Braking a windmill

[VinceB]

Hey guys,

I have a 8' wind genny, built exactly using Hugh's axial flux design (two rotors). The output is 12V and the rated power is 500 W (if I remember correctly), although on average I get 10-15 Amps, I have seen peaks of 35 Amps.

I want to have the possibility to brake it in case of high winds or battery overcharging, without having to watch and wait for complete stop before shorting it.

From what I understood from various sources, directly shorting the generator is not the best idea. I would like to brake the generator using resistors to lower the stress on the whole thing, but I seriously don't know what values would be necessary. I found an interesting 3- position switch which could be used to reduce the speed by steps...

The generator is connected to the power room with a 200' AWG #6 wire.

What values of resistors would be appropriate to brake this mill without too much electrical and mechanical stress?

Thanks,

Vincent

[electronbaby]

what is the length of feed line you have (wiring from the turbine to the batteries)?

[VinceB]

Two hundred feet of AWG #6.

I looked up at table which says that is roughly 0.1 ohm for one conductor.

[Flux]

Yes I was going to say about .2 ohms so you have enough with the line and the alternator resistance I think you can short it directly without worrying. It can be more of a problem with higher voltage machines where the resistances are relatively lower.

If it is the 5 phase machine with rectifier up the tower far the bigger issue is not to short the battery, you need a changeover switch or something that inserts a blocking diode before the short.

Flux

[electronbaby]

sorry,  reread your post and you clearly stated the distance.

I guess what i meant was,.. does that include the distance up the tower. My original thought was that you had sufficient feed line loss to allow the feed line to act as a resistor. Therefore shorting it out at the battery room would be ok. Its really hard to say because Im not familiar with Hughs 8' design. I dont know what size wire is specified for the stator and if its wired 3 phase or 5 phase. My best answer is to experiment. I would put a clamp on ammeter on each phase and short it. See what you get. Watch how the machine responds. Use ohms law to figure out the proper resistance based on your finding,.. or just take a good guess. I have been shorting out a 16' machine for years and its fine. I short it remotely out at the tower base. Never had a problem. My feed line is aprox 200' also. 10/3 SO down the tower (90') and 3 runs of 2AWG to the house. Having a lighter cable down the tower allows me to do this (its higher resistance).

[VinceB]

Yup, the total distance is 200' (tower height + line to power room).

The coils were wound with AWG #15 and it's a 5 phase machine.

I guess it's not that stator that I am afraid of burning; I just don't want to end up with flying blades when I short it if it's producing like 10-15 Amps.

[Flux]

I agree with your concerns, but in real life there doesn't seem to be a problem. I suppose torque loading is one of the things blades can stand better than anything else. Bending loads are far worse for them.

I once shorted a 1.5kW alternator on a bench test and the result was truly frightening, but despite the fears I got from that I find that the machine it is on stops fairly gently in about 10 rotations of the prop and seems happy even with a direct short. With your line resistance I don't see any problem and Hugh seems happy to short that design with no mention of added resistance.

Probably makes sense to wait for a bit of drop in the wind rather than do it during a strong gust.

Flux