Environmental Engineering Reference
In-Depth Information
Table 11.1 Main parameters of the Ginlong 500-A PMG
Parameter Value Comments
Rated shaft input power 510 W For blade design
Rated rotor speed 450 rpm For blade design
Rated input torque 14.8 Nm For blade design
Cogging torque 0.5 Nm For blade design
Generator inertia 0.006 kg m 2 Negligible in comparison to blade inertia
Generator weight 12.7 kg For turbine platform and tower design
Rated stator phase voltage 20 V rms line to line for rectifier design
Rated stator current 14.4 A rms line to line for rectifier design
Number of poles 16 Determines operating rpm
Stator resistance 0.35 Ohm For modeling PMG performance
d-Axis synchronous reactance L d 3.305 mH For modeling PMG performance
q-Axis synchronous reactance L q 3.305 mH For modeling PMG performance
DC bus voltage at rated speed 25 V Maximum rectified voltage
DC bus current at rated output 20 A Maximum rectified current
Permanent magnet flux density k pm 0.36 V/(rad/s) For modeling PMG performance
Data from manufacturer and measurements by Mohamed Fahmy and Nacer Benaifa
Fig. 11.3 Schematic of a
PMG wind turbine feeding
the grid. The filtering of the
inverter output and possible
isolation transformer are not
shown
rectifier
inverter
AC
DC
PMG
DC
AC
Grid
Ω, Q
DC bus
Table 11.1 shows that the cogging torque is small compared to the rated torque
even though the ratio is relatively high for this PMG, see Fig. 1.12 . The smallness
of the cogging torque means that it is only important for starting and low speeds, as
considered in Chaps. 6 and 7 . Furthermore, it is possible to design the control
system to counteract it, e.g. Favre et al. [ 4 ], but, of course, that requires the turbine
to have started and to be extracting power. Cogging torque is easy to measure,
e.g. Zhu [ 5 ], and is reasonably well understood, e.g. Guo and Chang [ 6 ]. In
practice, cogging torque can be reduced significantly, but it is very difficult to
eliminate without using iron-less generator designs, e.g. Islam and Sayeed [ 7 ].
Some small wind turbines use asynchronous or induction generators (IGs),
usually standard induction motors running ''backwards''. Their main advantages of
low cost, no cogging torque, and ruggedness were covered in Sect. 1.8 . Because
IGs are so common, a description of the basic operating principles can be found in
many texts on electrical machines, so only the features important for wind turbine
applications are given here. Figure 11.4 shows a typical torque-speed character-
istic for an induction machine in terms of the synchronous speed, n s , at which no
torque or power is produced or required. The synchronous speed is the speed at
which the rotor is in step with the electrical frequency at the induction motor
terminals. The same machine is a motor when operating below n s and a generator
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