1. A method of circulating heating fluid in a heating tube system by means of a circulating pump, characterised by the following features : a) the circulating pump is driven by an asynchronous machine at a speed dependent upon the heating load of the heating tube system ; b) the asynchronous machine is fed with a pulsed alternating voltage from an alternating voltage supply network by an intermediate voltage converter with a pulsed inverter circuit, the alternating voltage having a frequency proportional to the required speed and an amplitude that is proportional to the required speed, at least in the upper speed range ; c) the formation of a theoretical voltage having a binary waveform common to all outputs and firmly predetermined for all speeds, for each inverter output, which theoretical voltage has one binary state in the first half-cycle and the other binary state in the second half-cycle, with the exception of gaps in the vicinity of the fundamental oscillation zero passages wherein during each half-cycle the width of the central range which is free from gaps is substantially greater than the sum of the gap widths and the gaps are symmetrically arranged around the half-cycle centre, and d) the formation of a pulse-width modulated signal for switching the inverter circuit which operates towards the corresponding inverter output, this signal being formed from the respective theoretical voltage and high-frequency keying voltage of over 5 kHz, where the pulse width ratio corresponds to the required amplitude.
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