Oh, how to start?
Understand, that there are many different styles of alternators. But, the basics are the same.
Most of them do not need any outside electrical source to produce power. Just as the old generators did not use any outside electrical power to start producing.
That was what I meant by "flashing the field". Since a generator had no diodes, it could not be allowed to be connected to the battery when the engine was not running.
If it was, it became a very strong electric motor. So there was a relay in the voltage regulator that opened and disconnected it from the battery.
When you first installed the generator, you hooked a short piece of wire to the battery terminal on the regulator and momentarily arced it to the field terminal.
This put a high current pulse through the fields, which magnetized the iron parts of the field.
BTW, don't do this with an alternator, you can blow the diodes.
There was enough magnetism in the fields to start producing electricity when the generator started turning.
This voltage was fed back into the fields to produce more voltage, which made more until the relay had enough voltage to pull in and bring in the battery.
Most alternators will produce voltage the same way without being hooked to the battery.
So, even if the regulator quits supplying voltage to the system to turn it off, it will usually produce some level of voltage.
Lets look at a battery. To the electric circuit, it is a power source, a capacitor, and a resistor.
None of the numbers I am about to use are necessarily real world stuff, I use them because they make the math easy.
Now, consider a battery that it putting out exactly 12 volts. There is nothing in the car turned on, so it is not putting out any current.
Fire off the alternator. It starts putting out exactly 12 volts. Since the two voltages are equal, no current flows. As far as the alternator is concerned, its an open circuit.
Lets say the voltage regulator is set for 12 volts. Since there is no load, the voltage jumps up to the setting very quickly. As soon as it does, the VR tries to hold it there.
It shuts down the drive to the armature. Assume for an instant that the alt is not actually hooked to anything.
Since there is really no load, the inherent magnetism will cause the voltage to drift up to whatever its natural level is under no load.
The VR as done all it can do.
Now, lets let the battery be there.
As soon as the voltage starts to exceed 12 volts, current starts to flow into the battery.
A battery acts like a resistor. The higher the voltage put in, the more current it draws.
So, it very quickly loads down the drifting voltage to 12 volts.
Now, consider the voltage regulator. It cannot respond instantly. In fact, it better not.
If it tries to track and compensate for every little change in load, it will be constantly hunting back and forth, overshooting, undershooting, and might even go into oscillation.
Lots of times you can see this in the voltmeter in the dash when the turn signals are blinking.
The signals put a load on the alternator faster then the regulator will respond. So, the voltage dips closer to the battery voltage, and then comes back up when the signals turn off.
Even if the regulator could respond that fast, the magnetic fields in the armature take a finite length of time to build up and collapse.
Remember the windings in a alternator have a certain resistance which is in series with the load of the output.
Say the alternator is running, charging the battery, and running the computer.
The computer is drawing 1 amp @ 12 volts.
The battery is drawing 11 amps @ 12 volts.
Total, 12 amps.
The regulator is supplying the armature with enough current to put out 12 volts at that current level.
Which means that the alternator is being loaded down by an effective resistance of 1 ohm.
I hope you can see that if the load increased, in other words, the resistance went down, the output voltage of the alternator would drop for a second. The amount of power the regulator is putting into the armature is enough to create 144 watts.
12 volts times 12 amps.
If the resistance were cut in half, before the regulator compensated, it would still be putting out 144 watts, but now the load would be calling for 24 amps.
144 watts divided by 24 amps, equals 6 volts. If the battery were in the circuit, it would not get that low, because the battery would immediately supply the extra current until the voltage regulator caught up.
Now, consider what could happen if the battery were suddenly removed from the circuit.
Remember, even if you cut the wire from the voltage regulator to the armature at the same time, it takes time for the magnetic field to collapse.
So, you have an alternator being driven by the VR to produce 144 watts @ 12 amps.
144 watts divided by 12 amps is 12 volts.
Suddenly that 144 watts is being pumped into a 1 amp load. What is the voltage now?
144 watts divided by 1 amp equals 144 volts.........
Would it go that high? Probably not. but you will definitely get a heck of a spike.
Modern electronics have surge suppressors like Zener Diodes and MOVs to clamp high voltage and prevent it from doing damage.
I suspect that that is the reason you don't hear about people frying their systems by removing the battery anymore.
But, the farther back in time you go, the less sophisticated the protection you have.
I personally helped replace two different radios in friend's vehicles after they failed when a battery was removed with it running.
One was on purpose, to jump a guy off when they had no jumper cables, and the other one just fell out when he hit a bump.
I'm not sure at this late date if either car had a computer in it, probably not.
I personally had the output of my alternator go to almost 25 volts when a diode blew in my isolator.
It removed the battery from the circuit that the sense line was hooked to, so the alternator had no feedback to tell it what the output voltage was. I figured out that something was wrong when I smelled the other battery boiling.
Other than the battery, I never saw any damage.
So, you could say that its OK to remove the battery from a running truck, because all the electronics has internal protection to save it from high voltage.
But that is a little like saying its OK to overload that wire over there because there is bound to be a fuse in the circuit somewhere.
Read, study, figure out where I am wrong, and get back to me tomorrow. I am going to get something to eat, and go to bed.
Turkeys plus work beckon tomorrow.
tire... or A 60 means its WIDE
