Modern Automation Technologies Professional Magazine


НаименованиеThe effectiveness of implementing a system with variable-frequency drives


Among Russian cities, St. Petersburg is the recognized leader in using systems with variable-frequency drives in public utilities. Siemens Co. has accomplished several tens of heat-and-power engineering projects in this city. A relatively high cost of semiconductor transducers used to control the rotation frequency of asynchronous motors necessitates a careful estimation of the general economic effectiveness and payback time. Mere usage of the transducers is of little value. The ultimate effectiveness is provided by applying advanced circuit design, implementing energy-saving control algorithms, and utilizing available facilities (specifically, local-area networks) in full measure.

A variable-frequency system controlling the capacity of pumps mounted in one of boiler houses in St. Petersburg is built around a Siemens 160-kW transducer and a programmable controller. The controller implements a desired algorithm controlling the operation of three 100-kW pumps according to the season (winter or summer). Specifically, it selects an appropriate pump and switches it on. Also, the controller generates emergency and warning messages if the process and drive parameters become abnormal. The system was paid for less than a year.

More than ten similar systems have been introduced into boiler houses of St. Petersburg. Their payback time varies from 0.8 to 2.1 year. The simplest of them use LOGO microcontrollers and are fairly cheap. More sophisticated systems has two transducers controlling the frequency of, e.g., a ventilator or an exhaust fan; programmable controller; and auxiliary hardware. The drive parameters are read out with the RS-485 network, and the controller is linked to the transducers via the PROFIBUS-DP protocol (note that the controller may be switched to a higher level industrial network).

Systems configured with advanced variable-frequency drives and controllers offer a high economic effectiveness and a short payback time. This is achieved by reducing the energy consumption due to controlled operation, cutting operating costs, and increasing the service time of equipment owing to more distributed control of process parameters.

Variable-frequency systems
are easy and convenient to handle

System using a Siemens 160-kW transducer

By Mikhail Kozlov, Aleksandr Chistyakov

Mikhail Kozlov,  Aleksandr Chistyakov