High speed motor design for micro turbine

[johnboyr2]

Hi Guys,

I'm currently undertaking a project to design and build a high speed (approx 100,000 rpm) electric motor to drive a micro turbine. My skill are in the mechanical design side of things and I am afraid I am a complete novice when it comes to electric motor types and suitable applications!
I was wondering if anyone could point me in the right general direction to go as far as the type of electric motor design(s) that would be potentially capable of these producing these of speeds? It would also be useful if the function of the motor could be reversed to become a generator if mechanical input was applied.

Any guidance would be greatly appreciated!
Thanks

 

[cranky108]

I would think some type of DC motor would be a good choice, however a two pole 400Hz syncronous motor(If I calculated right) should have a speed of 24000 RPM.

You may have to consiter gearing to change the speed up or down.

 

[MikeHalloran]

So your motor coils would have to operate at 1.6667kHz, where their inductance starts to impose limits on di/dt, which in turn imposes limits on the torque that the motor can exert.

You could have chosen a less challenging airport.



Mike Halloran
Pembroke Pines, FL, USA

 

[johnboyr2]

Thanks for the suggestions, gearing may be an option!

 

[imcjoek]

I don't know how micro your micro is, but there are some hobby brushless motors that can turn ~50k+ no-load rpm.
Torque requirements will also bog you down.

http://www.neumotors.com/Site/Motors.html

Confirm electronic speed control commutation frequency limitations as well.

 

[crshears]

Hello, Johnboyr2!

I'm confused...

All the 'turbines' I've ever encountered have been prime movers that capture energy from a fluid stream of some sort and use said energy to drive a mechanical load of some kind, be it an alternator, dynamo, pump, antique De Laval cream separator, or whatever.

So...will the electrical machine you're speaking of drive the 'turbine?' [in which case the turbine wouldn't be a turbine, at least to my understanding...]

Or will the 'turbine' drive the 'motor?' [in which case the 'motor' isn't a motor, unless all my teachers were complete idiots, and I was an idiot to believe them...]

Which is it? Are you looking to drive a 100,000 rpm axial or radial compressor or some other load with an electric motor, or do something the opposite of that?

You wrote that you wanted to investigate whether 'the function of the motor could be reversed to become a generator if mechanical input was applied...'

I'm no mechanical guru or anything, but would it not be necessary to reverse the direction of rotation of your unit to have it swap functions between prime mover and load? For example, centrifugal pumps can, within limits, be used as simple hydraulic turbines, but they must be reversed to do so...I know that AC machines have been connected using phase-rotation switchgear to accomplish this, as for example in hydraulic pump/generation storage operations, so it's definitely doable; it's just not clear - at least to me - what you're actually trying to accomplish.

CR

 

[johnboyr2]

Hi crshears and imcjoek!

Apologies, I should have given a better description of what I am trying to do!
You are correct crshears, a turbine does extract energy from a working fluid and convert it to mechanical work, my wording was incorrect.
My task is to design a rotor and stator configuration that is capable of these kinds of speeds. At the minute its more of a proof of concept type of project with no exact size/speed restrictions,I have been given the option of either producing an electric motor that could be connected to a radial compressor to create boost pressure (basically an electric turbo charger), or producing a high speed alternator type device that could be connected to a turbine in order to extract energy from exhaust gas.
As far as size/power requirements go, I have not been set any specific targets, however I would be aiming to design something similar in size/power to a typical car turbocharger (whether it be the turbine or compressor side).

Imcjoek, Regarding the electronic speed control commutation frequency, again this is up to me! I am permitted to buy the power control circuits 'off the shelf' if i can find something suitable.
If i was going in the high speed generator direction and not the motor, i would require an electronic control setup that would be capable of taking the high frequency electric output and converting it to an appropriate AC frequency. It would need to be able to accommodate varying frequencies as the turbine may not always be rotating at a constant speed.

This is a completely new area for me, meaning I am having to start from the basics of electric motors/generators and work my way through the theory. Your time and replies are greatly appreciated!

Thanks, John

 

[ozmosis]

For an idea on high speed motors run from frequency converters, maybe look at Precise as a concept. These are high speed spindles but in principle, a high speed rotor/stator with something attached.

http://www.fischerprecise.com/

 

[waross]

Most motors do not require a reversal to generate. Most motors will generate if they are turned a little faster. Synchronous types, you just have to try to turn it faster. An induction motor has the advantage of a robust rotor and no electrical connections to the rotor. It may be driven by an inverter and there are inverter designs which will regenerate, allowing the motor to be used as a generator. An induction motor becomes an induction generator when the drive frequency is below the synchronous frequency of the motor.

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

 

[crshears]

Hello again johnboyr2 et al,

imcjoek wrote: 'I'm not sure how micro your micro is...'

...to which the OP responded with something about 'typical car turbocharger'. That being the case, I can see how what Bill wrote is very applicable...

It's a lot simpler to mechanically reverse the rotational direction of a large hydraulic pumped storage generation unit of the type I referred to in my previous post, since there need only be one set of water passages, and water passages are quite a bit more expensive than switchgear.

In this case however [given the machine's relatively small size] it would probably be far easier to maintain the unidirectional nature of the machine by piping it up in an H configuration. By this I mean that the machine would be located in the crossbar of the H, and flow through it would always be from left to right. The two top connections would go to a higher pressure system, and the two lower ones to a lower pressure system. The function of the machine could thus be swapped around by the simple expedient of opening and closing the appropriate vaves 'kitty corner.' The machine thus either harnesses the delta P or develops it, depending on how it's valved.

Were it desired to swap functions on the fly, meaning with the machine in operation and electrically connected to a grid or network of some sort, a second crossbar containing a recirculating valve could be piped up within the H; this valve could be opened so as to positively maintain fluid flow within the machine itself [thus precluding surge] during the relatively short transition from load to prime mover or vice versa.

The function of the 'reversing' valves as conceived is obviously not to reverse the rotation of the machine, but rather to swap around the machine inlets and outlets so the fluid flow through the machine is always in the same direction.

It might be worth it to look into using a turbocharger turbine equipped with variable nozzles as the mechanical portion of the machine so as to permit adjustment of the unit's characteristics, particularly for use as wicket gates to regulate the power output when operating as a prime mover.

Hope this helps.

Carl

 

[johnboyr2]

Hi Carl,

Thanks very much for your comprehensive reply! Your 'H' design is a very useful concept as far as machine adaptability is concerned. My project is actually slightly simpler than that initially, in that my prototype is not required to be able to switch from motor to generator or vice versa 'at the flick of a switch' It can be one or the other (and in the future the design could be altered to change it from one to the other, or to both as with your H concept).

As far as numbers go I reckon I need to produce/absorb approx 2Nm of shaft torque at around 80,000 rpm. from what I can gather a Brushless Permanent Magnet design would be the best way to achieve this?

Thanks again for all your replies!

John

 

[LiteYear]

It can be a motor or a generator

Generator is easy - a permanent magnet generator will suffice and a permanent magnet motor is very similar to a generator. You'd simply rectify the output and then use an inverter to produce your AC from the DC. That way it doesn't matter how fast it spins, you just get DC out. Voltage will be proportional to the speed and current proportional to the torque. The correctly specified inverter will happily accommodate the range of voltage and current. Grid-tie inverters for photovoltaics do this off the shelf.

Motor is trickier, and may require a step up gear box. Again a permanent magnet motor is probably your best bet. Think of the motors in a Dremel - I think they're up around 10000rpm, so maybe that's a start.

 

[johnboyr2]

Hi LiteYear,

Thanks for your reply, very useful information! Do you know of any companies that supply these grid tie inverters?

Thanks, John

 

[LiteYear]

Aurora and SMA are the big two that I know of. There's others...

 

[Clyde38]

Hi Johnboyr2,

We have designed several of these type of machines (up to 250,000 r/min). Some have been strictly motors and some have been just generators, however some are motor/generators. In all cases they have been 2 pole PM brushless machines. Both slotted and slottless typologies have been used. Rotor dynamics are particularly challenging (magnet retention, critical speeds, etc.). Losses caused by the rotational frequency will also have to be minimized. These issues can and have been addressed.

 

[johnboyr2]

Hi Clyde38,

Yes I have done a bit more reading around the subject now and have gladly found that it can be done!
For the first prototype I will be aiming to produce a PM device, (that most likely acts just as a motor for simplicity) to Spin at speeds up to 40,000 rpm and produce power around 50kw. Would you recommend a slotted or slotless stator for these sorts of parameters? I would like to keep the design as simple as possible for this initial prototype. As far as control electronics what method of position measurement would you suggest? Any recommendations would be greatly appreciated!
Thanks, John