Forever looking to save money, dairy farmers soon may be able to pocket up to half the energy cost of milking cows thanks to new technology developed by Cornell University agricultural engineers that provides energy- efficient ways to control vacuum levels on milking machines.

"Typically, the milking vacuum pump is oversized to accommodate any interrupted air flow, such as when milking units fall off a cow's udder," said David Ludington, Cornell Professor of Agricultural and Biological Engineering. Because those pumps generally operate at full speed and maximum capacity -- regardless of needs -- dairy farmers are forced to spend money on extra energy.

Agricultural engineers at the Cornell Agricultural Energy Program (CAEP), in conjunction with Jon Merrell of Merrell Farms, developed this new component for controlling the milking system vacuum using electronic feedback control and variable frequency drive technology. It is being tested and demonstrated on two dairy farms in New York (Autumn Ridge and Merrell Farms) and two dairy farms in Hawaii (Mountain View Dairy and Evergreen Hillside Dairy). The project was funded by New York State Electric & Gas Corp., Niagara Mohawk Power Corp., Rochester Gas & Electric Corp., the New York State Energy Research and Development Authority, Hawaiian Electric Co., Electric Power Research Institute and the Hawaiian Department of Agriculture.

Substantial savings for dairy farmers are possible because controls that use direct feedback from the vacuum milking system are linked to a variable frequency drive (VFD) on the pump motor. This type of drive, also referred to as an adjustable speed drive or a variable speed drive, must have proportional-integral (PI) capabilities, Ludington said.

In the new system, the capacity of the vacuum is set to match the actual air flow needs of the milking system. The conventional vacuum controller is set at a vacuum level slightly above the milking vacuum.

In this system, the voltage developed by the vacuum transducer, which varies according to the milking vacuum level, is transmitted to the VFD. During milking, if the vacuum level decreases from, for example, 14 inches of mercury to 13.8 inches due to more air entering the milking system, the voltage will decrease slightly. As the change in voltage is interpreted by the VFD, the speed of the vacuum pump is increased to compensate for the increase in air flow. Thus, the vacuum level is returned to 14 inches of mercury.

When the vacuum level increases, the pump motor speed is reduced, which allows the vacuum pump to match the actual air flow needs of the milking system.

At test farms, five vacuum pumps ranging in sizes from 5 to 20 horsepower are being controlled by VFD-PI technology. "The stability of the milking vacuum is equal to or better than that achieved by conventional controllers," Ludington said. Also, the vacuum pump noise is significantly reduced. "At the same time, the energy saved is at least 50 percent, sometimes as much as 70 percent. During milking the electrical demand is also reduced."

Ludington said that two of the farms are using VFD-PI control for both milking and washing, while the other two farms are using VFD-PI control for milking and a preset vacuum pump speed for washing.

The economics of the VFD-PI control system depend on several elements. "The two major factors are the hours the vacuum pump operates per day and the capacity of the vacuum pump compared to the actual air needed for milking," Ludington said. "The investment payback from energy savings alone is expected to be under four years with eight hours of operation a day. Operating this system for 18 hours a day, the payback could be within a year."