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Running a large DC motor from two generators?


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  #1  
Old 06-26-2010, 01:30:29 PM
jlrsp3 jlrsp3 is offline
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Default Running a large DC motor from two generators?

I have had this idea floating in my head for awhile. I want to take two generators and fit them with full wave bridges to develop 240 volts DC. I then want of parallel them and run a large DC motor.

What are problems facing me to implement this scheme.
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Old 06-26-2010, 03:07:52 PM
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Default Re: running a large DC motor from two geneators

Why do you need 2 generators to run your motor?
If they only put out 120 VAC each, you would need to connect them in series after rectifying the current to DC.
I see a strong potential for letting the magic smoke out of some, if not all, of your equipment!
This question / topic would be better on the "Sparks & Arcs" board. Lots of 'Resident Smart Guys' there that can give you answers with good reasons to back them up.
Your post has been moved.....
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Old 06-26-2010, 04:25:37 PM
armandh armandh is offline
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Default

when edison first put DC parallel generators online he had a problem of one getting all the load

sieries is easier

one current and voltage is related to speed&load

you would want a volt meter on each gen so that they can be governed to suite


keep in mind if the load is too big the larger is going to try and turn the smaller backwards

a big blocking diode across each output would prevent any reverse current

when rectifing ac b4 connecting in series the rectifier diodes provide the same function

if one generator quits the voltage from the other would travel through the failed generator's bridge rectifier providing half voltage to the motor

here some large storage batteries sound like a good idea
battery bank
1 motor
2 gens and 2 chargers

bell solved this one long ago
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Old 06-26-2010, 05:09:03 PM
pegasuspinto pegasuspinto is offline
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Default Re: Running a large DC motor from two generators?

240 volts AC rectified will turn into 340 (yes, three hundred and forty volts) DC, no matter if you start with parrallel or series generators.

Robert
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Old 06-26-2010, 07:07:57 PM
dalmatiangirl61 dalmatiangirl61 is offline
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Default Re: Running a large DC motor from two generators?

jlrsp3, why would you want 2 generators, that in itself makes your problem twice as hard.

Pegasuspinto, please explain why 240 AC rectified to DC becomes 340 volts?? I'm working on a 3 phase wind generator and was planning to rectify all 3 legs individually for 120 vdc, uhhh, did you just throw a monkey wrench into my plans?

Kristin
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Old 06-26-2010, 10:05:58 PM
Jack Hottel Jack Hottel is offline
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Default Re: Running a large DC motor from two generators?

I don't know if these will make you laugh or cry. From the Reference Data for Radio Engineers, for a full wave rectified sine wave, where A is the peak voltage, A average =(2/3.1416)A, A rms= A/1.414.
So A= 1.414A rms. Therefore for 240 rms volts input the peak output voltage is ~340 volts. Let me try some attachments
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Old 06-26-2010, 10:19:28 PM
pegasuspinto pegasuspinto is offline
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Default Re: Running a large DC motor from two generators?

We read AC voltage as 'RMS' (Root Mean Square) which is an average voltage. The most significant reason we use RMS is that the typical formulas, like ohms law and watts conversion, work correctly with RMS readings. If you rectify AC to DC with no load, it will end up AC's PEAK voltage is about 1.42 times the RMS voltage. let's say you want to charge a battery bank that should equalize at 130 volts. You take 1 phase rectified AC it will be very load dependent, but will slowly creep up to 170 volts. With 3 phase, it will be very close to 170 volts regardless of load. So yes, it will boil batteries, blow out light bulbs, and otherwise be a real problem if you were counting on 120 volts DC!
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Old 06-27-2010, 01:07:08 AM
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Default Re: Running a large DC motor from two generators?

Could one use transformers while still AC to bring the voltage down so that when rectified. the DC voltage is what was initially desired / specified?
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Old 06-27-2010, 01:16:17 AM
armandh armandh is offline
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Default Re: Running a large DC motor from two generators?

but most importantly why all the conversion
starting with rotary power and ending with rotary power
why not a jack shaft from engine to load
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Old 06-27-2010, 01:22:03 AM
jlrsp3 jlrsp3 is offline
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Default Re: Running a large DC motor from two generators?

I guess I need to elaborate or at least establish a recognizable reference point. Two generators of unequal wattage both putting out 240 volts AC and then each running through their own full wave bridge. Neither is able to run the 240 volt DC motor under load by it's self.

That is the concept but the actual wiring? Well I had this vague idea of just hooking the two bridge out puts to the motor with the assumption that the bridge diodes would prevent any shorts between the generators.

I am not quite following every thing armandh is saying. If I followed the series plan would I be rectifying two 120 volt sources and then running them in series to get 240 volts DC? I was not aware I could do that.

If that is the case I sort of understand what he is saying with the exception of two things. Where does the other half of the voltage go? And why would the bigger generator try to run the smaller gen backwards in the event of a overload?

I was thinking of using a battery bank to supply partial load and the bigger of the geneators to supply the rest. The load on the motor would be variable with no load about a third of the time so I was thinking the generator would also be able to recharge the batteries.

Then I got the idea of running two generators to supply the load.

Thanks all
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Old 06-27-2010, 01:40:37 AM
armandh armandh is offline
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Default Re: Running a large DC motor from two generators?

this is not really a good idea as one would likely end up with a generator frame hot to the other

best is commericial battery bank rectifiers [hi current chargers]
one powered from each generator and a motor at the battery bank voltage

the motor would be controlled with a bank of emitter followers paralleled and could have tack feedback or any other dc/dc motor control

converting theory to practice is the problem

what will the motor eventually do???

what about 2 AC motors coupled to the same load
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Old 06-27-2010, 02:46:14 AM
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Talking Re: Running a large DC motor from two generators?

Hey, it is simple... The OP has TWO AC Gensets, neither one big enough to run the load, so he wants to somehow combine the outputs, but doing it with AC is a Fools Errand, so he is thinking DC may be the answer. Well it is not quite as BIG of Fools Errand at DC, but How about you just Spend a few bucks and buy a BIGGER Genset, and forget all this Rube Goldberg, Haywire stuff...
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Old 06-27-2010, 12:41:53 PM
jlrsp3 jlrsp3 is offline
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Default Re: Running a large DC motor from two generators?

Hey gentlemen, I appreciate the feedback.. Bruce from Alaska made me laugh, he hit the nail directly on the head. At least for the time being, Armandh gives me food for thought.

The concept I proposed is a hazy extension of a more serious goal I am looking into. The basic idea is to get off the grid. Our house electrical needs are light, washing machine, gas dryer, refrigerator, several computers and a two horsepower submersible well pump. Every thing else is wood and propane.

I have two 240 volt DC motors. One is a five horse compound wound 20 amp the other is a thirty horse 104 amp shunt wound. The PLAN as it exists at this time is to buy enough automotive batteries to supply 240 volts to the five horse motor. It in turn is direct coupled to a five KW alternator. When someone or something in the house pushes a switch the motor turns on and supplies the necessary power. (Believe me I am the only person in the house who is interested in living off the grid).

As for the battery system I intend to treat it as a giant automotive system. As I understand it a automotive battery needs to be maintained at pretty much full capacity at all times. You can only afford to draw at the most 10% of its rated capacity before it needs to have a charge to back it up. Under these circumstances the batteries should last many years and apparently there are ways to combat sulfphation .

I also have a small diesel engine connected to a three KW alternator. The idea there is to treat this system in the same way as a automotive alternator,controlling the charge by turning the field winding on and off. The rectified 240 voltage will be 340 Volts at no load? That seems, on the surface, to be a good thing because I need around 300 volts to charge the batteries. The diesel would be sensed to the battery bank.

When I built the house I kept the lighting wires separate from the plugin wires. That’s basically why I need batteries, for lighting. As for the computers? Well I guess a small voltage inverter.

Obviously this plan can be manipulated in several different ways. I would be thrilled to hear from anyone who cares to comment on this.

As for the thirty HP motor, the full picture would be an extreme example of Rube Goldberg, Haywire stuff. I would be interested in Bruce’s reaction to that. Maybe I will venture into that later.

Best regards Scott
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Old 06-27-2010, 02:25:33 PM
pegasuspinto pegasuspinto is offline
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Default Re: Running a large DC motor from two generators?

Two facts to consider-
mechanical conversion of DC to AC (or AC to DC) with a motor/generator is horrilby inefficent. Also, you will need to come up with a way to regulate the speed of the DC motor accurately, and also to isolate the AC systems out while the motor spools up and down.

Direct charging batteries with rectified AC is kinda dangerous without regulation and intense monitoring-like i said, a 240 volt bank will quickly attempt to taper up to 340 volts DC in the system you propose.

Trying to generate any power on your own with combustion engines is horribly inefficent, causes infinately more pollution then the grid makes, and will cost a small forture compared to utility. I would love to hear from Bruce what a kWh costs him. But I would suspect the most finely tuned and efficent systems availible would cost 2-3 times what the same power from the utility costs. When you add conversion losses and the inefficency of what you describe would add up to, I would guess 10 times the utility costs.

I'm really not trying to discourage you-but my crude guess at your technical proficency level, I believe your best bet is to go with the pre-engineerd setups that are availible. IMHO, unless you have a particular need to go off grid, I would go with a grid-tie setup and sell excess power back to the grid, and buy it back when you need extra-and ditch the battery bank.

And no, I don't have pratically anything in the way of 'alternative' power. Every time I do the math, the payback is very poor-and you need to think of mantinence costs as well-hailstorm destroys your solar, or a wind gust takes out your windmill-and I just can't financially justify it.

Robert
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Old 06-27-2010, 03:43:59 PM
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Talking Re: Running a large DC motor from two generators?

We figure with Maintenance, Fuel, Useables, and a 50K Operational Hour Replacement Cycle, Electricity costs us about $.50/KwHr. Nearest Grid, (250 Miles away) is running abut $.12/KwHr. So Grid Power is about 1/4 the price.... Not very efficient, but better than no Electricity at all...
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Old 06-27-2010, 05:20:26 PM
Gil Garceau Gil Garceau is offline
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Default Re: Running a large DC motor from two generators?

Wow......
I hate to say it but the idea of using batteries to power a motor that powers a generator or alternator is nearly as inefficient as it could possibly especially when you consider that you are going to have something else (gas, diesel, wind, solar) to re-charge your batteries.

Why not just let the batteries supply power to an inverter that will run everything you have including your computer?

Better yet, Washington state has some of the more attractively priced and cleanest electric power anywhere. I am not sure of your precise area but as an example, I believe Puget Power is 70% hydro-electric, about 11-13% nuclear, and the rest is geothermal and other.

Living completely off the grid when the grid is right there in your neighborhood is way beyond anything sensible. How about selling your huge electric motors and build some solar panels and wind generator to help charge a battery bank along with a gas or diesel genset to back it up and fill in when needed and put an inverter on your batteries along with the proper switching to sell your excess power back to the utility as was suggested above.
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Old 06-27-2010, 09:48:05 PM
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Default Re: Running a large DC motor from two generators?

only with free energy is DIY generation a possible money saving idea

if you have falling water, wind, sun, bio-methane or geothermal ...GREAT
if you pay for fuel not so great

your local utility is the best source of cheap clean energy

we are at about 10.4 cents / kwh [12.1 cents after tax]

after some drunk took out a pole last night I spent 10% of last months electric bill in 11 hours of fueling the gen and that is with a reduced load

find the free fuel first
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Old 06-28-2010, 03:13:14 AM
jlrsp3 jlrsp3 is offline
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Default Re: Running a large DC motor from two generators?

Gentlemen, thank you for your input
Believe me I am well aware of the inefficiency of the system I propose. That is not something I am concerned about. What I am looking for is a rock solid low tech system that will last for years with minimal input from outside sources. If I am able to put this system into operation and solve the following relatively minor problems I will have a system that I will completely understand and can maintain without outside assistance.

1. Regulating the charging system by following the lead of a automotive charging design.
2. Coming up with a method that will sense a demand from the house and if necessary , give certain conditions priority. For instance if the well pump wants to start when the washing machine and the refridge are already running well then it will just have to wait.
3. Putting something together that when the battery voltage falls to a certain level the little diesel will come on line and help the batteries maintain the proper voltage and when the batteries are charged back up again shut itself off.

I am sure there are other things to consider but it’s getting late and I need my baby sleep. The technology is already out there just need to find it . Look guys, a person has to waste his time somehow right, call it a hobby. It’s something I want to do.

As I was writing this post it occurred to me that perhaps I am visiting the wrong web sight. I should be in some alternative energy place where they would at least sympathize with me.

As for efficiency get the thing going and then look for ways to make it better.

Every electrical book I have read says that a shunt wound motor operating within its load range and voltage is pretty much self regulating . The system , as rugged as it is will tolerate a little variation.

Fuel? There is wood gas, methane (my fuel of choice),bio diesel and other more exotic choices. Build it first and the fuel will come.

Scott
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Old 06-28-2010, 07:39:37 AM
armandh armandh is offline
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Default Re: Running a large DC motor from two generators?

dear scott

there are a bunch of realists at this site
they do not dis the idea of generation because they do enjoy the idea
they will however point out the obvious problems only to discourage belief in the unobtainable. your local utility has economies of scale one can never match.

here is what happened to a fellow I knew near Zürich
he and his neighbors were sold on the idea of a common heat plant to get some scale. it used fuel oil and solar for the source energy.

the fuel oil powered a good size diesel refrigeration compressor heat pump
the circulation water was heated by solar, engine heat and the heat pump
which scavenged the exhaust heat

the engine speed was controlled by the heat pump suction pressure.

a year or so in lack of attention to refrigerant level led to low refrigerant pressure over-speed and the diesel stuck a rod out the side

a regular boiler was installed while the lawyers discovered the various installing and engineering folk were long gone as well as the corporations.

had you mentioned in the beginning the end use was house hold power
I would have stressed dividing the load and if any single load is too large
get a larger generator but only as a back up

price wise you cant beat the local utility except with a surplus of energy such as a dairy farmer's free methane or a waterfall in the backyard.

you may find more boosters for your plans elsewhere among fuzzy thinking liberals but I doubt more truth.

best of luck
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Old 06-28-2010, 08:41:24 AM
Gil Garceau Gil Garceau is offline
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Default Re: Running a large DC motor from two generators?

Make no mistake about it..... Even if you generate free electricity, you want to be on the grid so you can sell your excess to the utility.

Also consider that if you have power from a utility at the street, you are really shafting your wife and kids if you make them depend on a rube goldberg rig that might just deny them of some conveniences that we have been accustomed to for the last 80-100 years. Even in third world countries they can plug a lamp into the same receptacle that powers the refrigerator. It is also nice to have simple things like electric clocks that don't require a gas guzzling generator to operate. If you are planning to move somewhere that has no grid this is good practice for that day. If it is pursuit of a hobby, also all good however I advise staying on the grid, selling excess back to the utility, and make it all seemless for the rest of your family. Your hobbies need not make them miserable.

Gil

Last edited by Gil Garceau; 06-28-2010 at 06:14:45 PM.
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