running the gen in sellected circuits. The gen is a 5600 watt 6500 peak with two 15 A brakers/outlets(oh ya 120VAC). One side goes to the house breaker panel (at this point for emegency use only we have a lot of power failures here, the last one lasted 4 days) the other does nothing and I would like to charge off of that one.

I'm starting with 6 12V bats, 2 sets of 3 wired in parallel. The transformer has 2 winings of 15 turns each of 14ga romex (i know that's not the best but it's what I have available), this is in the experimental stage. I plan to attach each winding to it's own set of 3 bats. Readings from these already refered to. Each set will then be hooked up to an inverter and each inverter will power one isolated circuit in the house. Hence two complete cucuits taken off the grid both of these circuits draw less than 300 watts with everything on. Right now I'm powering the trans off the grid from the mains, til I get things figured out.

Now if this all works out I plan on running the gen once a day (probably durring the late evening) for about 3 hours. Even though this is a gas gen it seems to do really good on fuel(I realize that this will decress as the load increases)it just seems like such a waste to have that thing running at full speed and capable of putting out 5K+ when we are only using say 2K. I'm wandering, anyway after the three hour run I would like to run my computer system (one circuit) and some lights(second circuit) til bedtime. I probably won't save any money, I'm just curious.

Hope this fills in some of the missing data, this hairbrained idea may not even work but I'll probably learn something in the process

Wildbill

 

[Flux]

Thanks, it makes sense. It's a bit crude but you will probably get away with it.

You can choose your volts to the nearest turn and if you don't let the batteries get too low you may not need a ballast or perhaps you could make your battery leads long and a bit on the thin side to give a bit of stabilisation. You may have to watch the final voltage and switch off if the volts go too high.

You may find that you need different number of turns on the mains and the generator. Also if you have any adjustment of the generator volts you may be able to set it nearer than one turn on the transformer.

If one set of batteries gets lower than the other you may have to charge one set to equalise them every so often.

Flux

[wildbill hickup]

Thanx,

Well at least that puts me in the 'Hey it might just work range' that's all I can ask for.

Thanks again

Wildbill

[wpowokal]

well said Flux.

Wildbill, just be conservitive on the number of windings initally, simple to add or remove windings.

When you are at the crossing the tees and dotting the i's stage of your project and using enameled wire you may find the turns ration a little different, i'm not 100% sure on that point. But hey give it a go.

Flux's comment about a ballast is very good, over current initally is a problem I had when I first built my charger. Since it was run off a generator the (AC 240V) voltage regulation was not all that great, I had to keep the generator speed down initally to limit the volts or I blew bridge rectifiers. I'm a greedy sucker and like to push everything to 120%, I think it's in my geens, that's my story anyway.

Fortunatly I now have enough (too much actually) Re comming in to not need to run a generator for now anyway.

I am happy to take 'e' mails if required, my board name at bigpond.com.au