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3 phase motor used as generator

James Rogers

Aluminum
Joined
May 27, 2009
Location
Asheville, NC USA
This might be the wrong place to post this but I didn't see anyplace else for a question of this nature.

We just went through a 4 day run without any electricity and I just wondered if a 3 phase motor attached to a gasoline motor would produce power? I use a 3 phase motor in a RPC capacity and produce power on one leg by running it on 2 legs to get 3 phase for the mill and lathe so, I figure it might work. If it is run with an outside source of power would it produce power on all legs? Would it need run caps?

I ask because I have a couple of Model A motors that I could use as a dedicated power source for a generator if this would work. I could machine a groove in the flywheel and use it as a pulley for a belt drive and gear the motor to run at or about 1750 RPM.
 
I just wondered if a 3 phase motor attached to a gasoline motor would produce power?

Yes but there is a catch.:D

It has to be connected to power and rotated at slightly over synchronous speed.

Some wind generators do this.

Tens of thousands of makers and users of alternators would be using ordinary motors if this was an easy or effective solution


John Oder
 
Unfortunately you need armature excitation, an alternator uses a DC field on the armature fed by slip rings.
The RPC gets rotor excitation from the single phase on the other two windings.
Minder.
 
Hi
No it will not act as an alternator.
To generate electricity you have to move wires through an electromagnetic field, or move the field past the wires..
There is no such field in an ordinary three phase or single phase AC motor which is just being spun with a petrol or diesel motor.
In a real alternator, the windings that generate the Voltages are stationary, and the magnetic field is produced by pushing DC current into a field coil on the armature .
When this armature is spun by means of the petrol motor, the magnetic field produced by its coil cuts the stationary windings and hey presto it generates electricity.
Because both North and South poles of the armature pass the stationary coils, an alternating current is generated.

This is exactly how a car alternator works. The three phases produced are rectified to DC via the diode block, and the regulator switches the armature field on and off so that the output voltage never goes above 14.2 volts, so as not to boil the battery.
The current from the car battery is fed to the armature field coil via carbon brushes on slip rings.
Wind generators usually have permanent magnet fields so that there is no need for a supply voltage. Again the coils in which the voltage is generated is stationary, and the permanent magnet field is spun by the wind driven propeller
 
Davycrocket

There is often a bit of residual magnetism present in an induction motor armature. Sometimes enough to initiate "self excitation" of the stationary windings, then the entire inductance/ reactance phenomenon takes palace, and "Viola" AC voltage and current output.

It remains that an inductance motor is a very unreliable AC generator source!
A reliable excitation field is needed.

Look at Win-Power pages for good ideas! I have a nice little 5 KVA device by that manufacturer. Perfect size, to small for just about any need, It should last a lifetime ;-)

CalG
 
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Whoops, sorry Minder
I wasn't elaborating on your post.
You must have put yours in whilst I was still typing mine.

But we are both singing from the same Hymn sheet. !
 
There are people who have successfully used induction motors as "island mode" AC generators (island mode means not connected to the grid). To do it, you have to set up capacitors that basically boost the residual magnetism in the motor to a level that can excite the windings. There are plans and papers written on this available at various wind power sites. It's tricky though, and a lot of it depends on the residual magnetism of a particular motor, something you cannot specify at purchase. So essentially, it's a bit hit-and-miss as to whether the motor you want to use is capable of doing the job, and there is no real way to know in advance without experimenting. The other pitfall is that they are extremely sensitive to overloads and can stop generating all of a sudden if subjected to one. I tried my hand at doing one with an old 3/4HP motor and some random starting caps I had laying around. I got it to work, but I plugged in a 500W heater to test it out and it overloaded, so the voltage dropped down to 10V; basically the amount it could generate from the residual magnetism. I admit to not taking much time to make it work more reliably, I just wanted to see if it could be done. It could, but it was a lot of bother for a little bit of juice.

There are other plans and papers I have seen on modifying AC induction motors by machining out the rotors and embedding permanent magnets, something the average homeowner is unable to do. But within this group, that might be a workable solution.

All in all though, it's probably cheaper and easier to just buy an alternator, or better yet, buy a backup generator. Clean, quiet, reliable.
 
Thanks guys, I guess I'll get one of the 10,000 watt powerheads from HF and belt drive it off one of the extra motors I have.

BTW, we are off the power grid again today. On for 24 hrs and back off.
 
I know this is an old thread but I myself have been designing a generator for home use over the years. This my results have left me with 18V DC input and 376V DC on the output. I have failed at my first sustainable attempt and have fried my equipment and prototype in the process. Essentially I have a 3 phase motor that I have modified and dedicated one of three terminals as my neutral and two separate terminals reading 188V AC each. When combined I obtain 376V AC. In theory your idea is obtainable with the right setup. My designed has actually become self-powering. No need for an external power source other than a quick jumpstart from a switching relay I have built. I have yet to discover it’s potential.
 
Just get a $50 craigslist generator and connect it across one winding. That's all you need for the engine motor combination to power everything else.
 
Just buy a used diesel backup generator from ebay, craigslist, many resellers out there....you can get a very reliable generator for small cost. Trying to build something yourself is very very difficult. Most modern generators have micro-computers to keep the engine running at exactly the same RPM continously regardless of load, in order to run the alternator and generate a good quality sign wave. Without this, the quality of electricity you generate will be so poor that your house devices will be constantly doing wierd things (flickering lights, devices crashing, etc...).

Also, you absolutely need a commercial automatic transfer switch, and you need to get permission to connect all this stuff to the grid. Without this you could easily cause the death of utility workers or yourself.
 
My designed has actually become self-powering. No need for an external power source other than a quick jumpstart from a switching relay I have built. I have yet to discover it’s potential.

This needs hi-lighted.....:nutter:

To the rest of you..."Please don't feed the squirrels"
 
This needs hi-lighted.....:nutter:

To the rest of you..."Please don't feed the squirrels"

Gee I was just going to mention the device I built that thru a simple radio antenna collect radio signals and convert it to power my home and shop! Plus have it connected to the grid so that the power company must send me $1000 every month. All built from parts available at the local hardware store for less than $29.99.:crazy:
 
You can now buy those neomydium button batteries by the box.

Perhaps someone determined to reinvent the wheel could shoehorn them into an induction motor?

I will not be the volunteer.
 
Thanks guys, I guess I'll get one of the 10,000 watt powerheads from HF and belt drive it off one of the extra motors I have.

BTW, we are off the power grid again today. On for 24 hrs and back off.

We went a full week without power, after an "inland hurricane" went through some years ago. I had a small Honda generator that did a magnificent job. One of the problems in an extended run, they can get expensive to feed.
 
Old thread....

The induction generator does in fact work. I have had one working.

Two requirements:

First, there needs to be a residual field in the iron, to produce s tiny excitation from which the generator will "build up".

Second, as mentioned, there needs to be a capacitor bank set up to supply exciting current.. The capacitance is fairly critical. Too little will not build up, and too much will produce a large overvoltage at light load.

The induction generator has some not entirely ideal characteristics. It will, if overloaded, "collapse", the voltage will disappear. This is because the field is not independent from the output, as it is with a generator that has an exciter built-in, or operating on the same shaft. An overload actually steals current from the capacitors that are "supplying" exciting current, causing the excitation to drop. The "collapse" is essentially a "reverse build-up".

It is, however, perfectly practical, and is a rugged, reliable, self-protecting generator that is being used in remote areas, typically powered by water power,, so that there is no fuel involved. It is very good for any place that cannot get spare parts easily, since it tends not to need them.

I would not try to power a home with one, the motor loads would require a rather large generator, and a standard type would tolerate motor starts far better, There is no reason to deal with the disadvantages if you do not need to,
 
I've not built one of these and do not have any immediate need to build one, but I have always been intrigued by them.

Re: having enough residual magnetism in the motor for it to self-excite, could there be an option to temporarily send a brief but strong "DC jolt" through the windings (maybe just one phase), while the rotor is stationary, but only moments before putting the rotor under power? That way you would not be counting on persistence of "long ago" residual magnetism.

Regarding the drop-out if subjected to overload, is there any sort of 'rough rule of thumb' on approximately how much power one of these is capable of producing (for a given size induction motor) before that drop-out point?

And any rough sense of whether, in trying to figure out how much mechanical rotating power is needed per kW output, any sense of whether these are worse (how much worse?) than a standard generator of similar kW scale? Motor size will of course relate to output in a generally proportional way, but could one "back calculate" that an _X_ kw motor would be expected to be able to generate out approximately _Y_ % of the motor's rated electrical load before reaching dropout?

I know this is getting OT from machine tool topics, but when I have tried to look up info on these IMAG (induction motor as generator) applications, information seems sparse and scattered at best, and I know we have some particularly bright and experienced minds here.

Thanks
 
I've not built one of these and do not have any immediate need to build one, but I have always been intrigued by them.

Re: having enough residual magnetism in the motor for it to self-excite, could there be an option to temporarily send a brief but strong "DC jolt" through the windings (maybe just one phase), while the rotor is stationary, but only moments before putting the rotor under power? That way you would not be counting on persistence of "long ago" residual magnetism.

Regarding the drop-out if subjected to overload, is there any sort of 'rough rule of thumb' on approximately how much power one of these is capable of producing (for a given size induction motor) before that drop-out point?

And any rough sense of whether, in trying to figure out how much mechanical rotating power is needed per kW output, any sense of whether these are worse (how much worse?) than a standard generator of similar kW scale? Motor size will of course relate to output in a generally proportional way, but could one "back calculate" that an _X_ kw motor would be expected to be able to generate out approximately _Y_ % of the motor's rated electrical load before reaching dropout?

I know this is getting OT from machine tool topics, but when I have tried to look up info on these IMAG (induction motor as generator) applications, information seems sparse and scattered at best, and I know we have some particularly bright and experienced minds here.

Thanks

One thing to consider is slip. If you have a 2 pole motor rated at 3450 rpm, it has 150 rpm of slip at full load, so you must drive it at 3750 rpm to get 60hz out. If the load is suddenly reduced, that slip goes away and gives you 62.5Hz, so frequency regulation is difficult.
 
I'm interested in this stuff from the self-sufficient shop perspective, but from my learnings, if you're looking to power your freezer, kitchen lights, and phone charger, a ready-made generator set is going to be your most economical solution.
 








 
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