Rewinding a fan motor to get a slow speed DC Generator

Chuck Morrison 2001

The electric power plant.

There are two types of generators that we can employ for our wind generator. In the automotive world, we know these as generators and alternators. Generators are properly known as dynamos. The main difference between them is that dynamos utilize a commutator to transfer power from the coils on the armature to the outside world. A commutator is a multi-part cylinder made of copper which allows the brushes to pull energy from the coils only when they are in one position relative to the magnetic poles. In essence, this allows you to "skim the top" of the electric cycles to produce DC. Usually, there is a commutator section for each slot in the armature, although there may be an additional slot. An alternator differs from this in that it has two slip rings instead of a commutator. The alternator produces AC current, which is then usually run through a set of diodes to produce DC.

From the searching you've already done, you are probably aware of how difficult it is to find a generator or alternator that generates an appreciable amount of power at ~300 rpm. How many rpms you need will depend on how many blades are on your turbine and how well they are designed and constructed, but 300 seems to be a good ballpark figure. So how is the search going ? Find any 300rpm motors out there ? I've heard that there are such motors out there being used for computer tape drives (big tape drives, small wind turbines) and battery powered floor washers (I've never seen a battery powered one myself). My own experience is that these things are rare. Most plentiful are auto and truck generators, alternators and fans.

The first thing to do is to wander around some junkyards and see what's available. Check out the name plates for voltage, current and rpm information. You can check the state of bearings, but I would advise replacing these even if they appear to be OK. They are used and you're proposing to put several million more revolutions on them. If you find a generator, motor or alternator large enough, you may decide to go with it, even though it's rated at relatively high rpms. Now you have a problem you must solve before you can use it. For wind turbine use, you have to get the thing to generate a decent amount of power at a usable voltage at a relatively slow speed. It would be preferable that this be somewhere around 300 rpm or lower.

Lowering the rpms.

The motor/generator you have is rated at a certain voltage, amperage and speed. These are all tied in together and changing one of them changes the others. A generator rated at 12 volts at 1750rpm, will generate power above and below this speed, but will only supply 12 volts at or above 1750rpm. With a load that draws 12 volts, it will supply power to the load at 1750rpm and above, but not below. This is called the "cut in" speed for that voltage.

To change the cut in speed for a generator or alternator you can

  1. Increase the field strength,

    1. Replace the magnets with stronger ones in PM generators

    2. Supply more power to the field coils in shunt generators and alternators. This may require replacing the field coils.

  2. Increase the number of field poles,

    1. Replace the magnets with more magnets (in pairs) with the same flux. This may require resizing the armature coils.

    2. Add more field coils in shunt generators and alternators.

  3. Increase the number of windings in the armature coils,

    1. More windings should give you more volts at a given rpm, all else being equal. You will probably loose power capacity if you have to reduce the size of the wire. Current is determined by wire size and since power (watts) = voltage x current (amps)... you loose if the current is reduced.

  4. or a combination of the above.

PM generator/alternators are preferred by wind enthusiasts. I've rebuilt one for slower speed and was able to do by replacing the magnets and rewinding the armature coils. Let's use that experience to examine this subject.

The motor I found was a model 7600 from Electric Fan Engineering. It was rated at 12 Volts, 50 amps and was a four pole permanent magnet fan motor produced for heavy trucks. It had two rather noisy bearings, but ran very fast when connected to a 12 volt battery, or fairly slow connected to a 12 volt, 3amp battery charger - which was very hot after about 15 seconds of running this fan. I was able to get a reading of around 15 volts when running it off a 1725 rpm motor at roughly a 1-1 gearing. Calling EFE only gained me the information that it was rated somewhere around 2000 rpm. Below is a picture of the insides of this motor. The shell casing is ~8" in diameter (OD), although it isn't exactly round. Note the four large pole magnets inside the casing.






The photo doesn't show the brush assembly or the shell ends and bearings. The four magnets are " thick and measure 2 " deep by 3 " long. The magnets are held in by double sided sticky tape and metal spacers. The entire motor weighed in at 45 lbs. The wire on the armature appears to be 15 or 16 gauge copper. It appears to have a rather complicated winding scheme which disassembling did little to demystify. I suspect that it utilized a frog leg winding scheme which is a combination of lap and wave coils. There were four wires coming from each commutator segment, completing two turns around 9 slots, giving a total of 8 turns (4 coils of 2 turns each). It was more complicated than this, but let's go with this simplified version to run through some calculations.

I did some quick calculations based on Mick Sagrillo's paper, which gave the following for just rewinding and not adding magnets.

Volts

Amps

Watts

RPM

Wire Gauge

Turns per coil

12

50

600

1750

15

8

12

25

300

825

18

16

12

12.5

150

412.5

21

32

12

6.25

75

206

24

64



Since I had been really happy about finding a 600 watt motor, I was less than thrilled about putting all sorts of work into it and getting a 100 watt generator out of it. Now of course at higher speeds more power is produced, so this isn't 100 watts max. However, current is dependent on wire size and 24 gauge wire will handle a lot less of it than 15 guar. It will have larger heat losses, especially at only 12 volts. So while this may generate 12 volts of power at +/- 200 rpm, it isn't able to supply the power the original motor/generator could, speed not withstanding.

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