In addition to the above very thorough answer, I need to add a few points.
Point 1: Stator current has nothing to do with RPM in an induction motor. The only thing that controls speed (as long as the motor is not overloaded) is the applied frequency, and the motors inherent slip. The current drawn is what is needed (again, up to the limit of the motor) to provide sufficient torque to drive the load.
Point 2: When you are dealing with a Variable Frequency Drive, you need to specify what mode the drive is running in before even considering relating measured current to the load. In simple Volts/Hz operation, the stator current measurement is generally accurate. Most modern drives though run in Sensorless Vector (no feedback element) or Flux Vector (with a speed feedback device). In either of those modes, and some manufacturers differently named but similar modes, you cannot actually measure the stator current with any degree of accuracy. In SSV or FV mode, the stator often draws just about full load current all the time if you just use a clamp on ammeter. The only way to measure the actual torque producing current is to use the diagnostics on the drive itself (if provided). They use the magnetic model of the motor to display the actual torque producing current and flux producing current as separate vector values.