Obviously, the resistive ballast incurs large power loss and significantly reduces the system efficiency. Fortunately, most discharge lamps are operated in alternating-current (AC) circuits so that inductive or capacitive impedance can be used to provide current limitation. AC operation also balances the wearing of two electrodes and maintains a longer lamp life. The inductor ballast represent the conventional ballasting approaches, and is known as magnetic ballasts.
Magnetic ballasts are operated in 50/60Hz line frequency. Every half line cycle, they re-ignite the lamp and limit the lamp current. Although magnetic ballasts have the advantages of low cost and high reliability, there exist at least three fundamental performance limitations due to the low-frequency operation. First of all, they are usually large and heavy. Second, the time constant of the discharge lamps is around one millisecond, which is shorter than the half line period (8.3ms for 60Hz line cycle), so the arc extinguishes at line voltage zero crossing, and then is re-ignited. Figure 4 shows the measured voltage and current waveforms of an F40T12 lamp operating at 60 Hz. After every line zero crossing, the lamp voltage waveform has a re-strike voltage peak; during the rest of the cycle, the voltage does not vary much. This causes two big problems: The lamp electrode wearing is significant, and the lamp’s output light is highly susceptible to the line voltage, which results in an annoying visible flickering. Finally, there is no efficient and cost-effective way to regulate the lamp power.