A Contactless DC Motor with a Soft Mechanical Characteristic
The considered motor is additionally fitted with a pulse-width modulator (PWM) and a winding current sensor. In the PWM, the sensor voltage proportional to the current is compared with a periodic sawtooth voltage, and rectangular impulses closing the switching device’s power transistors are generated. The higher the shaft torque and, hence, the consumed current, the longer the duration of these impulses, the smaller the average voltage across the winding, and the lower the motor shaft rotation frequency. Joint operation of the PWM and current sensor is described, and requirements for their voltages are formulated. Expressions for the converted (i.e., soft) mechanical characteristic, as well as for the characteristics necessary in designing motors and in converting the rigid mechanical characteristics of real motors into a soft characteristic are obtained. Examples illustrating the calculation of characteristics are given. The motor produces approximately the same small change of power in changing the load torque from 0.5 to 1.5 of the nominal torque as a series-wound DC motor, but has a design simpler than that of well-known contactless DC motors with a soft mechanical characteristic.