Analytical Determination of Mechanical Characteristics of Asynchronous Motors by Varying the Electric Current Frequency

摘要

The electric drives are still one of the fastest developing areas of the science at the present time. The asynchronous electric drives are most widely used in the industry and agriculture [1, 2] and they contain asynchronous motors of various types and powers. With the introduction of the static frequency converters the application areas of asynchronous motors broadened because by using them it is possible to achieve such a high electric drive rotation speeds which are unachievable for the direct-current motors. The rotation speed of the asynchronous motors at the present time is most often controlled by frequency variation. In this way a very wide speed regulation range, high uniformity of the regulation under changing loads, high speed stability, high efficiency of the electric drive operation and also the reversibility of the speed regulation in respect of the indicated speed are obtained. When only the frequency of the current f_1 is changed the speed of the rotation of the magnetic field of the asynchronous motor in the air gap changes in direct proportion, and its amplitude value Φ_m changes in the inverse proportion. In order to decrease the variation of the rotating magnetic flux when the frequency of the current is varied according to one or the other law the electric supply voltage of the asynchronous motor is also changed using the same frequency converter. The law according to which the supply voltage of the motor is to be changed is determined not only by the frequency of the current f_1, but also by the nature of the change of the static load of the motor shaft. In this way the sufficiently high compatibility of the mechanical characteristics of the working machine and asynchronous motor is achieved.