directly wire generator to motor to facilitate speed reduction?

[maxmitchell]

this is my first post, sorry it it's in the wrong spot.

I am working on redesigning a high speed gearbox, and would like to get away from gears all together. For this application thermal efficiency is absolutely NOT important. The high speed turbo-alternator spins at about 130,000 RPM. the low speed fan needs to spin at around 10,000 RPM. I would like to accomplish this speed reduction without involving expensive motor controllers / invertors / VFDs or similar. I had an idea on how this might be accomplished, but I can find little to no information online as to weather anything like this has been attempted, so I don't know if it would even work (though it does not seem far fetched to my mechanical engineering brain).

here is the general idea:

use a 1-pole PM rotor (one N, one S) on the high speed device. there will be a 3-pole stator with individual coil windings, lets call them "A", "B", and "C". they would be positioned at even 120 degree spacing around the PM rotor central axis. So, when the turbo-alternator is spinning at 130,000 RPM (~2167 rotations per second) I would have an AC current at a frequency of 2167 Hz in each of the three stator windings, at 120 degree phase shift from one to the other.

Now, instead of rectifying that into DC and using an invertor to spin the fan, here is the unique part of this idea...
The three windings (A, B, and C) would just be wired directly into the stator of the low speed fan motor. Since I want a speed reduction of about 14:1, I would design the fan stator to have 3*14 = 42 poles, and I would wind the 42 pole stator A, B, C, A, B, C, A, B, C (and so on all the way around).

(I think) this should set up a rotating magnetic field in the 42-pole fan stator that is rotating at exactly 1/14th the speed of the high speed turbo-alternator. Then I would use a squirrel cage induction motor rotor in the low speed device, and end up with a bit of a slip angle so my net speed reduction would be more like 13:1 (which was secretly my target reduction ratio to begin with).

Does this at all sound like something that might work? Keep in mind that I do not care at all about thermal efficiency. I do not need precise speed control of the low speed fan, it just needs to be about 13:1 speed reduction ratio.

 

[davidbeach]

It works in theory, but a 42 pole motor will be huge for its hp size.

 

[ScottyUK]

What power capability are the machines in question? The 2.1kHz frequency might be a problem with a laminated silicon steel core. What type of rotor do you propose - permanent magnet?

 

[maxmitchell]

David, I have a 42 pole alternator stator that I was considering using for an initial performance demonstrator (assuming I can find a squirrel cage rotor to fit snugly inside it). It is about 7 inches OD and 1 inch thick. It would probably be only an order of magnitude less power output than the final design (just my SWAG at it) Why is it you say that 42 poles would be necessarily large?

Scotty,
For the final product I need to deliver about 15kW to the low speed motor. I plan on a permanent magnet rotor for the high speed generator, and a squirrel cage rotor for the "low" speed motor. Do you recommend an air core on the high speed stator to avoid hysteresis troubles?

 

[Skogsgurra]

This kind of a solution was used BVFD (Before Variable Frequency Drives). Ususally in reverse order (high speed spindles for grinding Tools etc) and it should work also for speed reduction. No problem, at least not from a theoretical Point of view. But practical considerations (like what Scotty said) can make the proposition less attractive. David mentions the size of the motor and I can only agree. But certainly possible. And a lot more expensive than what you call "expensive motor controllers / invertors / VFDs or similar".

I need to ask why you cannot run the fan motor directly from the existing mains?



Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[maxmitchell]

The product operates away (sometimes very far away) from mains power sources.

So here is where my lack of EE knowledge will be obvious, but I thought that with high voltages (a couple hundred) and high frequencies (which would happen naturally in my case?) the motor mass would stay low. Such as the motors in modern hybrid and electric vehicles.

 

[Skogsgurra]

One possibility would be to use a variant of a stepper motor. They have teethed rotors and do not need many windings. What Power does the fan need?

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[maxmitchell]

Fan needs 15 kW

 

[maxmitchell]

Skogsgurra,

you mentioned:

"This kind of a solution was used BVFD (Before Variable Frequency Drives). Ususally in reverse order (high speed spindles for grinding Tools etc) and it should work also for speed reduction. "

I understand what you mean, but I would like to research the nuances behind how this was done. Do you have any recommendations for what I could web search for to find information on this? Perhaps a specific product or brand that employed this technique? Was there a certain terminology for this?
Whenever I Google anything like "directly wind motor to generator" all I get are links to people breaking the laws of thermodynamics.

 

[waross]

If you want a secret 13 to 1 reduction, you may be better to go with a multiple of 12 or 13. If your design is efficient, the 13;1 ratio will come within a couple of percent of your target speed. 60 Hz WRIMs often have a slip of less than 3%.
A 36 pole rotor with about 7% slip will give your target speed.
A 42 pole ratio will be slow and the slip will make it slower.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Skogsgurra]

I googled "high speed spindles" but did only find modern equipment.

There were around 3 810 000 hits. So it is possible that you can find older equipment further down the list. I'll let you do the hard work

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.