A electricity generating assembly includes a plurality of rotatable fan blades. In free gyro mode, the method includes the steps of applying a combination of signals to common electrodes 5 and of applying the combination in alternation to main electrodes 5. The circuit includes a comparator with first and second inputs that provides a pulse-width modulated output in response to comparing signals received at the inputs. The monitoring system includes an event signature recognizer; a trend analyzer; and a symmetry analyzer.