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Using OSI Watt Transducers on Variable Frequency Drives

June 28, 2016

PC5 Watt Transducer

The line waveform of variable frequency drives, “VFD’s,” can be severely distorted by voltage harmonics created by the drive and can cause erratic operation of equipment connected to these lines. Special attention should be paid in selecting and installing transducers to monitor these variable frequency drives.

Ohio Semitronics, Inc. offers two models of watt transducers for monitoring VFDs, the PC5 series and the P series. The PC5 series should only be used on the line side ahead of the VFD while the P series can be used either in front of the drive or between the drive and the load.

Recommendations in Selecting and Using Transducers for VFDs

  1. The PC5 series as indicated should only be used on the line side in front of the VFD. Also, select or use only externally powered models such as the PC5-___B, D or X5. Internally powered models, options A, C or CX5 can create problems since the internal power supply is connected directly to the distorted line voltage.
  2. Another problem can exist if you are using the 1mA output, option B with the PC5. The VFD can cause noise on this output option. It is recommended to use option BX680 which is a grounded 0-1mA output to eliminate this problem.
  3. The P series transducer was designed to work on VFDs, although precautions still must be taken as indicated in section 4.
  4. Installation
    • Mount the transducer as far away from the VFD as possible, preferably out of the VFD enclosure and in its own steel enclosure.
    • Ground the transducer’s metal case to earth ground and use star washers with mounting screws when mounting.
    • Route all transducer wires away from the VFD lines and twist together the voltage lines.
    • The output signal leads should be as short as possible, using only a shielded twisted pair, with the shield connected at the monitor’s chassis ground only.

      Output from Watt Transducer
    • Use only transducers which have a separate input for instrument power. Connect the instrument power to a clean 115 Vac 60 Hz power source. Use an RFI line filter on the instrument power input if available.