the circuit or the motor winding is supplied via an impedance z with half the intermediate circuit voltage 0 5 u d with the same signal waveform as the voltage u zb current i b which causes no damage to the bearing wl is injected by means of a circuit iq according to the invention as illustrated in fig6 and breaks through the lubricating film on the bearing wl in this case the lubricating film forms the capacitance c b mentioned in conjunction with fig3 and 5 the current i hf produced by the current source iq therefore flows as a bearing current i b via the bearing wl that is to say the capacitance c b however proportionally also flows via the capacitance c rh between the machine rotor ro and the housing or stator st the current i b produces a short in the lubricating film and prevents charging of the capacitance c b this prevents bearing internal lubricating film discharge currents and the bearing voltage ratio becomes virtually zero the injected current i hf is preferably a radio frequency current at a frequency of more than 9 khz however this preferred frequency may vary with the composition of the lubricating film the current source iq not only feeds the current through the lubricating film of the bearing wl but also.