The effects of ac current is related to amount of frequency strongly. If this amount be very low, the flicker of lamp illumination will be observable. The typical frequency range of observable flicker is from 0.5 to 30.0 Hz, with observable magnitudes starting at less than 1.0 percent.
The human eye is more sensitive to luminance fluctuations in the 5- to 10-Hz range. As the frequency of flicker increases or decreases away from this range, the human eye generally becomes more tolerable of fluctuations.
In discharge type lamp the traditional ballasts produce 100 pulses ( in 50 HZ) of current every second, and the phosphor’s light output drops drastically between pulses. High-frequency electronic ballasts which applied in compact fluorescents produce tens of thousands of pulses per second, and phosphor light output drops less than 2 per cent between pulses. This high-frequency excitation also eliminates the strobe or flicker effect that fluorescent lamps exhibit when used with electromagnetic 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.
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.
These drawbacks led to studying the use of high-frequency AC current to drive the discharge lamps. High-frequency operation not only results in significant ballast volume and weight reduction, but also improves the performance of the discharge lamp. The voltage and current waveforms are almost proportional with the same v-i characteristic of a resistor, although this resistor is not linear and varies as a function of time and lamp current. The re-strike voltage peak no longer exists. The recombination of ions and electrons in the discharge is very low. No re-ignition energy is needed. The lamp electrodes also sustain the electron density during the transition from cathode to anode function, resulting in additional energy savings. Therefore, the gas discharge itself is more efficient in high-frequency operation, contributing to an increased efficacy. The efficacy increases by about 10% when the operating frequency is above 20 kHz. Other discharge lamps have a similar characteristic. The high-frequency operation also makes the lamp start easily and reliably, and eliminates audible noise and flickering effect. In addition, due to the advances in power electronics, power regulation can be easily incorporated into the ballast, making intelligence and energy management feasible.