An uncontrolled  converter and a controlled converter connected in series in the rotor circuit of a three-phase a.c. asynchronous motor to feed the slip power back into the supply system are called a converter cascade or slip power recovery system.

The subsynchronous type of converter cascade can be generally used in all branches of industry. It is preferably used where a direct-current motor would be undesireable because of the necessity of commutator maintenance and a limited range of speed control is sufficient. This is the case, for example, with drives of centrifugal compressors, blowers and rotary pumps. The preferred power range is 500 kW to 10 MW.

As compared with throttle-type flow-rate control, e.g. for water pumps, the use of a subsynchronous converter cascade can yield an energy saving of 50 % and more. The same is true in comparison with speed control using rotor circuit resistors.

Particular advantage can be gained without difficulty also by converting existing slipring motor drives to subsynchronous converter cascades, if the motor power margin is not less than 10 per cent.

Further information you will find here: 

• Short description SCC  100 K