Abstract
Three phase induction motors are generally used as the marine electric motors and for various kinds of purposes, so the various kinds of motor characterstics are required. For that reason, it is very important that their starting characteristics are calculated with high accuracy not only in the application of induction motors but also in the design of induction motors.
The circle diagram methods for calculating the characteristics of induction motors are prescribed in JEC-37. The characteristics calculated by these methods are very accurate in the range from the no-load to the vicinity of the rated load. However, the results were believed to be completely inaccurate in the range of large slip.
These methods do not take into account the change of the circuit constants due to the magnetic saturation of the magnetic circuit caused by the load current. Accordingly, the calculated characteristics for the large slip, including the beginning of the operation of the motor, are quite different from actual ones.
Theoretically and experimentally it is found that the current and the power factor at the locked condition are varied depending on the locked voltage and the locus of locked impedance in this case can be approximated by a straight line, and the locus of the locked current phasor in that case shows a circle passing through the origin of coordinate.
Moreover, the loci of locked impedance phasor and load current phasor of special and deep-slot squirrel-cage induction motors are investigated.
By utilizing these results, the current diagram methods for obtaining the induction motor characteristics under the influence of the magnetic saturation by the load current are established.
A comparison is made between the results obtained by this method and the actual measurement. It is found that the proposed methods are more accurate in all slip range than the conventional method.
