A control technique for shaft-torque smoothing in PM-brushless small power cogeneration units

The paper deals with very small power co-generation units where a three-phase PM-brushless generator is driven by an internal combustion engine (ICE). A power range of 1.52.5 kW is considered for these units, which are mainly devoted to residential/household applications. The low-power values can lead to conveniently use a single cylinder ICE as prime motor, also in order to reduce sizes maintaining suitable efficiency values. For this kind of ICE the typical torque and inertia behaviour against shaft angular position can cause vibrations and angular speed oscillations. These last ones can be considerably smoothed by suitably controlling the brushless generator at fixed mean value of the electric power transferred to the grid. A simple and efficacious control algorithm is presented; aimed to yield a realistic reduction of the mechanical oscillations, this control technique is oriented to keep acceptable efficiency values of the whole apparatus unless excessive over-sizing of generator and front-end static converter. Simulation results are shown and discussed in order to show the effectiveness of the control.