Quiz: Reasons for dump load and furling?

[Ethan Brush]

It just occured to me that I dont know the precise reason for a dump load and furling/overspeed protection and if they are related. Who would like to take the quiz :) ?

Regarding Furling/overspeed protection, is this for:
1) mechanical limits of the machine, ie vibration, bearings, centrifugal forces.
2) prevent the machine from generating too much current and thus being damaged from overheating.
3) both 1 and 2.
4) other (explain).

Regarding a dump load, is this for:
1) To keep a load on the machine, to aid in overspeed protection. If so see answer to overspeed section above.
2) Because when turning without a load, voltage will rise dramatically potentially damaging winding insulation and other components.
3) both 1 and 2.
4) other (explain).

Regards,
Ethan.

[Rob Beckers]

Hi Ethan,

It's pretty much the entire list of what you mention. For turbines that have no other way to limit their RPM, they would quickly spin up beyond design limits and likely self-destruct when they are used without a load on them. Similarly, if they kept facing the wind, even with a load, their RPM would continue to climb as the wind speed increases. So at some point they have to 'shed' wind by furling, to partially unload the rotor and keep RPM in check. If you look at power curves you'll notice their output power going down as they furl, because less wind passes through the rotor (and angle-of-attack is all out of whack, so they'll produce even less than rated power).

-Rob-

[Ethan Brush]

Thanks Rob,

What about dump load #2? In other words, is a turning turbine ok with no load as long this doesn't result in a mechanical overspeed condition? Can I spin my turbine in a drill press up to around the max normal turbine rpm with no load?
Regards,

Ethan

[Rob Beckers]

Yes, you should be fine spinning up the turbine (without blades I presume!) in a drill press. The load is there to keep RPM in check.

You're right that the unloaded voltage goes up linearly with RPM. Twice the RPM, twice the unloaded voltage. So, in theory at least overspeeding could (literally) zap the alternator. In practice I doubt that'll happen, insulation failure voltages are very high even for what seems to be flimsy insulation. The whole thing would be more likely to come apart mechanically, because of forces on blades and turbine when overspeeding. Anyway, a drill press would keep RPM under control, so no problem with any of this.

-Rob-