The permanent magnet alternator is wired for 12, 24 or 48-volt battery charging depending on the size of wire and the number of turns per coil.
The alternator consists of a stator disc sandwiched between two magnet rotors.
There are 12 magnet blocks on each rotor. Each magnet block has a north pole and a south pole. The poles are arranged alternately, so north faces the stator on one block and south on the next. The poles on the other magnet rotor are arranged in the opposite polarity so that the north poles face south poles across the stator. In this way, a strong magnetic flux is created through the stator between the magnet rotors. The coils embedded in the stator are dimensioned such as to encircle the flux from one magnet pole at a time. As the magnet blocks pass a coil, the flux through the coil alternates in direction. This induces an alternating voltage in each turn of the coil. The voltage is proportional to the rate of change of flux. Voltage therefore depends on following factors:
- The speed of rotation
- The amount of magnetic flux and
- The number of turns in the coil.
- Speed -The voltage produced by the windmill alternator varies in proportion to the speed of rotation “rpm”. The shaft speed is a very crucial factor in all types of alternator and generator. The unit needs to make higher voltages at lower rpms, otherwise it is not suited for wind power use. This is also one of the main reasons why vehicle alternators are not suitable in wind power generation.
- The number of turns of wire in each coil is used to control the speed of the Windmill. If the number of turns is large, then the output will reach the desired battery voltage and start charging the battery at a low RPM. However if fewer turns of thicker wire in the coils are used then it will need to run faster. The number will have to be chosen to suit the rotor blades and also the battery voltage.
- Bearings - The blades and alternator are mounted on a hub, which rotates on a spindle with strong bearings. Besides having to withstand vibration and high rotation speed, there is a significant amount of thrust on the bearings from the wind, and it increases as the prop size increases. Vehicle wheel bearings are extremely useful because they are tapered and are designed to take the high axial thrust loads.