DC Bus Conditioners in VFD Drives
There are several issues involved in the design of the DC link supply of a variable-frequency drive (VFD).

In the most common VFD configurations, there is a DC link capacitor bank that limits the DC ripple voltage and supplies the reactive current required by an induction-motor load. The reactive current requirement generally determines the capacitor selection.

When power is initially applied to the VFD, the inrush current can damage the rectifier and the capacitors if it is not limited. There are a number of pre-charge schemes and other measures that are used to limit the charging current.

During steady-state operation, there is a pulse of current into the capacitors that occurs at the peak of each half-cycle of each phase of the supply voltage waveform. That is not a problem for the VFD components, but it represents harmonic distortion of the power current waveform. That can cause distortion of the power line voltage and have negative effects on power distribution components and other connected utilization equipment. It also results in a reduced total power factor. VFDs often have inductance added to the incoming AC lines or the DC link to reduce input current distortion.

There are two basic scenarios involving line voltage transients. High-voltage, short-duration voltage transients can damage the rectifier and possibly other components. Metal oxide varistors are often used to protect against that scenario. Line inductors are also useful in that regard. Lower-voltage repetitive or ringing voltage transients may have sufficient energy to charge the DC bus capacitors to a sufficiently high voltage to damage the inverter components. Input line inductors can also mitigate that problem. Sometimes, connecting a dynamic braking resistor across the bus can help. Ultimately, it may be necessary to eliminate the source of the problem. The source is often the switching of power factor correction capacitors somewhere in the power distribution system.

Of course motor power regeneration can also raise the bus voltage. Either dynamic braking resistors or deceleration rate limitation can be used in that situation. In some cases, braking energy may need to be returned to the supply using a regenerative input section.