The following was originally posted to the forums by cornbinder89 on 1/13/2013
I’m a firm believer that it is more important to know why than how. If you know why, you can figure out a way for how. If you don’t know why, then a procedure to do something is just trial and error with no reason behind it.
Unfortunately in today’s high tech world that is not possible for most of us on most things, so much the pity. That is one of the nice things about working on stuff from the beginning of the last century, it IS possible for someone to understand, if given the info.
I know I posted this somewhere on the old site, but it is long gone, so I do it again:
A generator "makes" electricity by passing a coil of wire thru a magnetic field, where a "north and South pole of a magnet are separated by the coil. You can move the magnet or the coil, it doesn’t matter but there must be movement.
Generators have fixed "poles" and the current coil rotates. To make more consistent flow, multiple coils are used on the rotating member. Each coil is connected to two copper segments on the armature 180 deg apart. The brush are placed so the coils on the armature that are in the height of the magnetic field (and making the most current) are connected to the output.
In the small generators IHC used, there are only two poles in the fixed field, so only two sets of brushes. Larger generator can have 4, 6, 8 or more poles. It just means how many sets of magnets there are in the generator.
In simple bicycle headlight generators, permanent magnets are used, but these are too weak to be practical for cars and trucks, so electro-magnets are used. These are coils of wire wound around an iron core. The more current you pass thru the field coil, the stronger the magnetic field, the more current the armature makes.
When the generator is at rest or very low speed, it is completely disconnected from the rest of the trucks electrical system. For this reason, you should never connect a battery charger to either of the wires on a generator. (alternators, on the other hand are always connected to the battery).
A small amount of magnetism remains in the iron core of the field coil, just as you can magnetize any iron object by subjecting it to a magnetic field. This is important as which pole is North and which is South is determined by the direction of current flow in the field winding. If the iron pole shoe has never been magnetized, and the generator has never been connected to an electrical system (remember, I said at rest, it is disconnected from the truck system) It can be either way. Lets say the two poles are in a vertical line, if the top one is North and the bottom South for the sake of this example, the generator will produce + current out the output stud and – from the case ground. If you reverse it had have the south on top and the north on bottom, – will be the output stud and + will be the case ground.
What Polarizing does is determine which way the poles will be. Because electro magnets polarity is determined by which direction the current flows thru the coil, you need to "tell" it how the rest of the trucks electrical system is wired. You do that by momentarily hooking it up to the electrical system when it is not turning. This passes current thru the generator (will will try and act like a motor, and will spin if the belt is off) and magnetizes the iron pole shoe.
When the generator is turning slow, The armature coils are passing thru the weak magnetic field left in the pole shoes, creating a small current in the armature winding.The output from the armature is still not connected to the trucks electrical system, runs to the regulator, which feeds this back to the field windings, making the magnetic field stronger, which increase the output, which gets fed back,and the cycle continues until the voltage exceed the "pull in voltage" on the reverse current relay (sometimes call the cut-out relay) in the regulator.
The voltage regulator used on most of the trucks is a "3 unit" type. This means it has 3 separate "relays" in them. They are the aforementioned reverse current, the voltage regulator, and the current regulator.
When the generator is at rest, the voltage and current regulators are relaxed, so the field is connected directly to the output of the armature. The reverse current relay is also relaxed so the output of the generator is disconnected from the rest of the truck.
Now we jump back to where the generator is making "juice" and the voltage is rising above the set voltage on the reverse current relay. This is a special relay as it has two sets of windings on the bobbin. One is a light winding of many turns of fine wire, the other is a heavy winding of thick wire. The heavy winding is connected so that any output from the generator to the trucks electrical system passes thru this winding. The bobbin is wounds so that in the forward direction (flow from the generator to the truck) the magnetism complements the magnetism from the lighter voltage winding, but any reverse flow (from the truck to the generator) will buck the magnetism from the voltage winding. Until the relay closes there is no current flow in the current winding, so only the voltage winding is trying to close the relay. The voltage required to close the relay is higher then the system voltage, but once the relay does close, current is now flowing in the current winding too and it keeps it closed even when the voltage drops, as long as current is still flow from the generator to the truck. If the generator slows, and current starts to flow the other way, the relay will open and the cycle will start again when the generator speeds back up.
As long as the voltage is below the set-point of the voltage regulator "relay" the relay stays in the relaxed position and full output is allowed back to the field coil. Once the set point is reached, the relay arm is pulled away from the top contact, but not all the way to the bottom one either, it "floats" in the middle. When this happens, a resistor is in series with the feed to the field coil cutting the amount of current and therefore the strength of the field. If the voltage continues to rise (because of increased speed or less load) the relay arm is pulled to the bottom contact, shorting out all voltage to the field, which collapses and the output of the generator falls, As soon as the voltage drops to the set point, the relay are rises off the bottom contact and the field is fed thru the resistor and more current is made in the armature, if still more is needed the relay arm rise to the top contact and full current to the field is restored. In actual operation, the voltage regulator spends most of its time "floating" between the mid point and full field, as required to maintain the set voltage.
Generators are not self limiting as to current like alternators are. If allowed to run "unchecked" they would over-heat the the armature coils, brushes and commentator. SO a current regulator is also included. It is wire exactly like the voltage regulator but instead of many turns of fine wire on the bobbin, a few turns of heavy wire that the whole output of the generator must pass thru (just like on the reverse current relay). It controls the field in exactly the same way as the voltage regulator, but based on current, not voltage.
When the engine is started and at low to mid speed, neither current nor voltage regulator are limiting the field as it take a full field current to make enough to charge at all. As the engine speed increase and the battery is not yet charged, it will be the current regulator that sets the limit of the field, once the battery voltage has come up to the set point, it is the voltage regulator that is controlling the output and the current seen on the ammeter starts to come back toward the middle.
Generators have segmented commutators that rotate. For this reason their top rated speed is often much slower then alternators. They tend to have larger drive pulley to keep the speed down. All this plus the fact that their are only 2 poles (where as alternator often have 9, 13 or more poles of three phase) means they make mush less current at low engine speed then most expect out of modern system. It is not un-common for them not to charge at all at idle, this is normal, the battery supplies the current needed to run the truck (and you will see the ammeter go to the – or discharge side) while this happens, and then when the speed increases it goes back to the positive on the ammeter.
Generators have some advantages over alternators as well as the above drawbacks. They can start charging a completely "flat" battery where as an alternator can not, their must be some charge on the battery or the alternator can not build enough voltage in the field to begin the charge. Generators give a long slow charge to the battery, which is better for it then trying to absorb a huge amount of current all at once. Generators (and their mechanical regulators) can suffer sudden voltage spikes (like the battery being disconnected while running) being hooked up backwards and other things that could kill and alternator. They can survive the large electromagnetic pulse of a nuclear blast that will destroy all un protected semiconductor.