A Simple Graphical Method to Assess Induction Machine Performance Using per Phase Current as the Controlling Variable in Lieu of PerUnit Slip

Fixed-frequency induction machines remain the most widely-used electromechanical energy conversion devices encountered in engineering practice; therefore a very simple and insightful assessment of machine performance should be of interest to faculty members and students across the engineering spectrum. The per-phase electrical model illustrated in IEEE Standard 112 has been used by most authors of engineering textbooks in a traditional analysis of induction machines almost exclusively motors using 'per unit slip' as the controlling variable. The concept of per unit slip is admittedly brilliant; however, the traditional method of analysis based upon that term is tedious, requires a repetition of the most cumbersome steps for each specific machine speed of interest, and lacks insight. In contrast the proposed analysis offers a graphical assessment of the same model using the magnitude of the current as the controlling variable. Analysts need only draw an Impedance Line and Resistance Line for a specific current magnitude upon a circular plot designated as the Lumped Loss Normalized Impedance Locus before completing an assessment of a machine. While valuable to electrical engineering power majors because of its simplicity and insightful approach, it is particularly-suited for engineers/students outside that specialty who wish to assess induction machine performance without having to deal with per unit slip, synchronous speed, and the manipulation of complex numbers associated with traditional analysis. With the widespread contemporary interest in “green” energy generation, an assessment of induction generators is increasingly important. Such an assessment is easily amenable to the method. It is important to note that for either motor or generator operation, there is one very definitive operating point on the periphery of the circular Locus for each specific value of current; one may easily visualize how that operating point moves around the periphery of the circle as the current varies.