Question about magnetic resistance 2

[worldchanger]

Engineers,

In a generator with stationary copper coils and a moving rotor arm of magnets, is there any sort of opposing force or resistance as the magnet passes by the copper coils? Lets just say we had a rotor arm with ZERO FRICTION--would it spin freely past the coils or do the coils act as a magnet would and resist the rotor? Please submit replies in as lay terms as possible, thanks, WC

 

[GOTWW]

I have Perminant Magnet DC wind generators. The brushless design has moving magnets on the rotor. The stator is wound copper and opposite of a standard motor, is located on the inside on the PM can. As for your question, if the stator leads are shorted, there is a very significant counter emf and opposing torque. However open circuited, the rotor spins freely. Shorting the stator leads is how you stop them in excessive wind. I one time watched a poor, hopeful hillbilly try to charge his battery using a standard automobile alternator and a hefty radiator fan blade. Everytime he connected the battery, the fan blade would stop dead. But it did look somewhat impressive spinning freely in the air.

 

[logbook]

When a magnet passes by a coil of wire the changing magnetic field "induces" a voltage in the coil. You will be able to google on "electromagnetic induction" and find lots of data. The voltage does not generate power unless you put a load on because there is no flow of electrical current. Thus just leaving the coils unconnected will give a very low mechanical load on the rotor. Now if you connect a resistor across the coils an electrical current will flow. Since the resistor will get hot to some degree, you must be generating power. Since the power has to come from somewhere (the rotor motion in fact) there is a mechanical load on the rotor. Changing the size of the resistor will change the mechanical load on the rotor. As GOTWW points out, too large a load will just stop the rotor dead in its tracks.

No load (high resistance) means no power delivered to the load.
Too large a load (low resistance) also means no power delivered because the rotor stops.
The optimum load gets the maximum power from the rotor and is somewhere in between zero and infinity!