HID lamps dimming in the public lighting installations dominated by magnetic ballasts

Due to their lumen per watt efficiency, the HID lamps are wide used especially in the outdoor applications, but also in indoor industrial or commercial ones. The ballasts necessary for their normal operation could be magnetic or electronic and in the last decade we assist to a long debate on which is a better option. The dimming capabilities are one of the most claimed advantages of the electronic ballasts. But, in the last years, several lower cost solutions for getting dimmability with magnetic ballasts have been reconsidered. In this paper, the possibility to dim lamps trough the voltage amplitude modulation by means an autotransformer is explored. This solution can be cheaper, robust and most suited for the low budget Africa´s lighting systems. Several experimental tests have been performed on two type of 250W HID lamps, a High Pressure Sodium (HPS) and a Metal Halide (MH), operated by a Very-Low-Loss (VLL) magnetic ballast. The range of the voltage modulation has been from 160V to 250V and the power consumption, the light output and the harmonics content have been recorded. The results confirm once again the validity of this proposed solution. The energy-saving per lamp by dimming to 160V is about 55 % for HPS lamp and 52.72% for MH lamp.