This is a frequently asked question on every DC drive application the Applications Engineering Group (AEG) does:

What happens in the Drive?

For armature (DCS800) and field (DCF800) supply drives, when thyristor power converters operate, the line voltage is short-circuited during commutation from one thyristor to the next. This operation causes voltage dips in the main’s PCC (point of common coupling). For the connection of a power converter system to the mains, one of the following configurations can be applied:

Isolation Transformer
ABB Isolation Transformer

Configuration A

When using the power converter, a minimum of impedance is required to ensure proper performance of the snubber circuit. A line reactor can be used to meet this minimum impedance requirement. The value therefore must not drop below 1% uk (relative impedance voltage). It should not exceed 10% uk, due to considerable voltage drops at converter output.

Note: On new designs or systems installed without reactors originally (namely GE DC drives like Siltrol or Silcomatic, Baldor DC including Reliance designs), B&D uses a 5% line reactor as standard. This design complies with both A and B configurations.

Configuration B

If special requirements have to be met at the PCC (standards like EN 61 800-3, DC and AC drives at the same line, etc.), different criteria must be applied for selecting a line reactor. These requirements are often defined as a voltage dip in percent of the nominal supply voltage. The combined impedance of ZLine and ZL1 constitute the total series impedance of the installation. The ratio between the line impedance and the line reactor impedance determines the voltage dip at the connecting point. In such cases line chokes with an impedance around 4% are often used.

Configuration C

If an isolation transformer is used, it is possible to comply with certain connecting conditions per Configuration B without using an additional line reactor. The condition described in Configuration A will then likewise be satisfied, since the uk is >1 %


It is ABB and B&D Technologies’ (ABB AVP) position that an isolation transformer is not needed to comply with the technical requirements of the drive if using a reactor. However, reactors do not change the induced ground currents or noise from being passed back up stream to the primary source. This usually affects other systems such as PCs, UPS units and other drive systems, etc. If other sensitive systems where being affected, then isolation would be the only choice.

Last Words

The key takeaway from all of this is that a reactor or isolation transformer must be used with the DCS800 drives. Unlike AC drives, this is not an option for reliable operation.