The electrical load causes the drag.. it's free spinning with the engine off - turn on an electrical load and try spinning it and it should be harder to turn.
It's the law of conservation of energy - you can't get 'free' energy, it has to come from somewhere, so it comes from the effort required to turn the alternator to produce the desired power output. Literally it's due to requiring a stronger magnetic field to generate the power..
Yep, in other words, "electrical magnets type stuff". 
I tested my refurbed alternator by spinning it on an electric drill. Apply a load (car headlamp bulb in this case) and you can certainly feel the extra resistance doing that.
Ok I'm getting there. So why can't I feel magnets spinning by hand? Are there magnets in there? Or is it electrical field type magnets?
There are no permanent magnets.
The rotor (bit that turns) has a coil of wire on it which creates a magnetic field when a current is passed through it (the regulator controls this, and the brushes pass the current from the fixed regulator at the back of the alternator to the rotating rotor).
The stator (outer part made of steel plates stacked up) also has coils of wire wound onto it (you can see these easily through the cooling slots in the case). When the magnetic field created by the rotor rotates within the stator, an electric current is induced in the coils in the stator. This current is rectified (turned from AC to DC) and powers the electrical items in the car and charges the battery.
So, until you have current in the rotor coil, you have no magnetic field, and it actually has to be spinning pretty fast for anything significant to happen even then (they typically spin at about 3 times engine RPM, so yours is normally doing about 18,000 RPM
).
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Topic: Battery light on (Read 10681 times)