There has long been doubt as to what size of contactor to mount on the mains side of a frequency converter. Danfoss makes both frequency converters and contactors and therefore found it natural to investigate.

The problem arises when contactors for the mains side are dimensioned according to the motor's output or nominal load current, and not to the frequency converter's maximum input current, I _L.N.

Incorrect dimensioning can cause the contactors to weld as a consequence of overload.

An Industrial Note (Danfoss IN.42.T1.02) provides a complete chart showing which contactors to mount in front of the different frequency converters.