Demystifying the Delco SI Alternator, Part I

By

ORC's own Junk Yard Genius

This Is A Delco SI Series Alternator

This Particular Alternator Is The Delco 10 SI Size Alternator. It comes in variations of: 10, 12, 16, 20 25, 27, etc. Each one getting bigger in output and most of the time, in case size.

IS AN ALTERNATOR DIFFICULT TO WORK ON? NO! It is as simple as a wiper motor or starter motor. The entire charge of your vehicle, every electron used by every system in your vehicle is produced by the alternator. Some people will argue this fact, but every electron the battery expends has to be replaced by the alternator.
So every electron your vehicle uses, looses or abuses is created in the alternator, so it's work is never done.

Here's a quick background on the principal of 'Induction'...

All modern automotive generators use an electrical principal known as 'Induction' (To Induce).

When a magnetic field is passed over a conductive material, an electrical charge is produced. The same with moving the conductor through a stationary magnetic field, a current is also produced.

Now we have some basic electrical principal under our belts, it's time for a history lesson..

For the first 400 or so years humans have been aware of electricity, but only in passing.

Along about 1800 or so electricity became well enough known that it was being harnessed in limited quantities for productive purposes..

Among the first of the useful inventions was Telegraph! A fine example of Electro-Magnets in action. Crude chemical batteries were used to make a circuit with one end being a contact key, and the other being an electro-magnet that made noise by pulling down a spring loaded arm.

We are going to use a version of that same electro-magnet barely changed from the early days when electricity was considered a 'Parlor Trick' at best, and the work of the 'Devil' at worst!

In earlier times, current was produced by collecting stray electrons with a 'Static Generator', or by crude chemical batteries. Then along came an inquisitive guy that decided to spin a magnet in a coil of wire!

In an instant the modern Generator is born!

Early generators were a simple type, simply a magnet in a coil of wire, or spun next to a coil of wire. Induction produced a gaugeable current, and if the coil was big enough, a current that would bite!

In the mid 1800 two things would happen to make electricity a viable energy source..

The first was a process to make plentiful steel that could be hardened where cast iron could not.
And the second was a viable way to rectify usable DC current at the armature.

Until the advent of steel that could be hardened, All magnetic fields for generators had to be produced using 'Load Stone', or naturally occurring rare earth magnets. Both of which were relatively scarce and very expensive when they were found. These expensive naturally occurring magnets were often set next to iron cores to focus the magnetic field, but soft iron will not hold a strong magnetic field for long when the natural magnet is removed.

With the advent of steel, which could be 'Normalized', then 'Hardened', the steel could be permanently magnetized, and rare earth magnets weren't needed except to magnetize the hardened steel parts, which would then become magnets themselves.

This made magnets plentiful and cheap.

The other invention was the rectifying commutator. For our purposes, a 'Rectifier', is a device that captures the usable parts of AC current and supplies them as DC current.

In the case of the rectifying commutator, it used a set of mechanical contacts on the armature to separate the positive & negative sides of the generator output, and supply it to a terminals for DC use.

The commutator was the standard for over 100 years! This little device was hard to manufacture, expensive, fragile, and needed frequent service.
The Commutator was the only way to rectify AC to DC until the invention of the vacuum tube, and that was even more expensive, fragile & hard to manufacture!

In the late 1950's and early 1960's the automobiles came with 'Electric Everything' up to three mechanical rectified generators were required to supply a vehicle. In the late 60's and early 70's 'Alternating Generators' started showing up on vehicles..

The early 'Alternating Generators' were quickly dubbed 'Alternators'...
Most of the early ones (and some today) were externally regulated.
In earlier times, the voltage regulator was a set of mechanical relays activated by electro-magnets.

To be useful, they had to be continuously 'Tuned Up', and one wrong turn of the screw driver could ruin your battery, generator or both!

With the advent of modern 'Solid State' electronics, the voltage regulator 'Tune Up' isn't required.

Some companies are still using the voltage regulator separated from the generator, but that's not needed on today's vehicles . The voltage regulator can be built right into the alternators case with no problem.

The SI series alternators are probably the most popular world wide.
Simple, powerful, easy to work on and modify, these little work horses are found in every GM vehicle for 25 years, and in some AMC vehicles, along with other manufacturers.

User modification of this type can be found in everything from water & wind generators, used as welders, supplies for AC & DC current for accessories of every description, from coffee makers to cardiac machines..
This is the first type of the wildly successful internal regulator design released by GM.
We are going to teach you how to trouble shoot, tear down, do a failure analysis, and modify this style unit for just about anything you want to do!

First, Take time to take a good look at your alternator..

• Is the case cracked?
• Is the unit excessively rusted or caked with grime?
• Does it have a 'Smoke' smell to it?
• When you spin the pulley, does it turn freely?

If any of these things are wrong with your unit, you may be better off starting with better quality unit in the first place.

Start with taking a good look at the back of the unit.
This image will tell you what most things are.

This particular unit is a very SI series alternator.

The design has changed very little from early on until the SI Series was superseded by the CS series. This unit has a lot of grease buildup on the Rectifier and the Regulator, both of which need to shed excess heat during operation. This buildup alone could have contributed to the failure of this unit.

This is the top of the alternator.

In this case, the rear half with the plug lines are pointing up along with the front half with the threaded ear. Notice the Delco information stamped into the case just behind the threaded mounting ear?
This information tells us right away:
1. The seven digit Delco part number.
2. The Rated amperage output, in this case 61 Amperes.
3. The Voltage of the system this alternator is intended for.
4. The Grounding configuration of the system this alternator is intended for.
5. The manufacturer internal date & type code for this alternator.

You can see the '61A' stamp at the top right corner. This means 61 Amperes.

Just below it you can make out '12VNEG' stamp in the lower right corner.
This means '12v' 12 volt system, 'NEG' Negative ground system.

This is the front of the alternator.

Notice the top mounting ear is threaded, and the bottom isn't. Also notice that with this type of fan, the cooling air is forced out and away from this end. That means cooling air enters the BACK, and is expelled in the FRONT, not the other way are around. This is quite the misconception about this, so now you know the truth.

When disassembling the SI series alternators, I prefer to start with the back half first. It really doesn't matter, I just find it easier to work on these things when I take down the back half first. Remove the four (4) case screws from the back side, and pull the back half off, as shown in the picture below.

This shows the back half with Stator still attached.

. This is pretty typical of what you will see when you open the case, Grease, Rust, Dirt, Dead Bugs are all part of the game. Set the front half aside for now, and concentrate on this back half. The second order of business will be to remove the stator. It's connected to the Rectifier by three studs and nuts.

Here is the back half with the stator removed.

The stator was connected to the Rectifier by the three studs that are bare now. There are only slight changes to the internal layout of Delco automobile alternators, but there are some differences.

Take a few minutes to identify everything. With your project, you may want to take digital pictures of your arrangement, so you can remember where everything goes in case you are called away or have to set the project aside waiting for parts to come in.

This is a picture of the Stator.

The stator is where the actual current is produced to run your vehicles accessories and charge the vehicle battery or batteries. These are loops of wire. The Rotor, or rotating magnet in the middle of the alternator, turns inside these loops, making a magnetic field move around these coils. Induction creates an electrical current in the coils, and that current is collected at the Rectifier. Notice the staggered placement of the windings. This stagger is there so all three don't peak in current output at the same time.

Above is a electrical diagram of a STATOR.

With DC Generators, the production, or output coils, were on the armature horizontality, and were prone to failure when spun very fast. Copper is a soft material, and would simply spin off the armature if spun too fast. So was the commutator, and commutators often had fairly spectacular explosions when spun too fast and decided to come apart!

Around 1,500 to 2,000 RPM was the limit for most DC Generators and were often under driven on gasoline engines to keep the RPM of the generator down.

That's why you often see very large pulleys on DC Generators. Commutators had several metal contacts trying to be held together by a non conducting material, and needed regular maintenance to keep them producing.

Combine that with in inherent strength of the SLIP RINGS on the rotor, the metal cage made by the magnetic pole halves, the larger sealed roller bearing surfaces, and the electromagnet winding being wound around the shaft instead of hanging off the side wound lengthwise, you can easily turn the alternator at twice engine speed, with three times the pluses, you have a generator that has roughly 5 times the output of early DC generators with none of the maintenance problems and weaknesses of the DC generators.

This is an electrical diagram of the Rotor.

Notice the rotor never ELECTRICALLY touches anything but the brushes? The Rotor is an Electromagnet, and it's entire purpose is the create a rotatable magnetic field. When the magnetic field produced by the Rotor passes through the Stator windings, a current is produced.

This picture shows the parts in the rear half of the alternator housing.

As you can see from the picture, there isn't a lot to the Delco SI series alternator!

Simple is good!

Most all of the parts for the small Delco alternators can be had at one discount auto parts store or another. This design is very adaptable for us 'Tinker' types! You would be surprised at what you can do with a Alternator! In this case, the Slip Ring Brushes are worn completely down, so I'm installing new ones, and the regulator is suspect, so it's being replaced.
With any luck, this alternator will last another 30 years!
If you are modifying your alternator for service as a welder, or for some other dedicated project where a regulator isn't really needed, you don't have to worry one minute about the regulator!

Here are the new parts ready to install.

Notice the back side of the old regulator? See the grime built up on what's supposed to be a heat sink for the regulator transistors? This could very well been what caused the alternator to quit working. The brush holder is fine to reuse if there are no cracks in the plastic or ceramic. Replace springs when you can. These springs came with the brushes, and as you can see, the old springs aren't even showing out of the brush holder, so the constant compression and heat have got the best of them.

This is a SINE WAVE output AC output from an alternator, as shown on an Oscilloscope screen.

As you can see, each time the Rotor's magnetic field changes from North Pole to South Pole over the Stator's windings, the polarity in the stator windings changes from Positive to Negative.

This change happens several times a second, so there is no mechanical way to extract the Positive Pulses. A non-mechanical, sold state device called a DIODE does the job very efficiently and quickly with no moving parts.

The Rectifier accomplishes this by using a 'Diode'. Diodes are simply one way gate valves for electricity. Current of one polarity can flow in one direction only, so the positive pulses are channeled in one direction, while the negative pulses are not permitted to exit the diode. When used together, Diodes can fully Rectify the AC Current into DC Current. That's why we call them 'Rectifiers'!

This is a picture of the RECTIFIER.

The job of a rectifier is to separate the Positive pulses from the Negative pulses in AC Current. This is what is called a 'Full Wave' rectifier, meaning it allows for the outgoing DC Positive, and to complete the circuit, allows the unused portion of the current to return to the regulator.

The middle is an overlay of a rectifier.

See the position of the diodes in relationship to the terminals? This should graphically illustrate the design of the Rectifier, and the placement of the Diodes to achieve a fairly constant DC output.

The bottom diagram shows a pure Electrical Schematic of the Rectifier so you will know what you are looking at when we move on to the 'Good Stuff'!

This is a diagram of the Delco 10 SI alternator.

You should be able to identify every part in the schematic, and all the sub assemblies by now. For the most part, we will be working with wiring diagrams like this one. Simple schematics are the easiest way to get one's point across! So, brush up on your diagram reading and stay tuned for next month's installment.