Maximum Photovoltaic Power Flow to a Separately Excited DC Motor.

In this paper the authors have designed an experimental system to optimize the matching of a separately excited DC-motor interfaced to two PV arrays. Special emphasis has been placed on matching the load line of the field winding which, if directly connected to the PV array, intersects the V-I characteristics of the array at a voltage close to the open circuit voltage resulting in a very small excitation current and hence poor motor performance. Step-up choppers were used as impedance transformers to match the load lines of the field and armature windings to the maximum power lines of the PV arrays. The motor was coupled to a brake torque meter acting as a mechanical load and the step-up choppers were PWM controlled by a personal computer. The system efficiency defined as the ratio between the motor output mechanical power (calculated using the motor torque and speed) to the input power available from the PV arrays, was evaluated for four control modes as follows: applying no control, controlling only the armature winding, controlling only the field winding, and controlling both windings. The importance of controlling the field winding was clearly illustrated whereas controlling only the armature winding didn't significantly improve the system efficiency and this was because most of the input power was wasted in the form of heat in the armature winding due to the poor field excitation.