Variable Frequency Drives

Vacuum pumps on dairy farms are typically sized to have sufficient capacity for the largest possible anticipated demand. Even though this demand occurs less than 1% of the time, the pump always operates at full speed. This condition wastes energy and is noisy. Now a new vacuum control system, using state-of-the-art electro-technology sensors with feedback control to adjustable frequency drives, changes the pump capacity to match the need. The energy savings are 50-80%.

The application of a variable frequency drive to the milking vacuum pump system of dairy farms offers an excellent energy conservation opportunity.The vacuum pump provides the crucial balance between the vacuum and airflow, which causes the milking action, moves milk from the cow to the holding tank and also moves water and sanitizing agents through the system during cleaning. Almost all conventional milking systems are intentionally oversized to deal with the highest anticipated vacuum capacity, which occurs only when a milking unit accidentally drops off the cow causing air to be drawn into the system.

Since the vacuum pumps under a conventional system run at full capacity, extra air is drawn into the system through a regulator. The variable frequency drive matches pump speed to the actual air in the system, allowing efficiencies approaching 100 percent. The variable speed drive modulates the speed and energy consumption of the motor to match the actual vacuum needed.

The amount of energy savings a dairy farm will realize using the new vacuum pump control system varies, but in every test case, energy savings were more than 50 percent. One farm with 825 cows reduced its kWh/cow-year-milking from 41 to 17, resulting in an energy savings of 59,400 kWh/yr.

Savings range from 50 to 80 percent depending upon the degree to which the pump is oversized and the hours the pump operates per day. Payback periods can be less than one year or as long as four years. Cost for a control system ranges from $1,800 for a 5 hp (3.7 kW) up to $7,400 for a 30 hp (22.4 kW) unit.

Keep in mind...

Your electric company may provide incentives for the installation of variable frequency drives on vacuum pump motors and ventilation systems. CET staff will help you with the process as needed.

The Energy and Small Farm Sustainability program is made possible by a grant from the Northeast Sustainable Agriculture Research and Education Program.