DC Link Scherbius Drive

This type of scheme is shown in figure. This circuit  allows both  sub synchronous  and super synchronous speed control. In case  of sub synchronous speed control  slip  power is  removed form the rotor circuit  and is feedback  into the  ac supply whereas n case of super synchronous  speed  control  the additional power  is fed into the  circuit at slip  frequency.

In this  drive  two controlled  converter bridge bridges are used  in which  bridge has firing angle  less than 90⁰ and bridge 2  has firing  has firing angle more than 90⁰ when sub synchronous  speed  control  is required. In  this case  bridge I works as a rectifier bridge 2 works an inverter. When bridge I has  firing angle more than 90⁰ and  bridge 2 angle  less then 90⁰ super synchronous speed control  is obtained and bridge 1 works as an  inverter and  bridge 2 works  as a rectifier.