This type of ballast is known by several names such as lag auto, lag, or high reactance ballast. It is used when the line voltage is 120 volts and socket voltage is in the 240 volt range. This ballast consists of two coils on a core of steel. Together, the tap and output coils transform the line voltage into the starting voltage needed. The ballast also limits lamp current.
Autotransformer ballast have the same operating and performance characteristics as reactor ballasts.
This type of ballast is normally used with mercury vapor and HPS lamps. These ballasts are also called regulated or autoregulator ballasts. CWA ballasts perform the basic jobs of current limiting and voltage transformation. In addition, CWA ballasts are always high power factor ballasts. They have starting currents that are less than the operating current.