Rewinding Generators/ Alternators For Wind Systems
© 1990 Mick Sagrillo
Wind generators run at fairly slow speeds: usually 250 to 600 rpm. Most people who design their own wind systems are stymied by the unavailability of slow speed generators. They usually choose to use an off-the-shelf generator that is stepped up to operating speed from the relatively slow propeller speed of a wind generator. But stepping up with gears, chains or belts introduces large inefficiencies, not to mention more moving components that need maintenance. There is another way around this problem: rewinding the alternator or generator for slow speed operation.
In its simplest form, a generator or alternator is merely a coil of wire passing through a magnetic field, see Figure 1, above.
When our coil of wire passes through a magnetic field, voltage is induced in that coil (suffice it to say that this is something akin to magic). The voltage induced in the coil is proportional to the number of turns in that coil, the flux density of the magnetic field, and how rapidly the coil passes through the magnetic field.
The current generating coils of wire are called the armature in a generator and the stator in an alternator. The magnetic field poles are called the field in either device. In a generator, the armature rotates in the stationary field. because it is rotating, heavy-duty brushes must be used to carry the current produced from the armature. An alternator is an inside-out generator: the field, or rotor, rotates in the stationary generating coils, or the stator. Because an alternator's field uses very little current, the rotor needs much smaller brushes than does a generator armature.
The design and construction of an alternator or generator is a considerable undertaking that could easily fill several volumes. However, there are several basic principles governing generators and alternators that we can use to our advantage in order to rewind an existing device for use at a slower speed. These principles incorporate the following generator characteristics:
- the RPM (speed)
- the number of poles
- the number of turns in a coil
- the magnetic flux density of the field
- the length of the armature or stator stack
- the airgap
- the current handling capacity of the wire
RPM & NUMBER OF POLES
All generators and alternators are designed to operate at a fixed optimum speed, called the operating RPM. This speed is what we wish to change to better match the operation of the wind generator propeller. One way of reducing the speed of a generating device is to increase the number of field poles.
If you double the number of poles in a given generator, you will:
(1) cut its operating speed in half for a given voltage: or
(2) double the voltage output of that device at its operating speed.
Unless you are building a generator from scratch, this is usually quite difficult to do. One exception is in a generator with main poles and interpoles. The interpoles can sometimes be converted over to main poles.
RPM & TURNS/COIL
The voltage induced in a coil of wire passing through a magnetic field is proportional to the number of turns in that coil. If we can double the number of turns in the armature/stator coils, we can either (1) double the operating voltage at a given RPM or (2) halve the operating speed of the generator at a given operating voltage.
RPM & FLUX DENSITY
Another way of increasing induced voltage in the armature/stator coils is to increase the magnetic field through which those coils pass. Field strength is related to the amount of current passing through the field relative to operating voltage; the more current you can push through the field coils (up to a certain point called saturation) the greater the flux density of the field.
If we can increase the flux density of the field, the induced voltage of the generating coils will increase. Field strength can be increased by decreasing the number of turns in the individual field coils. The field coil uses up some of the electricity produced by the generating device.
The ideal generator will use about five percent of its rated capacity in the field. Beyond this amount it becomes less efficient to the point where saturation is reached and the field becomes parasitic. Field coils are usually connected in series in a generating device. One easy way to increase the current draw in a set of field coils without rewinding them is to divide them in parallel. This series/parallel arrangement still allows for north and south oriented poles.
INDUCED VOLTAGE AND ARMATURE/STATOR LENGTH
Yet another way of increasing induced voltage is by making the coils that pass through the magnetic field longer. Doubling the armature/stator stack results in a doubling of induced voltage.
The amount of space between the field coils and armature/stator coils is known as the airgap. The airgap is necessary to prevent the coils from rubbing on the fields after both have expanded due to the heat given off by the electrical generating process. However, the airgap works against the flux density of the field: the greater the airgap, the greater the current needed by the field to overcome the airgap. Most alternators and generators have much larger airgaps than necessary due to sloppy construction. The airgap can be lessened by shimming the field poles with ferrous shimstock. The only way to do this is on a trial & error basis in small increments.
The current output of the armature/stator is entirely dependent upon the current carrying capacity, or ampacity, of the wire used. Ampacity is related to wire size. Comparing relative wire sizes can be accomplished by comparing the wire's circular area (called circ. mils), unit weight, unit length, or unit resistance. The following chart lists these relationships for wire sizes used in generators & alternators: Note that half sizes exist for most wire gauges but in the interest of clarity are not listed.
FIGURE 2: COPPER WIRE TABLE
|Wire Guage||Circular Mils||Pounds/ 1000 feet||Feet/ Pound||Ohms/ 1000 feet|
We have been talking about doubling the voltage or halving the RPM of a generating device by doubling the number of turns of wire in the coils. These coils fit into slots on the armature or stator. The slots have a given physical size that cannot be changed. Obviously, you can't fit more wire into a slot than it was designed for unless you use a lighter gauge wire. This is where the Copper Wire Table comes into use.
If you wish to double the number of turns in a coil, you must halve the size of the wire. This corresponds to three steps down on the wire chart. For example, say we have armature coils with 7 turns of #15 wire. The circ. mil area is 3.257. One half of this would be about 1.6. This area is equal to #18 wire. The new coils made from 14 turns of #18 wire would fit into the existing slots.
Note, however, that by halving the size of the wire, you also halve the current carrying capacity of that wire. There is no free lunch! If you want a slower speed, you have to give up something. This new wire size will limit the power output of the rewound generator.
Let's say that we have a 1200 RPM, 32 VDC motor that we want to make into a wind generator, (DC motors & generators are more or less interchangeable). The motor draws 30 amps. We want it to run at a maximum speed of 300 RPM, and we'd like to power our hot water heater with the wind generator. The heating elements in the water heater are rated at 120 volts. We take the motor apart and discover that it has two main poles and two interpoles of the same physical size as the main poles. The wire in the interpole coils is finer than that of the main poles.
We have pulled the armature apart and find that we have coils made of #10 wire with 4 turns/coil. What to do? Let's begin with the interpoles. If we rewind them to the same number of poles with the same gauge wire as the main poles, we have just doubled the number of poles in the generator. This has the effect of cutting the speed of the generator to 600 RPM, but still at 32 VDC. In order to get the speed down to 300 RPM, we need to double the turns of wire in the armature coils, from 4 to 8.
Wire size is reduced from #10 to #13. But we're still at 32VDC! If we halve the wire size again, we're up to 64 VDC. one more time and we finally get to 128VDC, close enough! But we've taken two more jumps in wire size, from #13 to #16 to #19, and doubled the turns twice, from 8 to 16 to 32. Our final armature coils would then be 32 turns of #19 wire. What kind of current can we expect out of this generator?
Doubling the field poles has no effect (in this case) on current. However, going to smaller wire gauge in the armature does. Going from #10 to #13 cut our current production from 30 amps to 15 amps. Two more jumps to #19 wire cuts our current output to 3 3/4 amps. Our wind generator will put out 4 amps intermittently at 120 volts with a top propeller speed of 300 RPM. This same process can be used in reverse to rewind a generator for lower voltage & higher current.
We have several old 12 volt, 100 amp Chrysler alternators in the scrap heap. We need an alternator for our hydro plant or wind genny to put out 24 VDC to match the PV array and inverter. New 24 volt alternators cost $400! What to do?
Car alternators possess several interesting features that can be used to our advantage. First, since we have several of these things, we have several lamination stacks at our disposal. If we take two of these cores, strip the wire and pop the rivets out, we can bolt them back together for rewinding. Since the lamination stack is doubled in size, we just doubled our voltage, from 12 volts to 24, without changing wire size. The same thing can be done with the rotor by merely feeding 24 volts into it. We'd need to use a 3-phase bridge rectifier in place of the usual voltage regulator. We can then proceed to rewind with different wire gauges to meet the RPM specs of our hydro or wind plant.
FOR THE LIBRARY
Anyone wishing more detailed information on rewinding can order the following republished out-of-print books from Lindsay Publications, POB 12, Bradley IL 60915. Both books cost $11.90 postpaid. Autopower, by S.W. Duncan, 1935 (Catalog #4791) LeJay Manual, by Lawrence D. Leach, 1945 (Catalog #20013)
Mick Sagrillo, Lake Michigan Wind & Sun, 3971 E. Bluebird Rd. Forestville, WI 54213 • 414-837-2267.