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VARIABLE-SPEED DRIVES

            10.16                      CHAPTER TEN

              The cable insulation problem must also be addressed. A 2000-V cable should be used
            (depending on the length of the connecting cable) for a 600-V IGBT inverter. This solution
            does not reduce the waveform rise time. Therefore, it does not address the motor bearing
            current or EMI radiation problems.


            CABLING DETAILS FOR AC DRIVES

            The cable connections from the inverter to the motor and ground are important details
            required to achieve a successful ac drive installation due to the fast switching rates. These
            details are applicable for the ac drives. However, they are mandatory for IGBT drives due
            to the higher switching rates. If this information is not properly followed, problems can
            occur in the motor and cable insulation, bearings, or EMI.
              The most important criterion for power cabling is the symmetry of the cable and
            grounding practices. The symmetry is required to ensure cancellation of any stray fluxes.
            This will minimize the bearing currents and EMI radiation. The components within the
            inverter should also be symmetric.



            CABLE DETAILS

            The continuous corrugated aluminum cable [NEC type MC (metal clad), Fig. 10.16] is best
            suited for PWM. It costs about 30 percent more than the “standard” variety with three con-
            ductors, one ground, and interlocked aluminum armor. Note the symmetry of the power and
            the ground conductors (only one random ground conductor is installed in most standard 60-


                                                         Ground conductor
                                                           Armor
                                                             PVC













                                                               Phase
                                                             conductor
                                                             PVC phase
                                                              conductor
                                                              insulation

                    FIGURE 10.16 Typical cable configuration used with VSDs.



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