How to Rewind an Alternator





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 note  of 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. :)



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    What is the current output after rewinding? How much does the voltage or current drop under load? I'm assuming the 35A alternator running at 1/4 the top speed will still only put out 1/4 the current, so roughly 8.5A. Is this correct? However, then it has to be energized at 2.5amps. I guess rewinding allows you to run it slower and get the correct voltage eliminating the need of a voltage regulator and losses, but I can't understand why a gear ratio/ pulley wouldn't accomplish the same thing. If car alternators worked so well, why wouldn't the airx wind turbines etc... use them. I'm not doubting the experiment just wondering if there are better ways to increase output without changing the number of windings and what is it that makes the AirX, $900 wind turbines etc... so much more efficient than an inexpensive car alternator?

    2 replies

    Alternator becomes more efficient at 2000rpm and higher, the amount of parasitic HP needed to energise the coil is negligible compared to the engine output.

    A windmill's range is from 50rpm, or cutin speed, to the max or furling speed usually 500rpm or less, but its largely dependant on blade dia and material, ie composite fibreglass 1.2m dia max - 800rpm max.

    In conclusion the wind doesnt consistently produce enough HP to overcome the HP needed for a geartrain to spin an alternator in its 3500 rpm range.

    I've seen them pop up in my neighbourhood with bicycle gearsets, they're never up and working for longer than 6months, by then the owners have figured out the above info.

    Hi Peter

    I see a company in the USA is producing a stainless steel rotor shaft and they fit rare earth magnets to replace the rotor winding to get rid of the loss associated with the rotor field.

    Or one could build a circuit that only energizes the rotor coil above certain speeds.

    Nice instructable.

    this is a very educational instructable. I love how you kept the facts including your lessons learnt. Very few people have the integrity to do just that. I applaude your remarkable effort.


    Neat description. I thought it would have been better if you had told us the number of turns. The microwave magnets work but they are shunted by the shaft running through the centre. I made an aluminium bushing for the centre of one of the claws, this lifts it away from the shaft. Can only get 20 turns x 6 for each phase I will be winding till next christmas. Let you know how it goes.

    1 reply

    Step 2:

    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.

    Haaa!! I was wondering where would you get the new wires to recoil...

    How much current does it take to drive the rotor field? I'm assuming you just put 12 volts across the rotor to generate the magnet field. Right?

    9 replies

    Roughly about 2.5 Amps, and yes, just 12V across the rotor field coil, its as easy as wiring the case of the alt to battery negative and then hooking the battery positive to the F terminal. The N is used to sense voltage for the regulator.

    So, it takes 30 watts to magnetize the rotor. Ouch.

    Here is another motor you might want to consider. The attached picture shows a power-steering motor from an 06 GM Malibu. This is basically a three phase servo motor.

    The picture also shows the amplifier next to the motor. The other little board is the hall sensor encoder board off the back of the motor.

    What makes this motor good for windmill applications is that it already has good magnets for the rotor. So, no slip rings or loss power to magnetize the rotor.

    I use to work for Motorola who made the amplifier for Delphi. I know that motor can handle 100 amps of current for short durations. It might be small, but it is really powerful. Just look at the size of the power connectors in the attached photo.

    Just goofing around, I smoked my voltmeter's fuse by hand cranking the motor while trying to measure current output.

    Anyway, just a thought that maybe there is a better motor to start your experiments with.

    Good Luck,


    petre says: no no you dont need 30 watts to magnetize the rotor. my onan manual says to use a 6 volt dry cell and a diode and a resistor in series with the batt. doesn't specify resistor size.

    more info would be welcome, like is the 6V batt in series with the 12V one or what? also where in the circuit do you put the diode and resistor?

    If you want 12 volts DC (really14 volts DC) from your alternator than you need to turn on the field from the battery you are charging when you sense enough rotation speed to make it worth while. You can do that in a number of ways such as a mechanical centrifugal switch or a sensor similar to a ABS sensor on an automobile. You could use an electronic circuit that uses a small amount of power to control a relay or power transistor that turns on the field circuit. That will save you from wasting power to the field circuit when the alternator isn't turning.

    petre says:OK my onan manual [ for a 120-240 volt alternator ] says if there is a loss of residual magnetism [voltage will not build up ] it may be necessary to flash the field. connect a 6 volt lantern battery in series with a diode and re sistor. diode is 120 amp[dc] 300 volt. resistor is 10 ohm,10 watt. batt + diode -- resistor----to pos brush. negative side of batt to alt frame. start the unit with no load connected to alt. momentarily touch resistor to positive brush. remove as soon as voltage starts to build up. use a volt meter to moniter the voltage. any questions call me at 412- 335-3508 i will be up until 3 am EST.

    Auto motive Alternator rotors need current applied continually while operating. Thoey don't retain magnetism like the old school auto generators do. Back in the day auto generators where re magnetized by flashing the field terminal from the vehicle battery momentarily.

    Those were DC generators not alternators. They got rid of those because the output came from the rotor and so required all the power to pass through the rotor commutator and brushes. However until the availability of cheap high current silicon diodes became available only DC generators were a practical source of DC current to charge car batteries.

    Yep, those might do the trick, as well as the Ametek range PM stuff. However its illegal to import left hand drive vehicles (we drive on the left here in South Africe), so I probably wont be seeing any Malibu's not just the '06 model. :)