REGULATION OF THE POWER FACTOR FOR THE ASYNCHRONOUS TIRISTIVE ELECTRIC DRIVE

Abstract

It is known that when an asynchronous motor operates with a constant frequency of the supply voltage, the losses in it in the steady state are a function of the moment of loading and the magnetizing current. Since the dependence of the losses due to the magnetizing current on the load torque is extreme, for each value of the load moment the engine losses can be minimized by changing the magnetizing current, for example, using thyristor voltage converters included in the stator circuit of the induction motor. Solving the task of optimization as multi-criteria, it is necessary to focus on control algorithms providing the value of the power factor of an induction motor close to the nominal one, or the minimum stator current. In this case, the efficiency of the engine also assumes values close to the nominal.
The purpose of the work is the development of a system for stabilizing the power factor of an induction motor, realized on the basis of a thyristor voltage converter.
The above results allow us to draw the following conclusions. It is true that in an asynchronous electric drive the equivalent phase angle of the load is adequate to the power factor. If the equivalent phase load angle is stabilized at a level equal to the nominal value, a practically constant and load-independent power factor is provided for the given motor with a change in the load moment in the range 0.1-0.8. It should be noted that the shape of the stator current, different from the sinusoidal, arising when voltage is regulated by a thyristor voltage converter, leads to additional losses in the asynchronous motor from the higher harmonics. This phenomenon does not allow to obtain a power factor equal to the nominal value without decreasing the efficiency in solving the optimization problem.
One of the ways to stabilize the power factor of an asynchronous electric motor working with a variable load is considered. The voltage at the motor terminals is regulated as an equivalent phase load angle characterizing the ratio of the active and inductive parameters of its replacement circuit. An algorithm for calculating this angle from the value of the current lag angle and the thyristor control angle is presented. The microprocessor control system of an asynchronous motor is considered, realizing the calculation and stabilization at a given level of the equivalent phase angle of the engine load when the load on its shaft changes. The results of experimental studies are presented.