Alternator secrets

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Rewinding Generators/ Alternators For Wind Systems
Mick Sagrillo
© 1990 Mick Sagrillo

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ind 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 therelatively 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.
ROTATION FIELD POLE and alternators that we can use to our advantage in order to rewind an existingdevice 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

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RPM & NUMBER OF POLES All generators and alternators are designed tooperate 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 FIELD POLE number of field poles. If you double the number VOLTAGE & of poles in a given generator, you will: (1) cut its CURRENT operating speed in half for a givenvoltage: or (2) PRODUCED double the voltage output of that device at its operating speed. Unless you are building a Figure 1. A generator is really wire moving within a magnetic field. generator from scratch, this is usually quite difficult to do. One exception is in a generator BASIC ANATOMY with main poles and interpoles. The interpoles can sometimes be In its simplest form, a generator or alternatoris merely a coil of wire converted over to main poles. passing through a magnetic field, see Figure 1, above. RPM & TURNS/COIL When our coil of wire passes through a magnetic field, voltage is The voltage induced in a coil of wire passing through a magnetic induced in that coil (suffice it to say that this is something akin to field is proportional to the number of turns in that coil. If we canmagic). The voltage induced in the coil is proportional to the double the number of turns in the armature/stator coils, we can number of turns in that coil, the flux density of the magnetic field, either (1) double the operating voltage at a given RPM or (2) halve and how rapidly the coil passes through the magnetic field. the operating speed of the generator at a given operating voltage. Thecurrent generating coils of wire are called the armature in a RPM & FLUX DENSITY generator and the stator in an alternator. The magnetic field poles Another way of increasing induced voltage in the armature/stator are called the field in either device. In a generator, the armature coils is to increase the magnetic field through which those coils rotates in the stationary field. because it is rotating,heavy-duty pass. Field strength is related to the amount of current passing brushes must be used to carry the current produced from the through the field relative to operating voltage; the more current you armature. An alternator is an inside-out generator: the field, or can push through the field coils (up to a certain point called rotor, rotates in the stationary generating coils, or the stator.saturation) the greater the flux density of the field. If we can Because an alternator's field uses very little current, the rotor needs increase the flux density of the field, the induced voltage of the much smaller brushes than does a generator armature. generating coils will increase. Field strength can be increased by RELATIONSHIPS The design and construction of an alternator or generator is...