Most batteryless wind systems use an inverter with maximum power point tracking (MPPT) to maximize system output. The inverter’s MPPT algorithm allows the blades to spin at the ideal rpm and still produce the output voltage within the inverter’s required range. Wind generator blades have a maximum efficiency at a specific ratio of blade-tip speed to wind speed. This is referred to as the tip/speed ratio (TSR). For example, if a turbine’s blades have maximum efficiency at a TSR of 6 and the wind is blowing at 10 mph, the blade-tip speed should be at 60 mph (6 x 10) for best results.
A wind turbine’s permanent-magnet alternator has an open-circuit voltage (no load) that increases with rpm. But doubling the turbine’s rpm does not double the output voltage—the relationship is not completely linear. This is where the inverter’s MPPT comes into play. It controls the load on the wind generator to keep the blades spinning at their optimal TSR and converts the voltage output of the wind generator to the voltage required for the inverter. Think of it as a variable-speed transmission for voltage, allowing the blades and alternator to operate at their maximum efficiency.
—Robert Preus
