A switchboard torque indicator for AC motors

The technique of calculating the air gap torque of an induction motor utilizing only the terminal voltages and currents has been known. for some time; however implementation of this technique in a commercial instrument that must compete in accuracy with electromechanical instruments raises a number of issues that must be resolved. Because the derivation of the theorem relies on assumptions of symmetry in the motor, questions have been raised as to its accuracy in real machines. Also, the effects of saturation and core losses usually are ignored but are always present, especially in "commodity"-type motors. There also are issues associated with the way data are processed, especially in the presence of strong harmonics. Finally, there is the question of how the system can be put into the field using only the information available to the user on the motor name plate. To clarify these issues, an experiment was set up using a PC equipped with standard A/D boards and a common 3/4 HP induction motor. The algorithm was implemented in C code on the PC. Upon the conclusion of these tests, the code was ported to a switchboard instrument and tested using a 15 HP motor. Results in both platforms were good, and the switchboard implementation has been adopted into the standard motor protection relay product.