Standard VFD installation to minimise EMC

Standard Induction Motor

  a. Determine the environmental requirements for the EMC levels.
        High sensitivity area will require installation to domestic levels.
        Standard environment in remote area will require industrial levels.

     
   b. The variable speed controller shall be connected to the supply in the manner described in the manufacturers installation guide for the domestic or industrial levels.
        Where an input filter is specified for compliance with EMC requirements, that filter shall be installed as close to the input of the VFD as possible with cabling and earthing as specified by the manufacturer.
        Where an output filter or ferrite ring is specified by the manufacturer for compliance with EMC requirements, such filter shall be installed.

        Note : Some Variable Frequency Drives require both input and output filters fitted in order to comply with  the industrial EMC levels. Other Variable Frequency Drives only require additional filtering to be added for compliance with domestic EMC levels.
        The length of cable on the output or the drive shall be less than the maximum length specified by the manufacturer.
        Note : The maximum length of screened cable may depend on the presence of input and/or output filters.



   c. The Variable Frequency Drive shall be connected to the motor via one or more screened cables.
        The screen is to be terminated to the VFD output earth or chassis via a screened gland or clamped by a copper saddle or clamping device as supplied/recommended by the VFD manufacturer.
        The other end of the screened cable shall be connected to the motor terminal box by a screened gland or similar clamping device.
        Where there is an output filter, output reactors, or output isolator in series with the output, such devices shall be mounted in the drive enclosure with the screen terminated after these devices, or they shal be mounted in a separate metal enclosure with the screen glanded or clamped to the enclosure at the input and also at the output of the enclosure.

     
    d.The motor shall be earthed to the output of the VFD and shall not be additionally earthed back to the supply. – There shall only be one source of earth connection to the motor.
        Where the screened cable is an EMC cable or a four core cable, the earth conductor shal be connected to the earth point in the VFD and to the earth point in the motor.
        Where a three core neutral screened cable is used, an additional flexible earth conductor of not less than half the cable core rating shall be run with the main cable to further reduce the earth impedance between the VFD and the motor.

     
  e.  Control cables connected to the VFD shall be screened with the screen bonded  to the VFD chassis.
        If the controls are mounted in a separate enclosure which does not have an independent earth connection, then the screen shall be bonded to the controls enclosure.
        If the controls are mounted in a separate enclosure which has an independent earth connection, then the screen shall not be bonded to the controls enclosure.