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
