Introduction: How to Rewind an Alternator

Picture of How to Rewind an Alternator

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

Picture of 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

Picture of 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

Picture of 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

Picture of 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

Picture of 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

Picture of 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

Picture of 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. :)


GregZ (author)2016-01-10

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?

petercd (author)GregZ2016-01-11

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.

Hiluxman (author)petercd2016-04-01

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.

wavestrock (author)2015-09-12

Very good

Mjtrinihobby (author)2015-05-25

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.

keith.cockburn.96 (author)2015-03-28


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.

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.

intesar (author)2015-01-24


turbo man (author)2014-12-28


unpro1 (author)2014-09-11

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

jimk3038 (author)2010-06-10

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?

petercd (author)jimk30382010-06-10

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.

jimk3038 (author)petercd2010-06-10

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 (author)jimk30382010-06-13

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.

petercd (author)petre2010-06-14

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?

obrienkims (author)petercd2014-08-05

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 (author)petercd2010-06-14

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.

static (author)petre2013-10-12

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.

obrienkims (author)static2014-08-05

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.

petercd (author)jimk30382010-06-10

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. :)

obrienkims (author)2014-08-05

Have you checked the three phase diode bridge rectifiers for shorted of open circuit diodes?

magickaldan (author)2010-06-08

You need to measure poweroutput with a load on it and measure the input from your batteries to see how much power the field coil is sucking up. See if it's putting out more than its using. Also if you can increase the voltage to your field coil then the voltage per rpm should go up too. So maybe a voltage doubler would be good for this?

magickaldan (author)magickaldan2010-06-08

Because all the voltage regulator on the alternator does is regulate the voltage to the field winding. The higher the rpm the less voltage to keep it at 12v.

obrienkims (author)magickaldan2014-08-05

Sorry but its the current to the field winding that is regulated by the value of the output voltage. They use a high power bipolar transistor to regulate the current to the field winding. Its true that the voltage across the field winding will go up and down but the transistor can only regulate the current in a circuit it doesn't care what the voltage drop across it or the field winding is.

petre (author)magickaldan2010-06-13

petre says: double the voltage half the current? is that it?

petercd (author)magickaldan2010-06-09

I'll probably do the full house output tests before I fly it, but first a 120 degree phase separation rewind will be in order. I checked the rotors consumption to be 2.3A at 11.4V dc. I've heard of power runaway leading to stator coil burnout if you connect the output to the rotor coil with out a regulator in between, dont know how true it is though. Thanks for the insights all the same.

fastrat2000 (author)2014-01-01

Did you test before you stared rewiring it? Okay I use to work on auto and still have dealing. The 70-80's had what Y winding these did build more volt at no or low loads, these are units was 30-70 amp about 2K engine RPM to get max, 2 or more step, so your need about 4K rpm or more to get full output. Now in late 80's to mid 90's they change two W winding, this make more amps at lower RPM, that and adding more polls these units make full power at or just off idle, so 800-1200 rpm rpm range, output is 80-140amp ez check is if fan is on the out side It Y, if inside, ( you likely can't tail it there ) its W, this should cover well in 2000's, so newer unit maybe better fit, I not sure if trying rewire is that helpful.

petercd (author)fastrat20002014-01-02

I pretty much just jumped in pulled everything apart and started rewiring, didnt check the old config but it came off a '70's Datsun 1400 and it had an external fan blade. I've shelved this from a practical point of view and thought it had more value in an academical way.

fastrat2000 (author)petercd2014-01-02

Well with out a base line it hard judge you mods to it, 2x raps would also mean 2x resistance too. Okay other thing was diode pack good, I just one diode goes bad it cut the out put down by more than 2/3. Even at 500 rpm I find it odd you showing a lost, but this so far under it min working speed too. This unit didn't get full output tail about 2000 rpm. Newer units Delta winding and have more polls, and make max power 800-1200 rpm range and have slower drop off, and output in 80-140 range ( common ), I think these could work a bit better older units.

mmorton (author)2013-10-15

Solargy Australia have developed a 5kW externally excited (4.6kW actually) generator/alternator. It produces peak output at about 2700 RPM and had an optional, CPU controlled load controller which is especially suitable for wind turbine applications. You can Google them on the web ( I think).

wga22 (author)2010-07-02

Do motors/alternators generally "burn out" because the enamel wears/melts away and the current shorts out? If one were to rewind, they would melt off the old enamel, then redip in enamel, then rewind? Any difference between AC and DC current motors?

static (author)wga222013-10-12

Universal motors (motors wit a commutator) function the same with AC or DC. That doesn't mean a tool with a universal motor can run with DC if it has electronic speed control, such tools are generally AC only. AC induction motor can't run from DC. A DC permenant magnet motor can't run off DC

static (author)wga222013-10-12

Unless it's an old hand rewinding motor model that they have rewound so often they could do so blind folded here is how a motor is typically rewound. The motor goes into the oven t melt off of much of the varnish as possible to make it easier to remove the old wire. after the motor is disabled to get to the windings. A reference point in center punched. on the frame.From that point the arrangement of the coils and w they are connected together is mapped. After that a coil of each size is removed as intact as possible. the wire gauge is measured the number of turns counted recorded the coils is saved to use to adjust the coil winding machine with. Replacement coils are wound with new wire. new insulating paper is placed in the slots in the frame the new coils are installed held in place by new wedges the coils are wired together a motor that can be hooked up to run on multiple voltages at multiple speeds has a nest of wires staring you in the face that's why that mapping step is important. After the coil are wired together a high frequency High voltage generator is used to test for faults. After it passes that test the motor returns to the oven to bake on a layer of varnish. Typically a rewind shop rewinds the same motor models so often they don't need to got through this process for each & every motor they repair, but will if they get a motor they aren't familiar with. The new hand without experience will do this until they gain experience. A motor can be rewound with simple tools & jig in the home shop easily. The biggest hurdle is getting the supplies

petercd (author)wga222010-07-03

Mostly yes, although its hard to cover all scenarios here, its usually a short in the windings or between the winding and metal like the armature/stator/rotor. Im not at all familiar with rewinding methods, but your description sounds about right. The main difference between AC and DC used to be that DC had brushes and commutator segments and magnets, and AC had the field wind and rotor wind with slip rings, although its not a hard and fast rule either. So even though you might have one of those old auto dc generators, its probably got a field coil hence not really suitable for windmill use. Even though motorbikes have "alternators", its not the same as car alternators because they spin magnets on the crankshaft rotor. They generate 3 phase AC which then needs to be rectified and regulated, hence the term alternators. As you may remember, the car alternator has a field coil which is energized by the battery, a luxury due to the larger more robust auto design.

Josehf Murchison (author)2013-02-23

Most of the alternators I have rebuilt only needed things like brushes, bearings, diodes or internal regulators.

numberonebikeslover (author)2010-06-15

I think, removing the inner rotter coils and putting magnets from loud speakers would make the rotter a permanent magnet. This is something i saw on internet long ago, but never gave it a try. Best wishes.

Its really works, but I used magnets from microwave oven they are bit strong the speaker ones.

hi everyone...i just went through the instructions..problem is how to opne the rotor coil further.i doubt while applying more force it might damage the poles of rotor.can somebody help me out how to open the rotor coil inside car alternator so dat i can place circular magnets of speakers that its strength will increase...thank you eveyrone

caluire24 (author)skishore22011-11-22

Did you find a solution I'm having just the same issue:
- removed the bearings (using a gear puller),
- removed the slip ring (unsoldering the coil wire links),
- cannot remove the two "fingers" parts: even with the use of the gear puller (begins to smash the axe).
Any solution may help, Thanks in advance,

Well I have not done it my self so I am rather ignorant than you my friend. I believe in learning by doing. Wish you all the best.

petercd (author)skishore22011-09-09

You need a gear puller, the small 2 legged one will do. I found it easier to remove the rotor finger part closer to the slip rings, there seemed to be a collar of some sort at the back.
First I removed the bearing then the slip rings then the rotor finger part, the coil came off easy enough so I will be saving its wire.

I just recently came across someone who'd done this with microwave oven magnets but the results were rather dismal in the authors opinion. Needless to say I didnt even bother pulling apart the rotor to experiment with this method. Thanks all the same.

Smartarts (author)2010-06-14

I've done a single phase rewind, 3 phase being too difficult for me to contemplate. For single phase you can use a continuous length of wire, feeding it down one slot then up the next and continuing round and round the stator until you have filled all the slots completely. I constructed a matching permanent magnet rotor using 36 neodymium magnets 25mm long and 5mm x 5mm section. This type of construction does create vibration at low speed due to cogging but this decreases as the speed picks up.

beko320 (author)Smartarts2011-10-25

can you tell us the results for volts and amps ?

Oorspronklikheid (author)2010-06-11

To make ripping out the old coil easier you might try turning your oven on max and putting the stator inside it to melt of the enamel.

databoy (author)Oorspronklikheid2010-06-13

I worked in the electrical industry. Placing the stator in an oven to melt the enamel will not make any difference. There is a wedge in the slots to hold the windings in place. Professional re-winders do not re-use the old copper. They just cut one side of the winding and tap out the windings. Professional re-winders use a bench form to wind the new windings then drop the windings into the stator slots. Then they use a wedge to stop the windings moving. Sometimes the wedge is just a thick piece of insulation.

honorman (author)databoy2010-10-07

Sorry, If it was taken from my post to reuse the old wire, of course this would never work, (dead short) plus cheap workmanship.

Also this was in the early 1950's that this man did this without any fancy jigs or maybe one he made, because he did take the motor back inside to his work bench to rewind it.

Its likely he learned this trade from someone else. this was small town Mayberry back then, 2000 people.

I then spent 12 years myself in electronic trouble shooting, later in life, then 33 year building New Homes

Oorspronklikheid (author)databoy2010-06-14

i got the idea from a company that regarly rewind motors in exess of 100KW(wich is kind of smal to them), so i thought it might work here too

petercd (author)Oorspronklikheid2010-06-13

Just a tad over the top? :) I was tempted to use a heat gun, but didnt know if it'd be a stuffup. Guess it could be kinda tricky pulling hot wire and perhaps too much heat could warp the stator plates, some of them have a varnish between them.

Silence (author)2010-06-15

Permanent magnet alternators are far superior to standard alternators, they're far more efficient and require no external source for the field. They're the usually choice for marine, RV and performance vehicle applications.

About This Instructable




Bio: general bloke type of tinkering
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