TSR and overspeed questions

[Dave B]

If a blade set is designed for a TSR of say 7 then why would this not be the maximum speed expected of this blade at a given wind speed loaded or un-loaded ? If I don't want my blades to turn any faster than 400 RPM then wouldn't I want to furl the unit at or before this windspeed ? I guess what I don't understand is that if an alternator is un-loaded but properly designed to furl at a given windspeed (rpm) why is there a possibility of over-speed ? Are TSR values only given for a certain load ? I want to know the highest RPMs attainable throughout a range of wind speed for a given TSR. Thanks for helping to clear this up.  Dave B.

[Flux]

Dave

The design TSR is the ratio of rpm to wind speed that a propeller will give maximum efficiency ( CP ).  If you load it too much it will become less efficient as it starts to stall. If not fully loaded it will run faster, there is a reasonable band where it will work quite well, but as you get well towards stall its output will fall very badly.

Similarly with reduced load the output falls slowly at first and with no load it will run away to such a speed that its power balances the losses. At no mechanical load these losses are mainly the drag component of the blades.

The degree of overspeed depends a lot on the TSR and blade profile.It may run away to several times the design speed. Usually the higher the TSR and the more aerodynamic the blades are the greater the overspeed. As a rough guide it is quite likely that a blade designed for TSR 7 will at least run to TSR 12 or even more on no load.

If you don't want your blades to run faster than 400rpm, you must furl when your generator load allows the blades to reach this speed. Rember that furling doesn't limit things sharply at the operating point and also if you loose the load on a machine designed to furl at 400rpm there could still be a serious overspeed as the thrust on which the furling depends is a function of the power extracted.

Try to make sure your blades can survive well over the rated speed to leave a margin of safety. High TSR blades at runaway will be very noisy and start to make helicopter noises.

Hope this helps.

Flux

[Chuck]

I don't believe The tip-speed ratio or TSR normally figures in a load, after all, everyone's load would be different (unless you're mass producing a particular machine).

I think the overspeed issue is this: The machine is designed to furl when a given wind speed (force) is pushing on a given area (the area of the rotor). If the rotor is unloaded it will be traveling faster than if it is loaded. It could be going twice as fast unloaded.

Usually I would think you want to figure the machine is loaded when furling. It should be loaded all the time, right ? Assuming TSR is unloaded, you have to guess at how the load is going to slow the blades (50% ?) and go from there.

I'm sure you could calculate the approximate reduction in rpms for a given load, TSR, blade diameter and wind speed. I know I'm in the ball park, with a 50% margin of error.  

Chuck