25-03-2022, 10:05 AM
(This post was last modified: 25-03-2022, 11:46 AM by Stuart Giles.
Edit Reason: missing word
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(24-03-2022, 07:30 PM)Bob Culver Wrote: Can any electronic whiz explain the regulators used with pm alternators? Would expect the regulator to interrupt output not short it to earth. Voltage spikes might be a problem but not insurmountable. On a small engine the constant power drain must be significant. Most bikes now run with permanet lights on so the waste dissipation may not be great. I became aware of a few years ago when a colleague cooked his motorcycle alternator and found it common for the make.
I wouldn't claim to be an electronic (or any other kind of) whiz, but the way it was explained to me is that the shorting instantaneously drops the voltage but only for the moment that the voltage had exceeded the Zener value. once the voltage drops (near instantaneous) the Zener diode stops conducting and the battery receives charge as the voltage rises back up to exceed the Zener voltage; this short to earth rise to exceed Zener voltage is a continuing process. There is usually a smoothing capacitor in the regulator which means that along with the Zener diode switching in and out of the circuit at high speed, the output looks like a steady 14 point whatever volts for a 12V system. The particular alty I've used is a relatively low power single phase unit @168W output, so something south of 1/4HP power input even when it's working flat out. So the isn't much power to sink on this one.
More powerful permanent magnet alternators are usually two or three phase output and it's usual to switch one or more phase inputs off (ie disconnect them) when there is low demand. I had an oldish Triumph twin motorbike with a two phase alty, one phase fed the headlight and nothing else, whilst the other charged the battery.