How does an alternator work?

I've been trying to figure out how an alternator works, but have come across some contradictory info regarding its power output. Any assistance to clear this up would be greatly appreciated.

From my understanding, alternators, ultimately, get their energy for output to the cars electrical system from the gasoline. As the gas combusts, the crankshaft, crankshaft pulley, and serpentine belt rotate, causing the rotor inside the alternator to rotate. The rotation rate of the rotor is dependent upon the diameter-ratio of the crankshaft pulley and the wheel at the end of the rotor. Now, the alternator is responsible for converting this rotational mechanical energy into electical energy. If the system only consisted of these parts, I would expect that the electical output from the alternator is soley dependent upon the engine RPMs. Therefore, a heavier electrical load would require more RPMs, and hence, more gas to maintain power.

However, a voltage regulator is part of the alternator system and feeds some current into the field coil of the rotor to produce the necessary voltage (this voltage is determined relative to the battery voltage and electrical energy needs of the car for any given moment), which produces the output current into the electrical systems. Due to the presence of the regulator and its operation, the alternator's energy output is not solely dependent on RPMs anymore because the regulator controls the return current to the field coil.


Questions:
1. Is my current understanding and description accurate?
2. In what manner does the voltage regulator supply this current? I realize that it's a complex integrated-circuit. Does the regulator always provide a direct current to the field coil, while modifying the amperage of the current? Or, is the amperage of the current constant, and it only sends pulses of current, like a digital signal, to the field coil?
3. If we add some extra load to the electrical system, does it necessarily mean that more gas (RPMs) is required to power the alternator and the electrical system's energy needs? Does it make sense that the energy needs to operate this extra load can come only from the operation of the alternator--can we keep the RPMs (gas usage) the same as it was before this extra load was added, but increase the current into the field coils via the regulator, which will produce a higher energy output? If this is possible, where would this extra current come from? I am certain that some fraction of the output energy is wasted due to the rectification of the AC current to DC at the diode-bridge. Maybe in simpler terms, can we reclaim any energy that is wasted by the alternator for free?

Hi, and welcome to the forum. You appear to already have a pretty good understanding of how the modern alternator works. The important thing to understand is that no matter how fast it rotates, it won't produce any output unless the field coil is fed with DC current, which in the first instance, when starting the engine, comes from the battery. After the engine is running, the alternator becomes essentially self-feeding, which is to say that a portion of the output is fed back to the input - i.e. the field coil. In order to increase the output current, it isn't necessary to increase the RPM, but rather just increase the current into the field coil, and that task is performed by the voltage regulator - it monitors the system voltage and varies the field coil current in order to maintain a relatively constant system voltage.

I've never had reason to put an oscilloscope onto the feed into the field coil but I'd expect it to be a continually-varying current, rather than pulsed. Any time the engine is running, even without any accessories switched on, the ignition system, engine control modules etc. will be constantly consuming current, so a continuous, though varied, output is required from the alternator.

If you add extra electrical load to the system, the system voltage will fall, so an increased output is required from the alternator - this is of course achieved by increasing the current into the field coil, at which time the alternator rotor has to work against an increased magnetic field, and therefore more fuel is required to overcome the increased drag. There are only two sources of current in the vehicle - the battery and the alternator - so when an increase in current is required, if you don't derive it from the alternator then you must derive it from the battery, so it's either a case of pay now, or pay later - pay now by consuming more fuel to supply the required extra current from the alternator, or pay later by using more fuel to restore the depleted charge on the battery, if you drew the extra current from that source instead.

For sure a lot of power is dissipated at the rectifier, for which reason the diodes will be mounted on a heatsink, and positioned within the airflow of the alternator's cooling fan.

It's an interesting question, but also a curious one, because if I was looking to recover energy from wasted heat within a vehicle, it wouldn't even occur to me to look at the alternator - I'd be looking at the cooling system radiator where, by design, we routinely and deliberately transfer massive amounts of heat energy into the surrounding air.

Selectron,

Your response was great. Thanks for taking the time to respond to my questions.

I've never had reason to put an oscilloscope onto the feed into the field coil but I'd expect it to be a continually-varying current, rather than pulsed. Any time the engine is running, even without any accessories switched on, the ignition system, engine control modules etc. will be constantly consuming current, so a continuous, though varied, output is required from the alternator.

That will vary from vehicle to vehicle. Many newer vehicles have the field wire controlled by the PCM which (for a '98 Grand Am 2.4L, since it's fresh in my memory) sends a pulse to the alternator of between 10% and 90% duty cycle depending on the desired charge voltage and the actual system voltage. A low engine RPM with high electrical load may be a duty cycle of 90%, while high RPM with low load may be a 10% duty cycle. When the frequency of the duty cycle is high enough, the magnetic field stays in place even after the pulse ends. It's like a household lightbulb. The alternating current is at a frequency of 60Hz, so the light is actually flashing 60 times per second, but it's so fast that the filament keeps glowing even when there is no voltage at all.

And the field wire is voltage controlled, not amperage(current) controlled.