Let’s cover some information on how a permanent magnet generator works, some of you might refer to these as permanent magnet alternator (PMA). The two most important things to know about in a generator are the magnets, which typically make up the rotor, and the electromagnets or coils that make up the stator.
Stator and Rotor, What are they?
Just to cover the basics, the stator stays and the rotor rotates. The stator is generally the housing on an radial flux motor, a pancake motor/generator is axial flux where half of the generator spins and the other ‘pancake’ is the stator.
A magnet has two poles, we commonly call them north and south, just like the earth magnetic poles. Magnets emit what we call magnetic flux and it flows from the north pole to the south pole. Flux flows or moves better thru iron than open air. Iron can direct magnetic flux, and this can be very useful when designing a generator. However, the flux must flow back to the magnet just like electricity, we call this the magnetic circuit.
Most PMA use rare earth magnets - neodymium are the strongest and the most common choice. Neodymium magnets range in strength up to N65. The stronger the magnets, ultimately the stronger the flux field hitting the electromagnets, the higher the voltage they will produce at any given RPM.
Electromagnets, sometimes referred to as coils, are split into two categories, iron cores and air cores (ironless). Iron cores are most common and highly used commercially. The iron can help intensify your magnet flux field by directing it and putting it to better use, increasing your efficiency. Air cores have no core material and are just a wrap of wire. Air core motors are just starting to be made with higher efficiency than those with iron cores; they do not have iron losses which make up the difference. Most of you interested in building a generator will start with an air core design, they are restively easy to design and if you do a good job it is possible to hit 60%, but typically I see anywhere from 30%-60% from homemade models.
What do the electromagnets do? When the permanent magnets are passing by creating an alternating flux field, this ‘excites’ the coil and a current is induced in the coil. Current is the real power, the voltage is the speed/force that the current has. The voltage is reversed with each change of polarity of the permanent magnet, this is why it produce A/C (Alternating Current).
The windings of the coil are very important to what power and voltage you want to create. First you need to determine your desired voltage, max current, and speed. The more wraps on the coil the higher the voltage will be produced with lower current, the few wraps would be a higher current while lowering the voltage. The power coming out remains the same, typically more wraps means more wire, more wire means more resistance, just keep that in mind.
Air Gaps and Flux
You are going to have to have some sort of gap between your stator and rotor as to allow them to spin. As I mentioned before magnetic flux does not travel through air the very well, we will refer to this as magnetic resistance. The smaller the air gap the less resistance and the higher the efficiency. Also you can use this to increase to decrease your voltage, move the stator further away from the rotor will decrease it, moving them closer together will increase your voltage.