Environmental Engineering Reference
In-Depth Information
Chapter 11
Generator and Electrical System
11.1 Introduction
Most early small turbines, such as Burne's, mentioned in the Preface, and the
famous Jacobs machines from the USA, used DC generators. Modern practice
favours three-phase AC permanent magnet generators (PMGs), with induction
generators, often just a standard electric motor running ''backwards'', as a fairly
distant second choice. There are novel sources of PMGs, such as domestic washing
machines,
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and an increasing number of them are being made especially for small
wind turbines. Many of these come from China which has most of the world's
reserves of the rare earth magnetic materials needed for their manufacture.
This change of generators has been driven partly by consumer demand—there
are many more AC-powered products than DC-powered—and partly by the
advances in power electronics over the last 20 years or so. Nearly all modern
generators are three-phase as this increases the ratio of power to weight and
produces a more constant shaft torque in comparison to fewer phases. Even for
grid connected small turbines, it is usual to rectify the varying frequency and
voltage generator power and then invert it to produce AC power of constant
voltage and frequency. This has become possible because of the significantly
reduced cost and increased capability of modern inverters. Usually this conversion
is combined with maximum power point tracking (MPPT) which aims to match the
generator power output to the blade characteristics so as to extract the maximum
possible power from the wind as its speed varies apparently randomly. MPPT is
becoming standard on turbines above the micro category. Many micro-turbines
charge batteries and use a much simpler control system. Another more
recent innovation is to combine the controller and inverter for grid connection.
This chapter was co-authored by Dr. Peter Freere and Professor Ed Nowicki.
1
For example,
http://www.sustainability.ofm.uwa.edu.au/
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