Common mode interference coupled via a buried armored cable

In an industrial installation a single combination of a three-phase motor and switched mode variable frequency drive is selected. The driver and motor were connected by an 85 m long armored cable buried in wet soil. The interference generating currents produced by the drive were measured in time domain. The transmission line parameters of the cable were determined separately in the laboratory. Its transfer impedance is remarkably low in view of the open structure. The external circuit was an L.shaped aluminum proffle in the laboratory. The actual installation has many cables in parallel, which also form a part of the external circuit. Preliminary calculations show that cable common mode current can be ‘a posteriori’predicted quite well with a reasonable guess for the effective soil parameter.