Re: Electrical Questions


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Posted by David Sherman on Monday, December 29, 2008 at 20:27:55 :

In Reply to: Re: Electrical Questions posted by Sam on Monday, December 29, 2008 at 17:02:26 :

That was the problem I had. There was no adjustment on my Motorola unit, though.

Here's the technical explanation, in case you're interested: No wound-field generator will start from a dead start unless there's a little bit of residual magnetism in the field iron. The reason we polarize old-fashioned generators is to get that residual magnetism pointed in the right direction. When you start spinning a DC generator with an electromagnetic regulator, any slight output at all gets fed right back into the field and boosts the field current, thus boosting the output current. Within a fraction of a second, it's up to full power.

Alternators don't do this very well, though. The output of an alternator (even an ordinary one) has to go through two diodes before it can get to the output lead. The voltage drop of a diode tends to be at least half a volt, even at minuscule current, and goes up to maybe a volt at high current. Thus, the alternator has to be spinning fast enough so that the residual magnetism in the field will produce an output voltage of at least two "diode drops" (a volt or so, total) before you'll get any current at all out of it, and thus any current that the regulator can feed back into the field to get the thing producing for real.

Another way of looking at it is that if you measured the output of an old-fashioned generator with no field current at all, you'd still find an output that increased directly in proportion to the RPMs, starting from zero, whereas if you did the same test on an alternator, you'd find that as you increased the RPMs, you'd get no output at all until you got to the point where there was enough internal voltage to push current through the diodes, and from then on, the output would increase proportional to RPM just like a DC generator. There are tricks the designers can play with a voltage regulator to get it to kick in at a reasonably low voltage, but they can't kick in at zero like a DC generator can.



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