Electric drives of the frequency converter-two-speed asynchronous motor system with switching structures

The paper considers issues of the enhancing immunity of the variable frequency electric drives (VFED) with two-speed asynchronous motors to power supply interruptions as well as those of improving their power efficiency. It proposes the concept of electric drives of the frequency converter-two-speed asynchronous motor system with switching structures. The distinctive feature of this approach is switching high and low speed windings in the process of variable frequency control on each of these windings. The electric drive with two frequency converters and winding supply from independent inputs and electric drive with one frequency converter featuring two DC links are considered. The paper provides results of experimental studies of VFED switching from the main to the spare input. Transient processes are recognized to be satisfactory. In terms of implementation of the proposed concept the method of variable frequency control of the draft units' performance has been developed; its distinctive feature is switching the frequency converter from the high-speed winding to the low-speed one (and vice versa) dependent on the unit performance. The variable frequency electric drive implementing this method is under consideration. The method of computing power efficiency of the introduction of VFED with two-speed motors has been developed. The researches have proven that power consumption may be reduced due to the increasing equivalent efficiency factor. The results of estimation of the technical and economic efficiency of the introduction are provided. For this purpose, effectiveness and consumed power are compared with similar exponents at the implementation of the known method at which variable frequency motor control is accomplished on the high-speed winding. The developed VFED is proven to be effectively introduced at forced draught fans of GOFHWB-100 boiler of the Magnitogorsk peaking boiler plant. The paper emphasizes that it is practically to use it at the draft units of the boilers with peaking loading.