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 In present-day frequency converter systems having a intermediate voltage circuit, in particular multi-shaft converter systems, system oscillations can form which are virtually undamped. This relates essentially to converters having an intermediate voltage circuit and a controlled feed in the form of a regulated mains-system-side converter, which is also referred to as an input converter.
 In principle, converters are used to operate electrical machines from a variable supply frequency. Such an intermediate circuit frequency converter makes it possible to operate an electrical motor, for example a three-phase motor such as a synchronous motor, no longer just directly from the mains system linked to an inflexible rotation speed, but by a mains system having frequency and amplitude which are both variable, and which are produced electronically, for supplying the motor.
 The two mains systems are the supply mains system whose amplitude and frequency are fixed, and the mains system used to supply the electrical machine with a variable amplitude and frequency. The two mains systems are decoupled via a DC voltage store or direct-current store in the form of what is referred to as an intermediate circuit. Such intermediate circuit converters essentially have three central assemblies:
 a mains-system-side input converter, which may be designed to be uncontrolled (for example diode bridges) or controlled, and in which case energy can be fed back into the mains system only when using a controlled input converter;
 an energy store in the intermediate circuit in the form of a capacitor for a voltage intermediate circuit or an inductor for a current intermediate circuit; and
 an output-side machine converter or inverter for supplying the motor, which generally converts the DC voltage of a voltage intermediate circuit to a three-phase voltage system via a three- phase bridge circuit having six active current devices, for example IGBT transistors, which can be turned off.
 Such a frequency converter system (converter system) is preferably used, inter alia, for main and servodrives in machine tools, robots and production machines since it has a very wide frequency and amplitude control range. Such a converter system is shown schematically in
 The converter system shown in
 By means of a regulated feed via the filter F and the mains input inductor L
 These system oscillations are stimulated in a particularly pronounced manner in the feed E. Depending on the control method chosen for the feed, two or three phases of the mains system N are in this case short-circuited, in order to pass current to the energy-storage inductor L
 Depending on the mains system voltage situation, the voltage is close to ground potential (approximately 50-60 V). Since the intermediate circuit capacitance C
 The resonant frequency f
 and where L
 This 