Abstract: A regulating device for a polyphase electrical circuit, having a motor with windings corresponding to the main supply lines for the circuit. Leads are connected to each of the windings and contain a relatively low m.f.d. capacitor for suppressing spikes and similar transients in the respective supply lines, and a relatively high m.f.d. capacitor for functioning as a phase converter in the event the current in the respective supply line is interrupted. By-pass circuits are provided for each of the low m.f.d. capacitors corresponding to a respective supply line, containing relay means in the circuit responsive to the supply line to by-pass the low m.f.d. capacitor in the event the current is interrupted in the respective supply line, thereby permitting the high m.f.d. capacitor to function as part of a phase converter.