Shortcircuited motorcycle alternator leads to RPM proportional losses?
This is an old thread but I thought I would add something as there seems to be a widespread misunderstanding of this and there is not much information on the web. I think the designers and manufacturers of these devices want to keep the general population in the dark. I think what most people are missing is that these "fixed field" permanent magnet alternators have an inherently poor load regulation curve. With no load the output voltage can vary from 20V at idle to well over 100V at high revolutions per minute. However as the load current is increased a magnetic field is created in the stator which is in opposition to the field of the permanent magnets. Thus even short circuited it will not produce more than 40A or so. When the short is made by an SCR with a voltage drop of 1-2V this is only about 80W. The SCR is only turned on for part of the a.c cycle as it only turns on when the battery voltage exceeds 14.4V. Before it turns on the current is being delivered to the battery and all the other loads, the SCR only turns on when the other loads cannot absorb the current. When the SCR does short the windings the current is so high the the field in the stator almost cancels out the permanent magnet field so the torque load on the engine is reduced. This may seem inefficient but remember even if the field was produced by a wound rotor with slip-rings and brushes like in a car alternator that would also require power and there would also be power lost in the regulator so it isn't as bad as would first appear.