Rotor current measurement of motors

Rotor current will be directly proportional to torque and stator current. In affect you could measure the stator current and calculate the approx rotor current. Measuring the rotor current directly with standard instruments will give errors as you are in affect measuring the current at whatever the slip frequency is.

For example, If the stator full load current was 250A (not sure what it is in this case) and the Stator start current was 400A (1.6x) then I would expect Rotor start current to be 1.6 x Rotor full load current = 775 x 1.6 = 1240A. Torque delivered would be around 1.6 x Full load torque, very typical for a ball mill or sag mill as Chris mentioned.

Correct measurement in the rotor current can be done with the likes of hall effect transducers or even shunts but not clamp-on ammeters unless they use hall affect or similar technology.

The liquid starter simply adds resistance in the rotor during starting to produce torque and depending on the electrode type and design, the start current and therefore torque should be similar with each new start. Generally the electrodes start wide open at start and slowly close up as the start progresses. Finally the rotor is then shorted as the motor gets to full speed and then runs like any cage motor. Differences in start current (eg As measured in the stator) would only be due to the electrode position at start being different each time. Liquid temperature will affect it only slightly.

The load will not vary the start current, only the resistance in the rotor and the speed of the rotor will affect that.