Power Quality in Power Systems and Electrical Machines, 2nd Edition

3.8 Inter- and subharmonic torques of three-phase induction machines

In Sections 3.4 and 3.5 the magnetomotive forces (mmfs) of spatial and time harmonics including the fundamental fields were derived, respectively. It can be shown that harmonic fields (where h = integer) generate very small asynchronous torques as compared with the fundamental torque. Sub- and interharmonic (0 ≤ h ≤ 1) torques, however, can generate parasitic torques near zero speed – that is, during the start-up of an induction motor. The formulas derived for harmonic torques are valid also for sub- and interharmonic torques. The following subsections confirm that sub- and interharmonic torques can have relatively large amplitudes if they originate from constant-voltage sources ...