Some tests of an exact practical theory of the induction motor

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Abstract
A polyphase induction-motor, fed with balanced supply-voltages, can be represented as a single-phase transformer having a variable load-resistance connected to the secondary terminals. The exact practical theory of the transformer that has previously been described1 can therefore be utilized to provide an exact practical theory of the induction motor; this is done in the paper.The method consists essentially in an application of Thévenin's theorem, and the theory is ranked as “practical” because the few parameters used can all be measured experimentally. A development beyond the corresponding transformer-theory is required for the induction motor, because the fact that the rotor can be turned with respect to the stator renders indeterminate the phase relationship between the primary voltage and the secondary open-circuit standstill voltage. A technique for measuring the correct phase-shift for the voltage ratio is described.The relationship between the open-circuit voltage-ratio and the locus of the stator current is demonstrated by tests on a wound-rotor induction-motor; these tests also illustrate the effect of saturation in the leakage-flux paths. Tests on a squirrel-cage induction-motor are also described. The voltage ratio of a squirrel-cage induction motor cannot be measured on open-circuit, but the phase shift of the voltage ratio can be inferred from the stator-current locus. Dynamometer tests are described which confirm the torque/speed characteristic calculated from the circle diagram. The influence of saturation of the main flux-path upon the characteristics is explained in the light of the theory.

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