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The equivalent inductance of three-phase induction motors is experimentally investigated in this paper, with particular reference to the frequency range from 1 kHz to 20 kHz, typical for the switching frequency in inverter-fed electrical drives. The equivalent inductance is a basic parameter when determining the inverter-motor current distortion introduced by switching modulation, such as rms of current ripple, peak-to-peak current ripple amplitude, total harmonic distortion (THD), and synthesis of the optimal PWM strategy to minimize the THD itself. In case of squirrel-cage rotors, the experimental evidence shows that the equivalent inductance cannot be considered constant in the frequency range up to 20 kHz, and it considerably differs from the value measured at 50 Hz. This frequency-dependent behaviour can be justified mainly by the skin effect in rotor bars affecting the rotor leakage inductance in the considered frequency range. Experimental results are presented for a set of squirrel-cage induction motors with different rated power and one wound-rotor motor in order to emphasize the aforesaid phenomenon. The measurements were carried out by a three-phase sinusoidal generator with the maximum operating frequency of 5 kHz and a voltage source inverter operating in the six-step mode with the frequency up to 20 kHz.

The aim of the article is to present the mathematical model of current distribution in rotor squirrel cage bars of the uniform air-gap three-phase induction machine, taking into account skin effect. In the analysis of electric machines, transformations can often play a useful role. The space rotating frame method is used in the generalised theory of electrical machines on which exists a large number of references1,2. In conventional machine analysis where only the fundamental component of the flux wave is considered, the d-q-0 transformation contribute a major simplification by changing the time-varying differential equations to constant coefficient differential equations for constant rotor speed (steady state), which are much easier to solve than the original ones. Unfortunately, with m.m.f. harmonics included, finding a transformation that simplify the analysis or facilitate the solution of the machine equations is a difficult task.