Characterisation of an Electric Motor Directly coupled to a Photovoltaic Solar Array in a Refrigeration Facility for Milk Cooling

A photovoltaic array directly coupled to an electric motor may become a simple and reliable system suitable for use in remote areas. Notwithstanding, in systems including no intermediate power conditioning device, the values of current, voltage, motor speed and electromagnetic torque at the operating point vary continuously as a function of the prevailing environmental conditions and the characteristics and configuration of the components integrating the system, i.e. the photovoltaic array, the motor and the loads. Consequently, these components must be studied in detail to guarantee the right system performance. This paper is centred on the analysis of a direct current permanent magnet motor integrated in a refrigeration facility for milk cooling. Motor parameters are estimated by analysis of motor losses, data coming from simple no-load, load and locked-rotor laboratory tests. A motor efficiency function is obtained based on easy-to-measure variables, i.e. voltage, current and speed values. The procedure for motor characterisation provides the system electromechanical characteristic curves necessary to define the system operating points under different operating conditions of interest and to estimate the milk facility efficiency by determining the coefficient of performance.