The advent of variable frequency drives (VFD's) in the last ten years has added tremendous flexibility to the machine operator. Being able to readily and conveniently change the operating speed of any electric motor, with a minimal capital investment, has a great advantage. Vincent routinely uses VFD's with screw presses, screw feeders, Fiber Filters, and pumps. A dozen VFD's, rated from 1 to 100 hp, have been accumulated for use with machines in our rental fleet.

While VFD's are thought of as drives for varying the speed of electric motors, they have a second important use. They can be set to alternate the direction of rotation of a motor. That is, most VFD's can be programmed so that they: 1. Ramp a motor up to full speed, 2. Allow it to operate at that speed for a given period of time, 3. Ramp the speed down to zero, 4. Immediately ramp the motor back up to full speed in the opposite direction, 5. Hold it running backwards for a short period of time, 6. Ramp back down to zero and, 7. Start the cycle over again. It is noteworthy that when you watch a machine operate in this mode, you never see it come to a full stop. The transition from forward to reverse is a continuous, smooth motion.

Dewatering of flows that want to blind the screen of a screw press can be made reliable using this technology. An example is seen at Foster Farms in California. Here they use Vincent KP-16 screw presses to dewater flows of chicken skins and water. The flow is pumped from the processing plant. The greasy material blinds the press in a few minutes, resulting in a big reduction of throughput capacity. By reversing the direction of rotation of the screw, the blinded screen is wiped clear, restoring the press to first class operation.

VFD's that drive the Foster Farms presses are programmed to run forward for 20 minutes and then reverse for 30 seconds, with the cycle repeating continuously. The result is a very satisfactory installation.