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GENERATOR AND ELECTRICAL SYSTEM
68
Type Advantages Disadvantages
Axial flux generator
with permanent
magnets (AFPMG) a
Asynchronous
generator (e.g. auto
generator)
DC generator
Induction motor as
PM generator b
� No transmission required
� No brushes required
� No cogging torque
� Can be build locally
� Availability
� Price
� Pre-assembled
� Simplifies the electrical
system
� Pre-assembled
� May not require transmission
� Price
� Availability
a) See description in the following paragraphs
� Availability of PM magnets
� Weight / size
� Transmission required
� Requires a current in the
field coils
� Brushes and slip rings
requires maintenance
� High maintenance
� Large sizes are hard to find
� Small sizes have limited
output
� Cogging torque
� High internal resistance
� Inefficient at high rpm
� Availability of PM magnets
b) Induction motor may converted to a generator and fitted with permanent magnets, e.g. from a hard
drive
Table 7.1: Results of generator survey
From the survey it is evident that the advantages of implementing an AFPMG in the wind
turbine are many. Its main disadvantage is the fact that it uses permanent magnets, e.g.
neodymium (NdFeB), samarium cobalt (SMCO) or ferrite magnets, as their availability in
developing countries is limited to some extent. In collaboration with EWB it is however
established that the advantages of the AFPMG design outweighs the availability issue and
the solution is therefore adopted for the wind turbine design. In situations where magnets
are unattainable they may either be provided by EWB or in cases of import prohibition one
of the listed alternatives may be implemented.