Windmilling effect on turbine generator shaft 1

[DHambley]

I have a question for anyone who works on smaller engines which you'd see in small business jets. Is there a constant geared ratio from the blades to the shaft which the generator is attached to in a typical turbine? If so, if a turbine is windmilling in a strong wind in the opposite direction of normal rotation the generator would rotate backwards at the same ratio also. This presents a possible problem for an idea I'm working on. I'm wondering if there's typically some disconnect device.


(This is for an idea I'm working on for a torque-ripple reduction device, but that's a big big different topic for another thread. I don't know anyone who would let me mess with their Citation so it's hard to get info.)

thanks

 

[thruthefence]

The Generator (commonly a starter/generator) is driven by the gas generator, or compressor spool, along with anything else on the accessory gear train, which would include the fuel control and hydraulic pump.

The 'windmilling' you see, out on the ramp, is the Fan, or power section; it is only coupled to the gas generator aerodynamically, by all that hot air that blows through the engine while producing power.

Forgive me if I have misconstrued your goals, but If your plan is to recover energy from the aircraft generator, while sitting parked, I don't believe there is enough wind force to overcome the friction of the gears and accessories. Much less the force needed to turn the hydraulic pump. This information applies to the engines I am most familiar with, the PWC JT-15-XX, The PWC 500, and the Honeywell TFE-731 series, all of which are used on one model or another, of the Citation aircraft you mentioned.

 

[tbuelna]

hgldr,

Turbofan engines utilize two types of generators on the EMAD (Engine Mounted Accessory Drive).

There are PMA's (Permanent Magnet Alternators) which operate at very high speeds (>20K rpm), and are gear driven at a fixed ratio usually from the HP compressor spool. Since a PMA's operating speed is a function of engine speed, they deliver variable frequency power output.

The other type of generator is an IDG (Integrated Drive Generator) which operate at lower speeds (<8K rpm). The IDG is gear driven from the same place as the PMA, but the IDG has a variable speed drive (usually a hydrostatic device) built into it that keeps the generator operating at a constant speed over the normal operating range of engine speeds. Thus, the IDG is able to produce power at a fixed frequency.

The PMA is much lighter and less complex than the IDG. But the PMA also requires additional power electronics to condition its output to something usable by the aircraft's electrical system.

There are not typically any disconnect or overrun devices in the accessory drivetrain. As thruthefence pointed out, I doubt that even a very strong wind would produce enough torque across the HP turbine to back drive the EMAD. Besides generators, there are many other significant shaft loads passing through the EMAD for things like hydraulic pumps, oil pumps, separators, etc. that the wind force would first have to overcome to backdrive the EMAD and HP compressor spool.

Hope that helps.
Terry

 

[thruthefence]

On these little Corporate jets I have experience with, when engaging the starter you get an in rush of around 900A @ 28.5V. This is on and external power unit, and pretty well regulated. (and as I said, this is the drive train we are talking about) The current drops considerably as the engine lights off, and becomes self sustaining, but that's a pretty good bit of horsepower to expect the wind to provide. (if indeed that is the PO's plan)

 

[DHambley]

thruthefence, recover energy from the aircraft generator while parked? I had to laugh, that idea was so bizzare.
No, That's far from my plan. The device I'm working on reduces torque ripple of the generator while in flight by a correction factor needed when the PMG output voltage signature is not a true sine wave, but somewhat trapazoidal which many of them are.
The problem I have involves the way I wanted to program a signature of the voltage pattern by rotating it at low speed while the turbine is off. A comment by an engineer at Cessna that, if it rotates backwards, it could mess up the oil pump.

 

[thruthefence]

Good to hear, i thought you may have been getting some "Green" grant money!

 

[tbuelna]

hgldr,

The guy from Cessna has a valid point. Turning the accessory gearbox backwards might create a condition where the oil scavenge pumps are pumping oil from the oil tank into the sump areas and filling them up. The only flow restriction between the scavenge pump discharge ports and the oil tank is usually an air/oil separator. So I can imagine it happening.

Good luck.
Terry

 

[thruthefence]

Couldn't you contact one of the many rebuild shops and borrow, lease, or buy a suitable component for your research? Build a simple test stand to rotate the armature as you describe? I would think that would be easier then instrumenting & turning an engine in reverse 'on the pylon'.