Reversing VFD

August 29, 2005
Revised January 2017

The advent of variable frequency drives (VFD’s) 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 our screw presses, and over a dozen VFD's, rated from 1.5 to 100 hp, have been accumulated for use with our rental fleet.

Besides varying the speed of a screw press, the FWD/REV button is used to un-jam a press. An additional mode, auto-reversing, is less well known.

Commonly when testing difficult-to-dewater materials, the press starts out operating well but soon the screen becomes blinded. There are several possible remedies for this situation. A solution which is easy to test is to stop the press and reverse the direction of rotation of the screw. Sometimes when this is done the screen clears and press liquor once again starts coming through the screen. This suggests that the press can be fixed with the VFD set for auto-reversing.

Most VFD’s can be programmed for auto-reversing so that the motor runs forward for a while, then a short period in reverse, followed by resumption of forward motion. 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. 

Our first successful installation using auto-reversing was at Foster Farms in California. Here water jets were used to skin chickens. The resulting flow of somewhat emulsified fat, chicken skins and water was pumped to Vincent KP-16S screw presses for dewatering. The greasy material blinded 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 was wiped clear, restoring the press to full capacity. 

VFD’s at Foster Farms presses were programmed to run forward for 20 minutes and then reverse for 30 seconds, with the cycle repeating continuously. It was run that way, 24/7, for many years, without motor or gearbox problems. The result was a very satisfactory installation

Today a typical installation is with forward motion set for two minutes at a relatively low speed, like 15 to 20 Hertz. Next the operation is in reverse at maximum speed (120 Hz) so that the screw turns backwards three revolutions. In order to minimize time spent on the reversing cycle, ramp downs and ramping up in reverse are set for only one half second, and ramping back up in forward, one or two seconds. (If your electrician is reluctant to set such aggressive parameters in a VFD, a younger electrician is called for.)

Issue 164