Electronic Analog Slip Calculator for Induction Motor Drives

For some schemes of variable speed control of squirrel-cage induction motor fed by a current-source inverter, accurate evaluation of motor slip frequency is essential for obtaining optimum torque output. For evaluating slip accurately, a digital speed transducer coupled to the shaft is generally required. Such a transducer and the associated digital circuits make the system complex and expensive. An analog speed transducer which is relatively cheap could be used only if the slip could be obtained accurately by some other means. This paper presents an inexpensive and accurate method of obtaining an analog signal proportional to the slip by using a simple calculator circuit that uses the dc link current, dc link voltage, and inverter frequency as its inputs. The slip calculator described here is capable of giving the correct output only under steady-state conditions. The design of the slip calculator is illustrated in the Appendix with the aid of a numerical example.