EMC and VFDs


VFDs in Cow Sheds (Dairy Parlors)

VFDs are used in Cow Sheds (Dairy Parlors)

VFDs are commonly used in cow sheds for a number of applications:

  1. Vacuum Pump. VFDs commmonly are used to control vacuum pumps to give a constant vacuum in the milk line. The VFD is installed with a vaccum transducer and the speed of the pump is regulated to keep the vacuum level at a preset level. This minimises the damage to the teat end that can be caused by excess vacuum. The use of a VFD makes the vacuum level independant of the number of clusters being used. Slowing the pump down reduces wasted energy, so the VFD saves energy as well as improving the animal health.

  2. Milk Pump. VFDs are becoming commonly used to control the milk pump with the speed adjusted to match the milk flow.

  3. Platform Drive. VFDs are used in Rotary Cow Sheds to control the rotational speed of the platform. Adjusting the speed of the platform enables all cows onn the platform to be milked within one rotation. If the rotation is too slow, the total milking time is extended. If the platform speed is too high, some/many of the cows will have to go round twice which also results in extended milking times.

  4. Water Pumps. VFDs are used to control water pumps, especially for washdown pumps tht are used on demand with variable flow. The sleep function of the VFD allows the pump to be stopped until a hose is turned ON, and by running in constant pressure mode with a pressure transducer, the speed of the pump is controlled to suit the required flow.


Stray Voltage in Cow Sheds (Dairy Parlors)

Stray voltages in cow sheds have been recongnised as a problem since initial research work done in New Zealand about 40 years ago. The research carried out was focused on electrical transients resulting from the switching of circuits and electric fences and did not involve VFDs as this technology was not a practical consideration in those days.
Little research has been conducted into the effects of the stray voltage caused by VFDs, but there are many actual installations where the VFDs have been proven to create a number of problems with the herd unless the VFDs are correctly installed. Typical problems are :

  1. Poor Milk Let Down.
  2. Increased Somatic Cell Count.
  3. Increased Mastitis incidence.
  4. Increased defacation while in the bales.
  5. Reluctance to enter the milking bales.

VFD Installation in Cow Sheds (Dairy Parlors)

Cows are very sensitive to stray voltages.
As a result, it is very important that VFDs installed in a cow shed must be installed to minimise the common mode conducted noise that can circulate through the shed and onto the platform.

  1. Use VFDs rated for "Domestic Levels" of conducted emissions.
  2. Install any EMC filters on the line side recommended for Domestic level emissions.
  3. Install any output ferrites (common mode filters) recommended for Domestic level emissions.
  4. Used high grade EMC cable between the VFD and the motor.
  5. Terminate the cable into the VFD using a suitable clamp around the screen to bond the screen to the VFD chassis. If there is no clamp, (small plastic VFDs), mount the VFD on a metal gear tray and clamp the cable screen to the gear tray as close to the VFD as possible. - DO NOT USE Pigtails!!
  6. Terminate the cable into the motor using a proper EMC screened cable gland. - DO NOT USE Pigtails!!

EMC Testing in the Cow Shed.

Use a wide bandwidth Oscilloscope with a 470 ohm terminating resistor across the input.
Measure the pk-pk voltage between the Rump Rail and the cow bail (on the platform).

  1. Measure the voltage with no VFDs running to get a base line measurement. If the background voltage is higher than 100mV, find out the source by turning things OFF.
  2. Repeat the test for each VFD by running that VFD on it's own and recording the pk-pk voltage.
  3. Measure the voltage with all VFDs running.
  4. Repeat this at multiple points around the platform, a minimum of three points, first, close to the entry point to the platform, second mid way around the platform and third, close to the exit point from the platform.
  5. If the voltage at any point around the platform exceeds 300mV, then alter the installation of the offending VFD to reduce this level. This may entail improving the screen bonding, and/or adding additional EMC filtering.