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Thread: redneck 3ph generator
04-22-2011, 10:31 AM #1
redneck 3ph generator
can a tractor be used to turn a 3ph motor to generate 3 ph power. im guessing if i turn a 1750 rpm motor at its rated rpm i will get 3 ph power at 60hz.
what hp diesel engine would it take to drive a 40 hp generator/motor?
would a 40hp generator of this type drive a 40 hp motor.
04-22-2011, 10:57 AM #2
No, you'd need to spin a generator or alternator to make power, not a squirrel cage motor.
I've spun a 15 HP 3 phase motor to nameplate speed and all I get is about 2 volts, and at speed it is 60 cycles.
The same motor runs perfectly when you give it power.
You can buy a ready made PTO driven generator for your tractor.
04-22-2011, 11:03 AM #3
04-22-2011, 11:52 AM #4
I have a Lincoln SAE 400 motor generator welder that I rigged up to run with the tractor PTO. The welder works good. I was surprised to check the motor leads and no voltage. I thought with all the magnetism from the DC, the motor would be excited.
A lot of these wind generators are just induction motors. When the wind has them running at synchronous speed, they put power into them and get more power out. The better windmills do the thing kind of like a backwards VFD, so the power all comes out as 60 hz.
04-22-2011, 11:59 AM #5
everybody is right!
correct, you won't get any output from the induction motor until it makes its magnetic field first. so if you had a way to get the magnetic field started I believe it would continue on as long as turning..... by the time you made a circuit to 'jump start'. the motor into a generator, you'd be much better of IMO to just buy a generator or take a generator off an old decrepit motor/generator and use it,
04-22-2011, 01:09 PM #6
Nope, an induction motor will not generate by itself. If the motor is driven externally it will efficiently contribute power to a connected electrical grid.
Second part of your question:
The 500 to 700% FLA initial starting surge of middling three phase motor raises hell on small plants like this. A 40 HP motor will nearly stall a 50 KVA diesel alternator - probably stop it dead. Larger means voltage and frequency dips. You probably need a 75 KVA alternator to start a 40 HP three phase motor across the line because of the starting surge. That means you need a 120 HP diesel engine to run it.
If the 40 HP motor has a large inertia load connected with it like a centrifuge bowl, flywheeled press equipment, loaded conveyor, etc you better be prepared for some major brown-outs. If you need fairly clean power while starting a 40 HP motor I suggest upsizing the alternator to 200 KVA.
If electronics are part of the system you better install a UPS between the electronics and the generating plant to keep them and an emergency light alive when during hiccups, spikes, drop-outs, and outages. You probably should look into some form of "soft start" if you want to run a 40 HP motor from a plant not much bigger.
But don't believe me. I know enough to make some guesses but not enough to reccommend a good system. Consult a portable industrial or emergency power expert. If you're on a shoestring, an expert can help you make the best of your available resources.
04-22-2011, 01:33 PM #7
Since this is a forum of machinists, it's probably appropriate to say this. I generally don't mention it to people who ask because most people lack the skill set to pull it off properly and end up doing something that causes damage.
There IS a way to modify an induction motor to become an "island mode" generator, but it involves carefully machining the rotor and installing permanent magnets to supply the excitation. There are several "home power" websites devoted to that concept with full instructions etc.
There are other sites devoted to a way of doing it by finding a motor with a high residual magnetism, then using capacitors to store and essentially amplify that into enough excitation to make it work. Very iffy though because everything depends on a random sampling of many motors to find one with enough residual magnetism to get it started. The proponents tend to gloss over that part but I know someone who did it and he had to go through testing about 20 AC motors to find one that worked, not everyone has that kind of time, resources and patience.
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04-22-2011, 02:22 PM #8
I suspect the closest one might come to 'redneck' three-phase would be a proper coupling of three identical single-phase generators, but before you run out and try to tie generators with synchronous belts or chain and sprockets, let's hear from the electrical engineers amoung us.
Oh, you would most likely need an o'scope.
04-22-2011, 05:23 PM #9
Just to be clear as mud, an induction motor by itself WILL be a generator off the grid. I think most cheap 3-4-5kw gas powered generators you buy are induction motors, and if they have a good instruction manual, will tell how to remagnetize them if they won't start on their own (as simple as clipping 12vdc across them to renew the residual magnetic field in them - same as we sometimes have to do with our ol' 8n ford tractor's generator. A capacitor is used to get em started by using this residual magnetic field to start current flow to build the field up to full level. Here is wiki-pedia explanation:
Squirrel-cage rotor - Wikipedia, the free encyclopedia
text: Induction generators
Three phase squirrel cage induction motors can also be used as generators. For this to work the motor must either be connected to a grid supply or an arrangement of capacitors. If the motor is run as a self exciting induction generator (SEIG) the capacitors can either be connected in a delta or c2c arrangement. The c2c method is for producing a single phase output and the delta method is for a three phase output. For the motor to work as a generator instead of a motor the rotor must be spun just faster than its nameplate speed, this will cause the motor to generate power after building up its residual magnetism.
some people have had real good luck making every induction motor they try work:
text: By adding capacitors in parallel with the motor power leads, and driving it a little above the nameplate RPM, (1725 RPM ones need to turn at approximately 1875 RPM, and 3450 RPM ones at 3700 RPM) the motor will generate AC voltage! The capacitance helps to induce currents into the rotor conductors and causes it to produce AC current. The power is taken off of the motor power leads, or the capacitor leads, since they are all in parallel.
This system depends upon residual magnetism in the rotor to start generating. Almost all the motors I've tried begin generating just fine on their own, with the appropriate capacitor connected of course! If it doesn't start generating, try speeding the motor up. That will usually get it going. However, it is extremely rare to find one that doesn't start.
If a motor doesn't start generating on the very first try, then apply 120 vac or even 12 or more volts DC to the motor for a few seconds. That will usually work to magnetize the rotor and your generator will start by itself from then on.
04-22-2011, 05:41 PM #10
You might be able to get power out..... But most commercially built alternators, or generators, use field coils connected to a voltage regulator. (Or an exciter controlled by the VR, and the exiter connected to the field coils, depending on the size of the unit) Otherwise how would you maintain a constant voltage with variable loading.
04-22-2011, 06:26 PM #11
Don't get me wrong, I'm not recommending anything here, just offering the facts.
If I were going to use an induction motor as generator I would surely research the implications of load changes and lots of other stuff.
Just first quick thought tho is it would probably be fine; the impedance of a 40hp 3ph motor is probably 0.2 ohms: the loads applied to it are probably >10 ohms so probably have little effect on changing the generated voltage output. My fellow hams using little 3-5hp motors run by lawn mower engines for field day events have loads that go from milliwatts to 3kw instantly when they fire up the transmitters and say they have no voltage reg issues. But I do not have first hand experience doing it.
First thoughts on inrush and starting issues seem to me would be no issues either; you turn on the tractor, turn on the PTO, spin the motor up - remember it ain.t generating yet - wouldn't it be sorta like a softstart circuit since the capacitor has to kick it in the but first to begin building a magnetic field? course you wouldl start it going with no load on it then kick on the breaker. the big inrush is caused by having 230v going into this .2 ohm load - until it gets spinning and the 1/s term kicks in. so probably can do it with my 23hp ford 8n. just wouldnt get more than 23hp of power out of the 40hp motor (tad less for its 90% efficiency)....
But come on, would a redneck generator really need to regulate the voltage well? If it did, would it be a redneck generator?
04-22-2011, 06:42 PM #12
If I was going to make an Island generator, or redneck generator to hook to my PTO, I would simply buy one of these in the proper HP range for $ 200 surplus and just use it. 3phase out the wires, no mods.
But first I would need to buy an island though. Wonder if there are any cheaps ones for sale......
04-22-2011, 09:33 PM #13
Well, well, welll...............
I have MADE a generator from an induction motor...... and it did work. You need some capacitance on the terminals to supply excitation, and you need to "flash" the coils to provide a starting field..... But it worked, putting out some power.
It is apparently done in South America as small scale hydroelectric power, and seems to work. Three phase motors are simple, cheap, rugged, and available, which makes them a good fit for the literal "backwoods".
There is even a paper which I used to have a link to from the EPRI or some such, on selecting the correct exciting capacitance.
That's as standalone. Obviously when grid-connected, you just have to apply enough torque to have "negative slip" and you will get power output. Just like many large wind turbines.
04-22-2011, 09:52 PM #14
all this talk about capacitors is makeing me dizzy.
what if i wired it into the grid just like a pony start 3 phaze convertor.(pony start being a 100 horse tractor) and then instead of disconecting the pony start i disconnected the incoming grid power. would that not excite the fields.?
04-22-2011, 10:07 PM #15
"Nope, an induction motor will not generate by itself."
Not to disagree with an expert but there are several web sites out there that
give explicit instructions on doing exactly this.
1) the power that can be drawn from an induction motor rigged as a stand-alone
generator is limiited.
2) the setup is quite critical, there has to be a capacitor across the windings and
it has to start on no load.
The typical use is small loads for field day ham stations for example. I've never
heard of it running on three phase service, or any large loads that included
A google search will turn up the sites.
04-23-2011, 12:01 AM #16
Firstly an induction motor does not have fields. there is a squirrel cage conductor in the rotor which gets current induced in it by current in the stator windings, you need current to make current with a generator like this. The capacitors are a low loss way of producing current which starts and keeps the thing generating power.
The unit could be pony started from the grid but there are all sorts of safety issues with this. The output would collapse as soon as the grid or load was removed, remember it needs current to generate.
With a proper generator in the 50 to 60 kVA range expect something like this.
Peak starting current will be about 3x FLA (motor full load amps) with a DOL start from a similar sized generator, generator voltage will drop to around 50% engine load will peak at 100 to 150% rated (depends of motors locked rotor power factor) The generator has a higher impedance than the mains which reduces the peak motor load current (LRA) Impedance has a resistive an inductive component in this case, the engine only sees the resistive portion of the impedance as load.
Last edited by HelicalCut; 04-23-2011 at 12:03 AM. Reason: spelling
04-23-2011, 02:49 AM #17
safety issues indeed! joe lineman disconects to do maintenance and gets the all clear only to get an unexpected blast from your end.
let me clairify, i have two dreams one is to have a huge shop in the country (where 3ph isnt even an option)complete with all the goodies. lathe , mill, welder, etc. the second is to go off grid. now how often will a guy actually be useing that big equiptment? not enough to justify getting 3 ph service even if it was available. power just went up 65% this month with no end in sight, so off grid is looking more and more attractive. the most economical solution is to generate my own heavy power when i need it. so when i mistakenly used the word "grid" to excite the fields, i was thinking about my own grid. kind of a mute point because as you explained an induction motor doesent have fields. most of the info on the net deals with smaller motors and loads, im still convinced it can be done though so keep the ideas coming in. thanks.
04-23-2011, 03:44 AM #18
I'm glad you understand why a generator can't just be hooked up to the grid. I have been where you are in the past and ended up buying an old generator. Over here 25 - 70 kva generators sell for a little over scrap $1K - $3 K depending on age.
2 of the last 4 generators I had anything to do with got scrapped because we couldn't sell them so I wouldn't be messing around tying up a good tractor when an old diesel or gas genset should be available fairly cheaply the cost of fuel over the running time of the unit will dwarf the initial capital outlay. The genset will probably use less fuel than the tractor, Another advantage of the genset is 480V power, that should be worth a saving in cable and the phases will be balanced no wild leg and the like.
There is nothing to stop you making an induction generator the real question is why when there are better options. Unless generators are outrageously priced in the US.
I could not say if an induction generator would start a large motor, it might just stop producing power under heavy load.
04-23-2011, 08:42 AM #19
04-23-2011, 01:30 PM #20
The very nice thing about an induction generator on the grid (as opposed to a residual magnetism/capacitance excited stand alone one) is that it basically CANNOT "island"....... (i.e. generate dangerous voltages on the local part of the grid).
When the grid goes "off", the generator can no longer generate unless the load and capacitance are just perfect..... very unlikely. Even if perfect, if anything changes, off goes whatever voltage is remaining... and it won't come back until the grid does.
Thus an induction generator can help satisfy IEEE 1547 & UL 1741, while still providing the capability for "low voltage ride through" to satisfy other requirements, if that is needed.