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.