In the old days, an alternator replaced the old-style generator, which was used primarily to keep a battery charged enough to crank an engine and keep the headlights bright enough to see at night. The alternator proved to be quite an improvement over the old-style generator, which needed new brushes every 15,000 to 20,000 miles. Generator repair was such a significant market that shops whose only business was to rebuild generators and starter motors flourished. The real reason behind their success in the past was that the average auto “mechanic” was less than competent at diagnosing “electrical” problems and preferred to leave that to “experts” while they focused primarily on mechanical repairs and tuneups. Many mechanical guys seemed to be unable to fathom that “electrical stuff.”

I'll never forget this old-timer who had worked on cars since he was old enough to pick up a wrench. He had never been to school and he worked barefoot on his dirt-floor, one-stall, wood-frame garage next to his house. The cars were lined up every day because he was good at it and he was honest. Sometimes he even took pies in payment if the customer didn't have enough money. We kids used to stop by his house and listen to him bang and grunt as he worked away. It was a mysterious place to kids, with the dirtiest dirt I ever saw. It was a mixture of oil, grease and discarded chewing tobacco drippings. We knew not to walk close to the big tree next to his garage because our feet would sink into the ground and come out all black and covered with smelly engine oil. I always waited for him to grind some metal on his grinder. The beautiful yellow/orange spray of glowing sparks always fascinated me. It still does to this day.

Occasionally he would engage us kids in conversation as if to confirm his diagnosis with someone else who wouldn't tell him he was wrong. One day I watched him check a “newfangled gadget” that replaced the generator by holding a screwdriver next to it. He explained that if the alternator pulled in on the screwdriver the alternator was “puttin' out.” I was quite impressed with his knowledge but I was only 11. Then again, if the alternator pulled in on a screwdriver, kept the battery from going dead and made the headlights bright enough, what other test did you need?

Have times changed now! The alternator not only keeps a battery charged and the headlights bright at night but now it powers onboard computers. What would that old-timer think about that? The complications of today's vehicle electrical systems demand more advanced testing than using a screwdriver to check the strength of a magnetic field. No longer can we leave electrical diagnosis and repair to others. No longer can we expect cars to be repaired by mechanics with no education. Times have changed a lot of things in the automotive and truck repair industries.

Let's see what we can learn about alternators in this three-part series. The term “alternator” to describe what we have been calling an alternator is now obsolete. We are now (since the 1996 model year) to call them GENERATORS. The name change more accurately describes what an alternator (oops - a generator) - does. (Sorry, old habits are hard to break.)

It is customary to identify a voltage source by the type of voltage it provides, AC or DC, and not by the way the output voltage is developed. A true alternator produces AC voltage at its output, whose frequency is dependent on speed of rotation of an electrically charged coil inside a stationary coil. Well, that kind of sounds like an alternator doesn't it? What's the problem? What we get from what we used to call an “alternator” is DC voltage, not AC. The term “alternator” doesn't fit. How could we have been so careless all these years? It should have been called a generator all along. Well, frankly I couldn't “care-less” what we called it as long as I could repair it. Yes, but we have all erred and caused great confusion within the industry ... lo, all these many years, say the “experts.” We must change ... and yes ... we will. It is a GENERATOR. 'Nuff said? Let's get serious about generators with this three-part series of articles: How they work (Part 1), how to test generators and the charging system correctly (Part 2), and how NOT to test a generator and the charging system (Part 3).

How Diodes Work
Generators need solid-state diodes to do their job. We know that a diode lets current flow in one direction but not the other. But, how does a diode respond to voltage? Let's look at both current and voltage with respect to the diode symbol shown in Figure 1.

The diode lets electrons pass through in the opposite direction the diode's arrow is pointing. For this to happen, a diode must be forward biased. That means the diode anode is more positive than the diode cathode. Forward bias allows electrons to flow through the diode. Without forward bias there is no diode current. If a diode gets reverse bias at its two leads, the opposite polarity of forward bias, the diode is cut off and electrons cannot pass through. This explains how current flows through a diode, which is necessary to understand how a diode in a generator makes it “generate” DC voltage and current. Without diodes, generators could not generate electricity. Let's talk about a generator. It's quite a story.

Generators
A generator contains several major components. The voltage regulator controls the amount of electrical energy produced. The rotor is a large spinning coil inside the generator. The outer ring around the spinning rotor contains at least three stationary coils called stator windings. When three stator coils are present the generator is a three-phase system. Some manufacturers use four stator windings and they are four-phase systems. The more phases in the generator, the more efficient it is and also, the more costly to build. Three-phase generator systems are the most common.

For a generator to generate electricity it must be electrically excited. It needs a battery to get going. Fortunately, every vehicle has a cranking battery. Battery energy is used to kick off the generator once the engine begins to run. Some techs have tried to take a weak or dead battery and place it in a car with the engine running to recharge the battery. Good luck because you will probably need it. First of all, NEVER - I say, NEVER - I say again, NEVER - disconnect a battery cable while the engine is running. The battery acts as a voltage stabilizer to help the generator keep its head. Take the battery away by simply disconnecting one of the battery cables and the generator loses its best friend. It then may go crazy and produce voltage spikes and a major energy dump into the generator and the electrical system, destroying the generator and a lot of electronics in the car. 'Nuff said? The other problem with putting a dead battery in a running engine (if you don't fry the generator and the car's electronics) is that the generator is looking for a little energy from the battery to get started charging. Dead batteries are notorious for having no “juice.” You can't start off a generator with nothing. It'll just sit there and spin (oh well, we try, don't we?). It is true, however, that some of the first “alternators” were able to charge a partially discharged battery because they were well built and could take the abuse. That is not true of today's generators.

Let's take a walk through the alternator (oops, there I go again; it's a generator, dummy). We'll look at how the component parts of an alt— ... err, a generator ... work together to charge the battery ... and ... electrically power the vehicle. That's right! The generator has two functions in a vehicle. The generator provides the electrical energy to operate the vehicle when the engine is running besides charging the battery at the same time. This makes the generator's performance during engine run very important. It ensures that the computer-controlled engine gets the power it needs to function properly to maximize fuel economy and minimize pollution. The battery is actually “off-line” when the generator is generating. If the generator cannot generate electricity, the battery takes over and runs the vehicle until the battery runs down. Then you are walking.

Figure 2 shows the rotor and stator windings in schematic diagram form. The stator winding is a delta connection because the three windings are connected in series to form a triangle like the Greek letter delta. Another stator winding configuration is called a “Y” (wye) winding because the three stator coils are connected to a central connection point. The name for this stator configuration looks like the letter Y.

Transistor Q1 is a power transistor located in the voltage regulator to control rotor winding current. Current flows up through the generator ground, through Q1 (emitter to collector) and through the rotor winding to B+. If you ever held a screwdriver next to a running generator you have felt the screwdriver being attracted by the intense electromagnet of the rotor inside the generator.

The rotor rotates within the delta stator winding to induce energy into the stator windings, using the principle of electromagnetic induction. The induced energy in the stator winding is a traditional AC sine wave with both a positive peak and a negative peak occurring each 360-degree cycle. The sine wave describes the voltage change and the current change in the stator winding. Sine wave stator voltage constantly changes amplitude going through a positive peak down through zero volts to a negative peak then back up through zero volts to a positive peak, etc., ... over and over again. The sine wave frequency depends on rotor rpm.

Sine wave stator current periodically changes direction. Stator current follows behind the stator voltage peaks because the inductance of the rotor coil opposes changes in current. That's what a coil normally does. During the positive current peak, the (+ alternation) portion of the sine wave, stator current flows in one direction to a peak. Then, it falls to zero and reverses direction during the negative, or - alternation, rising to maximum in the opposite direction at the negative peak. Then stator current falls to zero again, momentarily stopping and reversing direction. Current rises to a maximum peak current in the opposite direction during the + alternation, etc. ... over and over again. The question to be answered is: What does the stator do with all the sine wave energy induced into it?

The sine waves are presented to the positive and negative diode assembly or diode bridge shown in Figure 3. There are three so-called positive diodes because they connect to the positive terminal of the battery (B+). There are three so-called negative diodes because they are connected to ground, which is also the negative terminal of the battery (B-). The positive and negative diodes actually are the same type of diodes but are named after the battery terminal to which they are connected. A positive and a negative diode forms a series circuit network between B+ and B-.

At the common connection point between the two diodes is a wire from one of the stator windings. The stator's sine wave energy is connected to the common connection point between the two diodes. The job of the positive and negative diodes is to rectify the AC. Rectify means to change the AC to DC (direct current). The DC is then used to charge the battery and provide a DC energy source to power all vehicle electrical circuits. All the voltage energy put into the diode bridge drives the current through the vehicle electrical system. All DC current that runs the vehicle passes through the diode bridge. The diode bridge generates a lot of heat from all the hard work it must perform. If the heat is not dissipated adequately, the diode bridge will burn up and the generator dies - no charging voltage, no charging current.

Here is where diode knowledge comes in handy. The positive and negative diodes convert the stator AC to DC. Let's focus on only one phase or one positive and negative diode network for this explanation. When the stator voltage sine wave swings positive, the positive diode turns ON (forward biased) and the negative diode turns OFF (reversed biased). Refer to Figure 1 again for the polarity of voltage that forward biases a diode. The positive energy in the + alternation of the sine wave pulls electrons out of the positive terminal of the battery and through the positive diode flowing against the arrow. During this time the negative diode is reversed biased. Then the negative energy in the - alternation of the sine wave swings to a negative peak, pushing electrons into the negative terminal of the battery and through the negative diode flowing against the arrow. During this time the positive diode is reversed biased.

Each positive and negative diode network “alternately” rectifies its sine wave from its stator winding following a rotating order of 1, 2, 3 then 1, 2, 3, etc. A continuous stream of sine waves are rectified so that the current produced by the sine waves is a constant pulsating DC. Figure 4 shows the resultant “ripple” pattern riding on the DC charging voltage. Energy is directed by the diode bridge in the correct polarity to charge the battery in a continuously charging current through the battery being propelled by the charging voltage (also rectified by the diode bridge). The energy in the + alternation is directed to the positive post and the energy in the - alternation is directed to the negative post.

Electrons are pushed out of the generator's negative terminal and sucked into the generator's positive terminal in a continuous motion. At no time does any diode network stop contributing its share of electrical energy to charge the battery or provide energy to the vehicle. Each phase of the generator simply “peaks” in numerical order as long as the rotor is energized and turning. The end result is a pulsating DC voltage and current source at the generator terminals.

The purpose of the diode trio
The diode trio has a unique function in the generator. Notice that the three cathodes in the diode trio are tied together and go to the rotor winding's B+ side. The three anodes are connected to one of the three diode networks where the stator sine wave is applied. What's the reason? This is especially interesting since some generators do not use a diode trio; therefore, a diode trio isn't always necessary for a generator to generate.

The advantage of using a diode trio is most evident when the diode trio fails. The generator light comes ON at idle and goes OFF when rpm increases at acceleration. The purpose of the diode trio is to increase generator output at idle since at low rpms a generator may not turn fast enough to produce the energy needed to charge the battery and operate the electrical system. Increased generator output at idle is accomplished by the diode trio taking some of the stator sine wave energy, rectifying it to DC and sending it to the rotor winding for increased excitation during low rpm (idle) conditions. By applying an increased amount of B+ to the voltage side of the rotor winding, more current is drawn through the rotor to increase excitation. During times of high rpm the diode trio isn't needed but continues to contribute to the overall generating efficiency of the generator.

In an upcoming issue, Part II will cover how to test the charging system. The third part of this series will cover how NOT to check the charging system. See you next time!

Vince Fischelli is president of Veejer Enterprises Inc. (www.veejer.com) in Garland, Texas.

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