Wound rotor motor is not the best option for SAG mills because the slip rings always are a problem even with liquid resistors. Best option is synchronous motor in combination with variable frequency drive. However if you already have the wound rotor motor the ERS operation is simple.
Instead of dissipating the rotor power in a resistor we can use it to power a 3 phase inverter that returns the power to the AC power source. The line commutated inverter is connected to the same feeder that supplies power to the stator, usually a transformer is used so that the effective voltage input at primary side be 80-90 % of voltage at inverter input this ensures that the firing angle is reasonably close to the permissible limit while reducing the reactive power absorbed by the inverter.
As per Torque / Slip characteristic for AC Motor, the value of the Max. Torque can be developed is constant while the Starting Torque occurs @ S=.1, (T proportional to r2 and S also proportional to r2 where r2 is the rotor resistance, the ratio r2/x2 when equal to 1 gives the max. Torque w.r.t Slip at Starting. Wound rotor motors are suitable and recommended for application for MV drive where it is required to be started on load such as ID. Fans, S.D Fans, Drill, etc.
As you aware the torque is directly proportional to the rotor resistance "r2" & varies with slip "S", hence injection of resistance into the rotor via Slip Rings, High Starting Torque can be got while the Speed, efficiency and Starting current will be reduced. Therefore resistance is the most practical method of changing the torque (i.e. wound rotor Slip ring Motors). Moreover, the Max torque can be achieved at starting when rotor Resistance "r2" = The Stator impedance, at starting S=1.
On the other hand, the slip of the Induction Motor (speed) can be changed by "extracting" electrical power from rotor circuit, more extraction increases the slip. By using thyristorized Slip-Recovery Scheme "i.e Kramer Scheme" feedback of Power from rotor circuit to supply circuit which also known as "the slip Power recovery scheme". The scheme is simply consists of rectifier and an inverter connected between slip-rings and the A.C Supply circuit. The Slip Rings voltage is rectified by the rectifier and again inverted to AC by the inverter and feedback to supply via a suitable Transformer. Such arrangement gives good efficiency with high cost due to Rectifier and Inverter.