Induction-motor characteristics at high slip

THE practical work of inventing the polyphase induction motor, by Nikola Tesla and Galileo Ferraris, was followed by farseeing analytical studies of motor operation. Masterly theoretical analyses of the induction-motor circuit were made by B. A. Behrend, Alexander Heyland, and others. These men, through their insight and understanding, were able to devise ingenious methods for theoretically predetermining operating characteristics of induction motors bv employing readily determined constants. For many years these methods of predetermination were accepted and used in practical work. However, as knowledge has expanded and a higher degree of accuracy becomes requisite, it has been found that the older methods of analysis produce inaccuracies which are too great for modern requirements. The trend away from the older methods is clearly shown by abandonment of the use of the classical “circle diagram” in cases where highly accurate results are desired. The use of the former methods of predetermination results in inaccuracies due to several factors, the most important being: (a) unjustified treatment of the exciting current, (b) the assumption that circuit parameters are constant when they are not, and (c) failure to consider the stray load loss. The errors resulting from wrong assumptions may not prove serious for light loading of the motor, but discrepancies increase as load and slip become greater.