First, let us make sure we understand how an automotive alternator works. Here is a block diagram of the typical automotive alternator charging system:
This shows an alternator with a built-in Voltage Regulator (VR). Basically, the voltage regulator senses the battery voltage on the S wire, and controls the current in the rotating field winding (Rotor) via F1 and F2 . The IG wire simply switches on/switches off the VR circuit so that it doesn't drain the battery when the vehicle is parked. The current produced by the alternator goes to the battery via the B wire.
When the alternator is spinning, the rotating magnetic field created by DC current flowing in the Rotor through the slip rings induces three-phase AC currents in the Stator windings (stationary part of the alternator) which are rectified to DC by the six-diode bridge inside the alternator. The important take away is that a
small DC current in the rotor winding (F1 F2) makes a much
larger DC current (with a bit of ripple on it) at B. Another important point is that the VR
senses battery voltage and
controls the average field (rotor) current.
The wiring is similar if the VR is in a separate box, mounted external to the alternator. In this case, F1 is sometimes connected to the B wire inside the alternator, and the VR controls F2 by
sinking current, or F2 is grounded inside the alternator, and the VR controls F1 by
sourcing current.
To demonstrate the key elements of how a charging system works, I made this simple LTSpice simulation. It has much simplified behavioral models of the VR, alternator, battery, and some static loads, but it behaves similarly to the real thing..
The alternator is modeled as a current-controlled current source with Iout = 30*Irotor . This is a simplistic, but quite accurate model of the 14V 60A Prestolite Alternator in my Cessna 182. It assumes that the Alternator is spun up at an rpm where it can produce full output (not at idle). The typical resistance of the Rotor of a 14V automotive alternator is about 8 Ohms. The inductance of the Rotor is quite large, measured at 1H.
If you bypass the regulator, and apply full battery voltage directly to the Rotor (between F1 and Ground), the Rotor current is ~2A, and if you spin up the alternator to ~5000rpm, the alternator puts out it's full rated near 60A. It acts very much like a current source. If you open-circuit the alternator output, the voltage will soar to over 150V (its compliance). In normal operation, it is the battery that holds down the alternator output voltage (and filters the ripple). Think of "Load-Dump".
The VR is modeled as a voltage-controlled switch S1 (which is what it is) with a bit of hysteresis. In this example, I set the trip voltage to 14.22V with 4mV of hysteresis. To make the switch turn off when the battery voltage is above 14.24V, I set Ron=10meg. When the battery voltage is below 14.20V, I close the switch by making R0ff=10mOhm. Note that this is a classic bang-bang house thermostat type of control system. The VR either applies full battery voltage to the Rotor, or zero volts to the Rotor. The average Rotor current is the result of the VR acting as a Pulse Width Modulator (PWM). You need to understand this if you are mucking about with the VR.
The battery is modeled as a huge capacitorC1 , with some internal resistance. The static load (represents the things in the car that draw current) is shown as R2.
Now look at the first plot pane, which shows what typically happens right after motor start. Cranking the starter pulls some charge out of the battery, making its initial voltage ~12V or lower. This is sensed by the VR, and it applies the battery voltage to the Rotor. As the engine comes up to fast idle or faster, the alternator cranks out nearly its full rated output of 57A, about 14A of which goes into R2, and the rest is available to charge the battery. The time scale on this sim is arbitrary for illustration only, but see V(bus) green trace does during the first 20 sec. The "battery" is being charged at a rate of ~57-14 = 43A.
At 20 sec, the battery V(Bus) green trace voltage exceeds the setpoint of 14.24V, and the VR begins doing its thing. Note the alternator output current I(B1) red trace decreases
asymptotically approaching the 14A static load current. The battery voltage is maintained within a few tens of mV. The VR is now regulating the battery voltage. If more loads are turned on, the VR increases the average Rotor current to cause the battery (bus) voltage to stay constant.
Now look at the second plot pane. I plot the detail of the voltage at F1 (output of the VR) as V(f1) green trace at the 20 second point in the previous plot. I also show the current through the Rotor as I(L1) red trace. Note the low-pass filtering effect of the inductance of the Rotor, and the catch diode D1. Does this remind you of what happens in a Switch Mode Power Supply? Note that the VR makes PWM that is not constant frequency; it is whatever it has to be to make the average Rotor current be what it has to be to create enough current at the B output to just keep the battery voltage near the VR's set point. Remember that I(B1) is 30 times I(L1).
I have used a 'scope to look at the PWM at the field terminal in my airplane and various cars. Once the battery recovers after the initial charge up, the PWM rate varies between ~30Hz and ~150Hz. This is the ripple attributable to the PWM; nothing to do with the 3-phase, full-wave rectified ripple which I am ignoring in this simple simulation.
In the next posting, I will show the details of some real VRs.