Alternator Field Feed

I'm not certain what you are asking here since you've mixed a few terms here. So I will go through a quick discussion of an alternator.

In all automotive alternators that I'm familiar with the rotor magnetic field gets created by the current running through the brush contacts, into the electromagnet windings of the rotor. Now since the voltage induced in the stator windings follows Faraday's law of induction, the magnitude of the rotating magnetic field and the angular velocity of the rotor sets the induced voltage magnitude. So as the engine velocity changes, the rotor magnetic field inversely changes to maintain a consistent peak output voltage. Now if there were only one set of stator windings then the mechanical load the alternator would present would peak only twice on each rotation as the rotor pole crosses the single stator. By placing three separate stator windings mechanically oriented 120° from each other, a more uniform mechanical load appears on the rotor shaft and three sine wave stator outputs, phased 120° from each other gets rectified to then produce a much closer to DC output level.

Thanks for your response. To clarify, here is a photo of the alternator. You may be able to see that the alternator output (3-phase) is rectified by diodes encased in finned aluminum heat sinks. This provides power to charge batteries. You will see that there is a little black thingee in front of the three alternator output terminals that is jumpered to the voltage regulator with a metal strap. The little black thingee is a rectifier. It feeds rectified AC to the voltage regulator.

My question is this: since this thingee is right adjacent to rectified AC why would there be a requirement to provide a separate rectifier for the regulator/field?

I ask this question because I intend to do away with this thingee altogether as I think that it is redundant. But just to be sure, I'm asking people who know.

It's not redundant, it's feedback. The output cannot be regulated unless the regulator knows what the output is doing.

Thanks for your patience.

I intend to discard the internal regulator and replace it with an external regulator. I plan to take the "feedback" from the alternator output terminals. I can't see any necessity to provide a separate rectifier just for the regulator - ?

Oh so you mean redundant after you've made your circuit modifications. That maybe true. I recommend that you make a schematic sketch of what you originally found wired up in your alternator before you modify it. This schematic along with your desired modification schematic will help you with solving any unforeseen problems in your project.

Good Luck

That "thingee" is another diode. You'll have no output if you remove it.

The separate diodes that supply the regulator is really a convenience of the way cars are generally wired. Since the ignition switch only switches +12 volts into various places, having the main diode output, and thereby battery, connected straight to the regulator would have the alternator drawing power from the battery all the time - unless a relay was used to isolate power to the regulator.

Using a separate diode set means that the ignition switch can provide +12V power to the regulator input via a light bulb to initially "excite" the field and offers a second benefit in that the "alternator light" goes out when the alternator starts to produce output.

It basically saves a relay and some electronics to extinguish the light.

Early auto alternators had an external electro-mechanical reg & 6 rect diodes- then came inbuilt transistorised regs-also 6 rect diodes- then int regs with 9 rect diode - these extra 3 diodes provided field power once the rotor was turning as against the 6 setup needing batt power. It is easy to provide your own reg to get output power desired- many years ago I made an external transistor reg to replace a failed orig electro-mech device- it worked well & the circuit was published by a leading ozzie electronics mag. I also used an car light switch(with inbuilt dimming function) as reg on a electric motor driven alternator used to quickly charge car batts- thus alt amps output could be set by simply turning switch to vary rotor input.

This feature is not redundant:

The Field supply is thru this set of 3 rectifiers and is necessary. You will find that these are connected before the main rectifier diodes. The reason is to prevent DC from the battery to come back and feed the field when the alternator is at stand-still (not generating)--> will just heat the field and drain your battery if the field is connected directly in front of the rectifier.

Clarification: When you start the car, the alternator is supposed to get excitation from the remanent magnetism in the rotor and stator irons This small initial voltage is used to build up the voltage that will feed the regulator and will be by-passing the regulator control circuit, to the field directly so as to boost the voltage to reach the required(set) voltage (or a percentage of it) . At that point, the regulator takes over and controls the field current.

If you take the time to draw the circuit as it is, you will see that the field is actually supplied via the regulator...The main diodes operate as blockers at the start until the voltage has built up.

{ imagine that you do not have a battery voltage at the start }

Both you and redfred suggested that I make a schematic. Now that I have done that, coupled with your information, the matter is now clear.

Thanks to everyone for your assistance.

Glad I could help.