Inrush Current of Induction Motor on Applying Electric Power

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The transient currents flow into the windings of the induction motors when electric sources are suddenly applied to the motors. These transient currents are called the inrush currents of the induction motors. In this paper the characteristics of the inrush currents are studied and the formulas to calculate these currents are shown in several forms. The formulas are led from the equivalent circuit of the induction motor of which circuit is assumed to be linear and the characteristics of the inrush currents are discussed according to these formulas. Sequentially it is obvious that the inrush current is composed of a steady alternating current component and two transient components. Both transient components are direct currents at starting of the motor and are sinusoidal alternating currents on running and in either case the transient components rapidly decrease with the time. The maximum value of each transient component takes place at a special speed of the motor, not at starting point, therefore the inrush current for the pole change type motor becomes large value on changing the poles from the low speed side to the high speed side. The basic formula presents the current flowing into the motor winding when the voltage is suddently applied to the induction motor on running at a given speed. To calculate the constants including in the basic formula are so complicated that the simplified formula is led in order to get the approximate value of the inrush current at starting of the motor. The simplified formula is applicable within error of about 10 per cents or less of the values obtained by the precise formula, and available easily by giving only the winding resistance and the leakage reactance of the motor. The peak value of the inrush current is affected by the phase angle of the applied voltage, and the peak value of the starting inrush current for the actual motors arise at about 0.5 cycle after applying the voltage. It is noted for the motors with the deep slot or double slot rotors that the rotor winding resistances and reactances change due to the motor speeds. The tendencies of these values are studied and the methods to calculate these values are indicated. Since results of the inrush currents calculated by the formulas are obtained according to the assumption that the magnetic circuits of the induction motors are not satulated, the actual inrush currents are expected to be larger than the results obtained by formulas.
公益社団法人日本マリンエンジニアリング学会の論文

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糸井宇生
川崎重工業 (株)

Inrush Current of Induction Motor on Applying Electric Power

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