Transforming the Mechanical Characteristic of Induction Motors
Abstract
The article considers a method for transforming the rigid mechanical characteristic into a soft one for induction motors additionally fitted with a frequency converter, pulse-width modulator (PWM), and current sensor. In the PWM, the sensor output voltage proportional to the difference between the consumed current and no-load current is compared with the value of periodic sawtooth voltage to produce voltage impulses, which are supplied to the converter input. As the shaft torque and, accordingly, the consumed current increase, the mean voltage at the converter input, the motor power supply voltage, its frequency, and the shaft rotation frequency decrease. Joint operation of the PWM and current sensor is described, and requirements for their voltages are formulated. An expression for the transformed mechanical characteristic is obtained. An example of transforming the mechanical characteristic of an induction motor is given, and the characteristics of such motor are calculated. In comparison with a series-excited DC motor, the considered motor shows a smaller change of power in changing the load torque from 0.5 to 1.5 of its nominal value.
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