Variable Speed induction Motor Drives Use Electronic Slip Calculator Based on Motor Voltages and Currents

In certain types of adjustable frequency induction motor drives, optimal control of the motor can be best implemented by using information on the motor slip frequency. Unfortunately, this generally requires the use of an electromechanical speed transducer coupled to the shaft. The presence of this transducer spoils the general characteristics of ruggedness and mechanical simplicity typical of an ac drive. This paper discusses an improved method of obtaining the slip information without the use of rotary transducers of any kind. By sensing the electrical quantities applied to the motor (voltages, currents, and phases) and by performing simple signal processing operations on the sensed quantities, an analog signal proportional to the slip level is derived and used in motor control functions.