alternator field coils

[Birdseye]

does anyone have an idea of how much current is flowing through the field coils of a typical 60 / 80 amp alternator whenthe engine is idling?

and how does the battery light circuit work. the simple diagram I have of an alternator suggests that it just works on the voltage difference across the ballast resistor in which case I would expect it to fade out not sharp[ly switch off as it does.

 

[lw395]

The field coils are actually the rotor.
The current could be as much as 5A when the alternator is working hard at low RPM.

The battery light (charge Light) circuit works a bit like this.
The alt is not generating, the battery drives current through the bulb and into the regulator and hence into the rotor.
That current is about 0.2A, it's controlled by the 1 or 2 watt bulb.
As soon as the alternator is spinning fast enough, the aux diodes put current into the regulator, so as soon as the current rises to 0.2A or whatever the light goes out, as the revs increase a tiny bit more the current rises, so the field is stronger, so the current rises more. Hence the light goes out suddenly due to positive feedback as the revs increase.
The current thorough the rotor stops rising when the regulator sees 14V and throttles the current back.

HTH.

 

[halcyon]

The field coils are actually the rotor.
The current could be as much as 5A when the alternator is working hard at low RPM.

The battery light (charge Light) circuit works a bit like this.
The alt is not generating, the battery drives current through the bulb and into the regulator and hence into the rotor.
That current is about 0.2A, it's controlled by the 1 or 2 watt bulb.
As soon as the alternator is spinning fast enough, the aux diodes put current into the regulator, so as soon as the current rises to 0.2A or whatever the light goes out, as the revs increase a tiny bit more the current rises, so the field is stronger, so the current rises more. Hence the light goes out suddenly due to positive feedback as the revs increase.
The current thorough the rotor stops rising when the regulator sees 14V and throttles the current back.

HTH.

You also get negative regulators that sink the rotor load, as opposed to positive that feed the rotor, the field diode then provides the feed back to turn ignition light off.

Brian