Quoting Bryan's signature line:
" NEVER argue with stupid people, they will drag you down to their level and then beat you with experience"- Mark Twain
I 'm not sure why I'm bothering, but lest Tcmtech's postings leave the wrong idea about how alternators really work, this might teach a thing or two:
Here is a very simple simulation of an automotive charging system that models a 14V car alternator as a
current-controlled current source. The Voltage Regulator is modeled as a voltage-controlled switch, which opens when it senses an input voltage exceeding 14.2V, with a tiny bit of hysteresis (just like a real car Voltage Regulator). The battery is modeled as a huge capacitor because it
ACTS like one. No, I'm not advocating calling a battery a capacitor...
Look at the first plot. To start, the voltage of the battery (red trace) is initialized to 12.6V. Initially, the alternator supplies its full output current (lt. blue trace) to charging the battery. As the system voltage reaches 14.2V, the Voltage Regulator begins switching on/off the alternator's field voltage (Pink trace), thereby reducing alternator's field current (dark Blue trace). As the battery becomes fully charged, its current goes to zero, so all that is left for the alternator to do is to supply the static load of the system (~2A).
The next two plots show expanded time scale, the second just as the system voltage reaches 14.2V where the PWM duty cycle is still high, and the third, where after the battery stops accepting any charge current, all the alternator has to do is to supply about 2A to the 7Ω static load. Note that initially, the alternator is going flat out, with the field current limited only by the DC resistance of the field winding. Once the PWM gets going, note that it is the field inductance and amount of hysteresis built-in to the Voltage Regulator that determines the PWM rate.
**broken link removed**
**broken link removed**
To anybody who works on automotive and aircraft charging system (like me), this is a faithful simulation of what actually happens with the exception of the time scale along the bottom. I just sped up the time it takes to charge a battery so the simulation doesn't have to run as long as the physical system....
The reason I posted it is to show that even a simplistic model can show the workings of a physical system. I especially wanted to show that the alternator
ACTS as a
current-controlled constant-current source. It is the feedback provided by the Voltage Regulator that turns it into a
constant-voltage system, contrary to all the blather by tcmtech.