This invention relates to ballast apparatus for operating high-intensity-discharge (HID) high-pressure sodium vapor lamps and, more particularly, to a lead-type ballast having an improved power factor over the life of the lamp.
The conventional lead-type ballast has been utilized to successfully operate high-intensity-discharge lamps for many years. The circuit is characterized by a capacitor connected in series with the lamp. The capacitor functions to control lamp operation to provide inherent power factor correction. The lead-type ballast has been used, in recent years, to ballast HID sodium lamps. The HID sodium lamp is characterized by having a voltage that increases with age. As a result, limits in the maximum and minimum permissible lamp wattage and lamp voltage have been defined in the lamp industry as established by the American National Standards Institute (ANSI). These limits take the form of a trapezoid and HID sodium ballasts used today are designed to have the lamp operate within its boundaries over the life of the lamp.
To start the lamp, an electronic starting circuit is typically included as part of the ballast apparatus which provides pulses of sufficient voltage and duration until the lamp starts. The lead-type ballast is popular for use with the HID sodium lamp because it is intermediate in terms of cost and power losses while providing good wattage regulation. However, as the voltage of an HID sodium lamp increases with age the effect of the serially connected capacitor in maintaining power factor for the ballast apparatus is decreased.
Other ballasts have been utilized to operate HID sodium lamps, such as the magnetic regulated or constant wattage ballast which includes a voltage regulating section which feeds a current limiting reactor which maintains the desired power factor through the majority of lamp life. The drawback of this ballast though is its relatively high cost and comparatively much higher power loss level than the loss level that can be attained using the lead-type ballast.
One such lead-type ballast is disclosed in German Pat. No. 1,802,011, dated Oct. 11, 1967, issued to Nuckolls. In the Nuckolls patent a high reactance transformer with a condenser is included in the secondary circuit, by which an adjustable degree of concentration is achieved in the secondary magnetic circuit that provides control of the lamp current to compensate for any changes in the supply voltage, as shown in FIG. 3 of the Nuckolls patent. A bypass condenser for high frequencies is also included across the primary of the higher reactance transformer.