Discharge lamps are of negative resistance characteristics and for this reason they should be used with a current limiter. The tool, which is connected between the lamps and network, to limit and adjust the current of discharge lamps by the help of inductance, capacity or resistance elements, is called ballast. Ballasts may not only be produced from any of the resistance, inductance, or capacity elements, but also be in such structure to encompass a few of these elements as well as most of the auxiliary components such as transformers, flicker suppressive, cold ignitors, interference attenuator, etc.
A proper ballast should:
Ballast losses feature to be one of the most significant factors to affect the system efficiency. Those ballasts used with fluorescent lamps are classified for energy by CELMA. According to this classification:
Those devices that provide voltage jumps for ignition in discharge lamps, without any need of pre-heating in electrodes, are described as ignitors. For metallic halide lamps and sodium vapor lamps the voltage value that is necessary for starting discharges is higher than the mains voltage (700 – 5.000 V), so these lamps are used with ignitors. The duty of ignitor is to supply a high voltage value as required for igniting the lamp. Ignitor must be deactivated as soon as the lamp is ignited. To achieve this, the features including the voltage control in lamp ends, lamp current and time function, etc. are used. Such ignitor that is not deactivated as soon as ignition occurs causes decrease in lamp life.
High pressure sodium vapor lamps and metallic halide lamps require a specific igniting voltage ranging from 700 to 5.000 V to be on. In choosing an ignitor to be used for a given lamp the power and type of such lamp are determinant. Ignitors ensure an ignition up to a mains voltage of 190 V. There are three types of ignitor: series, parallel, and semi-parallel. The most preferred one of those is the series ignitor.
Discharge lamps, together with auxiliary equipments used, draw active power as well as reactive power from the mains network. Due to that the used ballasts are of inductive characteristics, the inductive reactive power so drawn must be compensated. In compensation a specific condenser with a different capacity must be used for each different lamp type.
A condenser is basically generated through putting an insulator between two conductive plates. Air, mica, paper, oil, and glass-like materials are used as insulator. Capacity value is directly proportional to the surface of conductive plate and dielectric coefficient, but inversely proportional to the distance between plates, and denotes the electric load storage quantity.
If a condenser is charged and deactivated, then a slow and automatic discharge may occur between plates in the course of time, in which case the dielectric material between such plates corrupts. To prevent this condition a 'discharge resistance' is connected between plates, whereby the condenser is discharged through such resistance.