torque conditions the system normally assumes that calibration is unnecessary however if the difference between the speed reference e g the radar or gps reference and an axle speed signal is greater than a selected value typically about one per cent while the axle is operated at a relatively low tractive effort or horsepower less than about 4500 lbs and at a speed of more than a selected minimum such as about 5 mph with all speed sensors operative then calibration is desirable transient conditions are eliminated by requiring the above conditions to exist for some selected time interval such as 20 seconds if calibration is needed a need cal flag block 78 is set to allow a forced calibration to occur a forced calibration of wheel diameter only occurs when the locomotive is operating in propulsion or electrical braking accordingly before implementing a forced calibration the control system controller 26 must determine if one inverter can be disabled without effecting locomotive operation an exemplary set of conditions to evaluate locomotive status is whether the locomotive is in motion and speed is greater than about 5 mph air brakes are not in use the rate of change of speed is less than.