Introduction: How to Rewind an Alternator

About: general bloke type of tinkering

This will cover rewinding of an alternator stator for use in a wind turbine generator.

Traditionally, the use of car alternators is discouraged in favor of homemade slow running axial flux generators. However I bought a small Hitachi alternator for 1/3 of the price of one rare earth N35 Neodymium magnet, typically one uses 24 of these magnets in a wind turbine, so it would take about 45yrs to recoup the costs of the magnets alone.

In other words, not gonna happen.

Im using a Hitachi alternator from an early 1970's 1200 Datsun that had an external regulator mounted on the fender in the engine bay.

I figured this being my first foray into alternator rewinding, it would be much easier learning on a simple little 35Amp alternator like this. The downside is the rather low output and I probably wont go to all the trouble of mounting it on a pole too.

Be that as it may, it serves as an example of how alternator rewinding is done, whether it is just repairing for auto use, or modding for windmill use.

Step 1: Pull It Apart

This is relatively simple, 3 long bolts and the case is apart.
Then remove the field connection brushes from the rear of the case, it has the letters F and N on the connector, being field and neutral.

Once everything is apart, check the bearings for wear, they should turn freely without noise of grinding. They shouldn't spin which would indicate that they are dry.

Step 2: Ripping Out the Old

I'll start off with the caution.
The wiring varnish/shellac has sharp edges, so be careful when removing the old windings.

The original windings were 13 turns of 1.2mm dia and I reduced the wire diameter to 0.75mm in order to double up on the windings, fitting in 30 turns in each stator slot. This is an attempt to increase the voltage which has the benefit of generating 12V at much lower rpm's.
In their designed for application, alternators generate 12V at about 2000rpm and higher. This is totally impractical for a windmill whose blades might only turn at 300 ~ 700rpms.

TIP of the Day
...take noteof the original style of windings as well as the start and end connect positions, in order for the new rewound stator to match up in the original position.

This stator 4 coils of 13 anti-clockwise turns for each of the 3 phases, so we need to keep a similar format in order for the rewound stator to fit.

Step 3: Star or Delta Windings

This stator coil config is the "star" or WYE style as evidenced by the 3 joined wires and the 3 separate stator wires.

Simplified explanation is that star gives more voltage and delta gives more amps for a given rpm.
Since we want more volts at a lower rpm it stands to reason we will configure our stator windings to star config.

If your stator had only 3 connection wires with 2 wires joined on each connection, then you would have a delta connected stator.

Step 4: Sliding in the New

By the way, I salvaged the enamelled copper wire from an old fridge motor, for what its worth.

In this 3 phase 4 coil wind in 24 stator slots, we start by inserting the 1st coil and skipping 2 slots for the other 2 phases.
Even the start wire (lead in) and the exit wire (lead out) are separated by 2 slots as can be seen in the pics below.

120 degree phase separation mistake.

It has been pointed out to me that my phases haven't been wound with a 120 degree separation, more like 60 degrees, which will most likely result in low voltage output.

I should have left an empty slot between the lead out wires, so a rewind will be necessary to correct this mistake.

Step 5: New Rethink

I decided to add slot-insulation just in case of a wire short to stator edge or something else.
So I used Ganex DMD 2-3-2 0.18mm which is half the price of the Nomex stuff yet still good till 150 degrees C.
DMD stands for Dacron-Mylar-Dacron, for what its worth.
I also used a "D profile" fibreglass rod called a topstick to keep the wires in the stator slots.

Lastly the windings are treated with a shellac or varnish to keep them in place and also to prevent loose wires from vibrating which might cause shorts etc.

Step 6: 120 Degree Phase Separation Rewind

I decided to rewind the stator with the correct 120 degree phase separation and check performance before jumping to any conclusions.

The exit wires to the diode pack need a 1 slot spacing between them for the 120 degree phase power output. As they were in my old configuration in step4 was more like 60 or 90 degrees which resulted in the strange cloverleaf like coil spacing you can see in the pics in step5.

So if it looks odd it probably isn't right and wont work as intended.
For interest sake, the previous config yielded 12.2V @ 500rpm and the new proper phasing yields 17.6V at 500rpms.

As I said in an earlier step, pay attention to the original wiring, including the lead out wire spacing that I missed, take plenty of pics, because somebody else's eyes might pick up something yours missed. :)

Step 7: Closing Up

First install the Diode pack, then once the diode pack is installed, the end coil wires can be soldered on.
Slide the rotor in to check for fit, nothing touching or scraping, and then the front case half can be installed.

Lastly install the brushes for the field rotor coil and hook up all spade connectors and plastic covers.
The N Neutral conn is used as a voltage sense for the regulator, the F field goes via the alt warning light on the dashboard to the regulator.
The case is connected to batt neg and the big threaded rod from the diode pack goes to the batt positive.

Step 8: Conclusions and Final Thoughts

Initially I wound the stator incorrectly for the standard 120 degree 3 phase output, which resulted in a rather dismal output of 12V open voltage at 500 rpm's  on a drill press under no load.
This was with an external 12V batt on the field connection to give a full field excitation.

Final testing has now yielded 17.6V open voltage and charging at 1A, giving a theoretical power output of 20W max.
However, the field coil required 11.45V @ 2.5A to achieve this, meaning the input power required is more than the output generated.

So, it will be rather futile to try this configuration as a windmill, unless you are able to modify the rotor with magnets of some sort. Even so this is a small output alternator and probably wont yield great power.

Feel free to experiment with the larger 65A  and greater type of alternators like the AC Delco 10DN or SI (internal regulator ), but don't be too disappointed  if the output isn't what you'd hoped for.

You can also use the Ametek range of DC treadmill motors which wont suffer from the parasitic power loss of the field coil on alternators.

When considering standard DC motors for use as a windmill generator, you need to divide the voltage into the working Rpm's of the motor in order to ascertain if it will be suitable.
For example a 220V  2850 rpm motor will have a volt to rpm ratio of 12.95. Multiply this ratio by your required voltage ie 12 and you discover that it will generate 12V at roughly 155rpm.
The lower the better. :)