Alternator Basics

Introduction

Most people who build wind turbines want to use them to generate electricity. There are two types of device commonly used to generate electricity, these being alternators and dynamos. Both these devices perform the same task (covnert kinetic energy into electricity) but do so in different ways and produce different types of electricity.

Basic principle

When a magnetic field cuts across a wire (typically a coil of copper wire) an electrical current is induced, this is called electromagnetic induction. The amount of electrical current and voltage will depend on the strength of the magnetic field (the flux), how quickly the magnetic field moves, how many winds in the coil of wire and the thickness of the copper wire.

Alternators

Alternators are the most common type of generator on the planet, almost every car on the planet has an alternator which is powered by the engine and used to charge the battery and power the cars electrical systems. Alternators produce alternating current (AC) which will need to be rectified to direct current (DC) if you want to charge batteries.

The alternators used on cars are built with cost and longevity in mind, instead of magnets they use some another series of coils of copper wire to create the magnetic field. As a small amount of electrical energy is diverted to coils to maintain the magnetic field the efficiency is not very high. Alternators built for use in cars are designed to operate in the range of 1,000 to 7,000 rpm, this is too high for a wind turbine. The poor efficiency and requirement for high rpms means that alternators intended for use in cars are not suitable for use with wind turbines.

Permanent magnet alternators (PMAs) contain a set of magnets (typically Neodymium rare earth magnets), as the alternator spins the magnets are passed over a set of coils of copper wire and an alternating current is produced.

Dynamos

Dynamos produce direct current (DC) which has the advantage that it does not need to be rectified to charge batteries. However, they do have a big disadvantage in that they use a commutator and brushes which will wear out quickly if in constant use.

Any DC motor can be used as a dynamo, all you need to do is to spin the motor shaft and it will generate electricity. Most of the cheap 'can' type motors are of no use for generating electricity as they need to be spun very fast to generate a high enough voltage and brushes wear out very fast. Some people have had success using DC motors from treadmills and old computer tape drives.