Frequency converters for multiple soft start of synchronous motors of voltage 6, 10 kV and power 0,63…12,5 MW are designed for sequential frequency-controlled start of electric drives of heavy-duty machines with high moments of inertia and constant static moment: drum mills, crushers, conveyors, tube furnaces, sinter exhausters, large-sized smoke suckers, fans and others.
The following converter circuit configuration is used: controlled rectifier — current inverter controlled by motor. It consists of a controlled rectifier with input current-limiting reactors, smoothing inductor and bridge inverter with natural thyristor commutation. During frequency-controlled start a linear and smooth motor speed increase is achieved accompanied by current limitation to the level of not more than 1,2 times rated value. When the rated speed value is reached a special phase adjuster synchronizes the mains and the motor voltages and triggers a bypass contactor.