Vacuum Regulation with a VFD Controller: Preliminary Tests and Modeling of the Vacuum System

Using a variable frequency drive (VFD) in order to drive the vacuum pump of a milking machine allows a dramatic reduction in energy use, while still producing equivalent vacuum stability. The VFD technology is able to adjust the rate of air removal from the milking system by changing the speed of the vacuum pump motor. A PID controller was developed in order to command the electric motor driving the vacuum pump. The PID controller used by the vacuum regulating system was tuned using the Ziegler-Nichols tuning rules for the frequency response method. In order to proceed to a more systematic approach a mathematical model of the vacuum system was developed, assuming that the system consists of a single air tank, provided with a vacuum pump port and an air-using port. In order to validate the model and study the system’s response to vacuum variation due to a pulse air leak the detachment (fall-off) of one teatcup was simulated; the teatcup was detached for 10, 20 and 30 seconds respectively. During the fall-off tests the rate of air flow into the system was measured by the means of a rotameter and the vacuum level was recorded. The experimental results were compared with the ones predicted by the model and it was concluded that the model accurately describes the response of the system.