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Generators & Motors General Discussion Antique Generators, Light Plants and Old Electric Motors: Questions and answers about restoring and showing old power generation systems.

Generators & Motors General Discussion

3 phase motor to generator conversion?


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  #1  
Old 06-10-2008, 10:44 PM
DirtbikePilot DirtbikePilot is offline
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Default 3 phase motor to generator conversion?

I have a 52 hp, old school, open frame 3-phase motor. It's a GE, 440 volt, 860 rpm, and has 3 slip rings in the back end. Is it possible to wire it to generate electricity? The slip rings aren't connected at all right now and it was freshly rewound back in the late fifties, or so it appears. The tag has both primary and secondary amp ratings if that helps. There are three wires coming from the field coils. I thought I read something somewhere about doing this. Any ideas? Thanks!

Chris
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Old 06-10-2008, 11:37 PM
DirtbikePilot DirtbikePilot is offline
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Default Re: 3 phase motor to generator conversion?

Here is a picture of the motor.
Attached Thumbnails
motor.jpg  
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  #3  
Old 06-11-2008, 12:23 AM
Jim Rankin Jim Rankin is offline
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Default Re: 3 phase motor to generator conversion?

By the tag (860 rpm) it isn't a synchronous motor or it would be rated 900 rpm. The synchronous motors are the closest to a standard generator and would only need a voltage regulation system to control the excitation of the field on the rotor and a prime mover to turn it at synchronous rpm.

It must be a wound rotor induction motor that MAY have the rotor circuits shorted or connected by resisters on the rotor to create a certain fixed "slip" which is why the sliprings aren't connected right now.

My understanding is that by using the sliprings and brush gear, different resistances can be substituted during operation to make it have more slip at startup to keep amperage draw down/soft start the load and then become a lot "tighter" once the load is close to full speed for increased efficiency.

Most standard induction motors have a fixed amp/torque/speed curve that can only be changed by changing the load (a transmission or clutch of some sort) or the voltage (a soft start of some type), but a WRIM's rotor circuit resistance can be changed by an external controller to accomplish the much the same thing as a modern soft start or variable frequency drive does today with a standard induction motor.

The thing that will determine whether you can use it as a synchronous generator is how many poles the rotor has if you energize it with DC . When it's running as an induction motor, the stator "induces" the poles in the rotor windings, while in a synchronous motor or generator, the poles are created by the design of the windings and iron on the rotor.

With 3 sliprings, who knows how it's wound. If it had 2 sliprings, it still might have an odd number of rotor poles. For that speed motor (8 pole stator/900 rpm synchronous speed), you would want it to have 2 rotor poles and turn at 900 rpm to generate 60 hz 3 phase electricity from the 8 pole stator (you called it the field, but anyway, the main windings of the motor).

2 pole turns 3600 rpm with 60 hz AC (3000 rpm with 50 hz)
4 pole turns 1800 rpm with 60 hz (1500 rpm-50 hz)
6 pole turns 1200 rpm with 60 hz (1000 rpm -50 hz)
8 pole turns 900 rpm with 60 hz (750 rpm-50 hz)

In general the wound "poles" are on rotor of a generator and stator of a motor.
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Old 06-11-2008, 12:47 AM
Jim Rankin Jim Rankin is offline
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Default Re: 3 phase motor to generator conversion?

http://www.gemotors.com.br/products/motors/wound/

It should have an 8 pole 3 phase winding on the rotor.

It's a beauty! Could always use it as an induction generator in parallel with the utility power if you have a source of prime mover power to run it faster than synchronous speed so it will generate back into the power line.
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Old 06-11-2008, 02:02 AM
rrfxxxr rrfxxxr is offline
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Default Re: 3 phase motor to generator conversion?

An idea about the 860 RPM, could it have more than 8 poles.
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Old 06-11-2008, 02:15 AM
rrfxxxr rrfxxxr is offline
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Default Re: 3 phase motor to generator conversion?

Quote:
Originally Posted by rrfxxxr View Post
An idea about the 860 RPM, could it have more than 8 poles.
Just googled 860 rpm 10 pole and found this link
http://www.electricmotorsint.com/specifying_a_motor.htm

So i guess i answered my own question. Under score approximations.

"The speed of the motors is determined by the number of poles and the frequency of the supply.
The following approximations are applicable.
50 Hertz60 Hertz
2 Pole2880 rpm3460 rpm4 Pole1440 rpm1750 rpm6 Pole960 rpm1150 rpm8 Pole720 rpm860 rpm10 Pole580 rpm690 rpm12 pole480 rpm570 rpm
The actual full load speed will depend upon the motor design."
I know some ceiling fans have something like 24 or more poles to run slow.
Later
Sbw
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Old 06-11-2008, 10:39 AM
DirtbikePilot DirtbikePilot is offline
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Default Re: 3 phase motor to generator conversion?

I've done some more research and thinking about this. I read that the windings are probably connected in a Y or star configuration, but I don't know that for sure. I don't know how I could energize the rotor with DC if that were the case. I might be able to find and tap the center of the Y configuration in the stator windings though, and collect three phase off the rotor. Would that work? Thanks to all for your input!

If I could get this to work, maybe I could belt this to my fairbanks morse zc-346 and make a neat generator display.
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Old 06-12-2008, 08:01 AM
Jim Rankin Jim Rankin is offline
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Default Re: 3 phase motor to generator conversion?

I think you're out of luck unless you reconnect either the rotor or the stator to a 2 pole field. With an 8 pole stator and an 8 pole rotor, you would have to turn it mighty slow to get the frequency right.

If you use the stator as the field, your output would be seriously limited because of the lighter? wiring on the rotor. Ok for a demonstration I guess.
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Old 06-12-2008, 08:08 AM
Jim Rankin Jim Rankin is offline
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Default Re: 3 phase motor to generator conversion?

OK, just really thought about it, If you want to try the stator as the field, use only one line through to the neutral. Leave the others disconnected. What you have if you use all 3 lines is 8 poles x 3 phases = 24 poles!

It would run some lightbulbs no matter what the frequency is.
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Old 06-12-2008, 10:18 AM
KidDynamo KidDynamo is offline
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Default Re: 3 phase motor to generator conversion?

Let's look at this another way. Many synchronous 3 phase motors that run on 60 HZ are rated at 3450 or 1725 rpm, etc, yet they CAN be spun and excited to create 3phase 60 hertz current. If we used the prevailing thoughts of this thread, we'd say it wouldn't work yet we know it does.

Maybe this has already been posted but see if you can figure out how many poles you have, etc. I 'spect your 860 rpm includes the slip. At any rate, the rules about poles and rpm still hold.

If this thing wasn't run of 60 hertz originally, what the heck did it run on ? I guess if you had a tag reading for hz, you would have said so.

I can't say for sure but I would guess that it might be worth an experiment to spin it at 900 rpm and excite it like the old home-spun single to 3 phase "converters" where a single phase motor is used to spin and 3 phase motor. I don't have a diagram handy but two of the 3 phases are hooked to the single phase current and thus the 3rd phase is created.

You would need enough h.p. to at least spin your motor up to 900 and a little more to make some power. Of course this is making a BIG assumption that it has the number of poles for a "900 rpm" motor. You'll have to figure that out, for sure.

If it does work, and I think it might, then you'd need to calculate what kind of wattage the winding are capable of sustaining but it would probably excede the typical display array.

Anywhooo........that's what comes to mind at this time. Keep looking !!
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Old 06-12-2008, 10:44 AM
DirtbikePilot DirtbikePilot is offline
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Default Re: 3 phase motor to generator conversion?

Yeah, it is a 60 hz motor of course. I suppose all I really need is a way to get it excited and keep it excited. I'm excited. Does that help?

The biggest puzzle for me right now is how the heck would I get DC into the three windings of the rotor if I'm to use the stator for power generation. DC has 2 poles, not three, and I need an even number of slip rings to make that work.

Another idea would be to do something like back feed grid power, using the grid power to get the motor running, then overspeed it with an engine and feed power back to the grid. But, in a display, where would the "grid" power come from?
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Old 06-12-2008, 01:50 PM
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Talking Re: 3 phase motor to generator conversion?

It would seem to "Me", that what you would want to do, is isolate "one" of the
three Rotor Windings, and feed that with the output of your Voltage Regulator.
Then turn the rotor at 900 Rpm, and check the Stator Windings for Voltage,
at 60Hz. If that was successful, then look at what you need to do, to drive
the input of Voltage Regulator from the output of the Stator Windings, so as
to get the appropriate Stator Winding Voltage. Then look at how much load,
you intend for your display, and set the appropriate Primemover HP to deliver
that load.
__________________
Bruce in alaska
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Old 06-12-2008, 05:36 PM
Richard W. Richard W. is offline
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Default Re: 3 phase motor to generator conversion?

May this website be of any help.

http://www.qsl.net/ns8o/Induction_Generator.html

Richard W.
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Old 06-13-2008, 12:37 AM
DirtbikePilot DirtbikePilot is offline
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Default Re: 3 phase motor to generator conversion?

Richard- I think that site hit the nail on the head! It gives such a simple solution to my problem.
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Old 06-13-2008, 07:58 AM
armandh armandh is offline
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Default Re: 3 phase motor to generator conversion?

I always thought a DNR type motor would work best
http://www.maschinensucher.de/ma2/bi...1-english.html

these have a dc field with a slip ring fed stator
co wound on the stator is a DC generator with commutator and movable brushes to vary the DC output [and speed]
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