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Reducing Vfd Stray Voltage Problems In Dairy Sheds (Parlours)


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VFD induced stray voltages can cause major issues in cow shed (dairy palours).

Typical symptoms are :

  • Interference with ear tag readers
  • High Somatic Cell Count in the milk
  • Agitated Cows - reluctance to enter the milking bales
  • Extended milking times due to poor milk let down
  • Excessive defecation while in the bails
  • High Mastitis incidence.


There are some basic fundimental issues when using VFDs in dairy sheds that must be adhered to when using PWM VFDs :


  1. The connection between the VFD output and the motor MUST be using a screened cable with the screen continuous the whole way.
  2. The screen must be terminated in the output of the VFD by a circular clamp around the screen to the chassis of the VFD, or, if there are conatcors on the output of the VFD, the screen must be bonded to the metal gear tray the the VFD is mounted on by a circular clamp or saddle clamp.
  3. If there is an isolator on the output of the VFD, (typically mounted near the motor), the screen of the cable going to the VFD and the screen of the cable going to the motor must be bonded together by a circular clamp(s). This can be via a flat metal plate, or bonded together.
  4. The screen of the cable must be bonded to the motor. This can be by means of an EMC gland into the motor terminal box (assuming metal box) or via a clamp onto the frame of the motor.
  5. The EMC filter in the VFD must be enabled, or an external EMC must be fitted.

Where these steps have been correctly taken, and a problem still exists, there are a number of steps that can be taken, but there must be a continuous screen path between the VFD chassis and the motor for these additional steps to work properly.

Potential improvements :

  1. Add low frequency common mode chokes to the output cores going to the motor. This usually entails one or more turns of all the three phase conductors through one or more common cores. (NOTE: output conductors only, not the earth conductor). High inductance EMC ferrite cores can be used but the best results are from specialised cores designed for this purpose. - consult your VFD supplier.
  2. External EMC filter on the input.
  3. dv/dt filter on the output of the VFD
  4. Sinewave Filter on the output of the VFD. (use sinewave or dv/dt, but not both.)


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