Abstract: A mobile land vehicle having an engine arranged to provide motive power to the vehicle and to pump or pumps in a hydraulic work circuit; the engine governed to operate at a given speed. An alternator is also driven by the engine and connected to provide alternating current to high intensity discharge lamps which are capable of operation only between a relatively narrow range of current frequencies. The drive means to the alternator produces a frequency near the high end of said range when said engine is at or near its governed speed and the frequency approaches the lower end of said range as the engine speed is lugged down under full load.