A switchboard torque indicator for AC motors

The technique of calculating the air gap torque of an induction motor utili zing only the terminal voltages and currents has been known. for some time; however implementation of this technique in a commercial instrument that m ust compete in accuracy with electromechanical instruments raises a number of issues that must be resolved. Because the derivation of the theorem reli es on assumptions of symmetry in the motor, questions have been raised as t o its accuracy in real machines. Also, the effects of saturation and core l osses usually are ignored but are always present, especially in "commodity" -type motors. There also are issues associated with the way data are proces sed, 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 informa tion 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 o n the PC. Upon the conclusion of these tests, the code was ported to a swit chboard instrument and tested using a 15 HP motor. Results in both platform s were good, and the switchboard implementation has been adopted into the s tandard motor protection relay product.