Civil Engineering Reference
In-Depth Information
FIGURE 7-9
Scherbius adjustable-speed drive system injects variable frequency voltage in the wound rotor
circuit.
7.2.6
Variable-Speed Direct Drive
The generator that operates directly at the turbine speed is extremely attrac-
tive. This is possible particularity for small machines where the rotor speed
is high. The direct drive eliminates the mechanical gear altogether, and needs
no power electronics. This results in multiple benefits:
•
lower nacelle weight.
•
reduced noise and vibration.
•
lower power loss by several percent.
•
less frequent servicing requirement at the nacelle.
The last benefit is particularly attractive for offshore installation.
The low rotor-speed requirement for large rotors imposes a design lim-
itation on the electrical machine. That is, the generator must have large
numbers of poles. Such machines must have short pole-pitch, resulting in
poor magnetic design. To circumvent such limitation, the permanent mag-
net and wound rotor synchronous machines are being considered for
1.5 MW direct drive generators. Another possible solution is the axial gap
induction machine. It can be designed with a large number of poles with
less difficulty compared to the conventional radial gap induction machine.
The axial gap machine is being considered for direct drive marine propul-
sion, which is inherently a low-speed system. For small gearless wind
drives, the axial-flux permanent magnet generator may find some interest
for its simplicity. A 5 kW, 200 revolution-per-minute laboratory prototype
of the axial-gap permanent magnet design has been recently tested,
how-
ever, significant research and development effort is needed before the vari-
able-speed direct drive systems can be commercially made available for
large wind power systems.
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