For supplying a load, such as motor for rail vehicles, with direct voltage pulses with a variable frequency from a direct voltage source, a pulse generating member is connected between the source and the load and delivers direct voltage pulses with a variable frequency to the load. The pulsing member can be switched for the pulsing in groups or symmetrical pulsing. When pulsing in gropus, it delivers pulses in groups of two pulses with a constant time interval T1 between the two pulses of each group. A control arrangement is provided for switching the pulse generating member to symmetrical pulsing when, in the case of pulsing in groups, the group frequency exceeds a value of substantially (K/2T1), where 0.5 is K 1. Immediately after switching to symmetric pulsing, the working frequency is brought to (K/T1).
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