Electrical Braking Strategies for Wind Plants

[TomW]

Originally posted by Flux on Tue Dec 19th, 2006 at 03:06:17 AM CST and attached to this story:

http://www.fieldlines.com/story/2006/12/19/11312/235

Perhaps a bit about brake switches would be useful.

If you have copied a design by DanB or Hugh, then you can use a brake switch to hold it stationary or stop it from speed. The alternators are efficient enough to do this.

With Hugh's normal design with rectifiers on top you need to short the dc. To do this without shorting the batteries you need a change over switch ( with or without a series blocking diode). Hugh has a diagram in his plans. It is a bit hard on the rectifier, but as the design uses lots of 35A bridges it copes with it. Diodes have a short time surge rating much above the average rating.

If you have the 3 phase available then it is far better to short the ac.

If you have done your own half baked version of an axial machine, you should find that a brake switch will hold it stopped in normal winds. You may or not be able to stop it in a high wind and you would need to satisfy yourself that it will remained stationary in very high winds. If it will not stop within a few seconds from speed then you had better turn the brake off or it will fry the stator. If the brake will not stop it from speed then you would need to prove to yourself that it will hold it stationary in a very high wind. If it does start up and you are not there to turn the brake off, bye bye stator.

It takes a lot of magnet and copper to be absolutely sure it can be totally reliable with a brake switch.

Many motor conversions may not stop from speed but may be safe when left stopped. This depends on the proportions of resistance and reactance in the windings and you will have to decide by experiment.

DC motors may stay braked with a short before the rectifier, but every case is different, if it gets away it will destruct.

With DC machines DONT try to stop them from any significant speed, the excess current will murder the commutator and brushes, they just can't be designed for gross overload.

You have to know your machine and act accordingly. Try to find out what happens in reasonable winds, even then a machine that seems as though it will always stay braked may get away in a severe storm.

Flux

[jmk]

 Work on getting the tail to furl properly too. This way you have a second or even a first pertection if the machine is left on.

[blueyonder]

 this post  has got me interested. its a old post but hope i get a response.

 playing with the turbine on the ground before i get mast etc.

  fitted a brake switch to the ac. and hand spinning i flick the switch and it works great.

 i keep trying to turn it but its a hard drag to get it going round very slow.

  so the i fixed the prop on and was doing a bit balancing.

  turning the prop with my hand it was going very well fast considering.

 flicking the brake switch made it slow down but i can still turn it easily with my hand on the hub . i don't understand what is happening.

  prop off works great. prop on not very good.

 at first i thought it was only braking on one or two phases.

  but checked it all out . and still doing the same thing.

  i did think maybe the prop is acting like a flywheel.

  . connecting dc wires to a 20watt bulb makes it very hard to get up any speed with my hand  . but the bulb will go from orange to bright white but making it hard to turn.

  voltage at this point is about 10 vdc. cant get it to go any faster. to much drag on rotor as yet havent put it on a 12 volt battery. still doing other things .

 the more i understand now the better it will be when i make a twin rotor.

  but i am learning .the fun way. cheers john.

[TomW]

blue;

I find that the dual rotor axials I have had installed [2] were not braked to a complete stop by shorting. They would always turn some but never much above 40 or 50 rpm. Seemed safe enough that it would keep it from going runaway in any reasonably sane wind speed.

I think it is just how it is. More like being slowed than stopped. I was concerned, too and DanB assured me thats how they work. Never lets them get any speed up.

Cheers.

TomW

[blueyonder]

thanks tom . i am sure it is working ok.

  in life things im not sure about i like to ask.

 so building a gen is very new to me but none of my friends know anything about them.

  so its good i can sum times ask here.

  but i think i am just asking for reassurance sum times.

  maybe im lacking in confidence .  so thanks for the reassurance.cheers john.  

[DamonHD]

Hi,

I am using a crowbar circuit as an automatic emergency brake for my very small turbine:

http://www.earth.org.uk/wind-power-pilot-autumn-2007-MotorWind.html

See the circuit diagram in the middle of the page.

It doesn't completely stop my turbine but it does slow it a lot.

Rgds

Damon