Inrush currents in the dc intermediate circuit of a motor controller necessitate
the incorporation of protection circuits. These circuits usually take the form
of a resistor inserted in series with the intermediate circuit, and the resistor
is short-circuited after completion of inrush by a switch positioned in parallel
with the resistor. To reduce cable-transmitted electrical noise an earthed noise-decoupling
capacitor is added to the intermediate circuit, but it also requires protection
against inrush currents. To minimise the number of components, a protection circuit
is described which combines noise filtration and inrush protection. According to
the invention, the inrush resistor has the form of a resistor network and is connected
to a switch which via a controller alters the resistance of the resistor network,
so that when the switch is open, the resistance to a differential current is greater
than the resistance to a common mode current, and when the switch is closed, there
is greater resistance to a common mode current than to the differential current.