VFD for overload control

August 25, 2009                                                                                                                                                                                                 ISSUE # 214
                                                                                                    VFD FOR OVERLOAD CONTROL

We took a count, and VFD's have been mentioned in sixteen previous issues of Pressing News. This shows the importance of Variable Frequency Drives. Their cost has
followed the same pattern as PC's: each time you look at one, the price has come down.

Originally, VFD's were a replacement for a mechanical variable speed drive, like the old Reeves Drive. This had relatively few applications with Vincent screw presses.

We hit pay dirt as more and more applications were found where screw presses could be made to operate if they were driven by a VFD which is programmed in an auto-reversing pattern or step mode. Here the press is allowed to run forward until the screen becomes blinded. Then it automatically goes backwards for a few turns before resuming the forward motion. During the reverse cycle, the material in the press is used to wipe clear a blinded (covered over) screen.

Lately we find that VFD's are being used to replace an electrical starter or breaker. This is especially true when we get into large drive motors, 40 hp and up. Since the VFD is
programmed for a soft start, the motor sees a much less severe duty. In addition, a VFD offers superior protection against motor overload and power surges.

A very significant development is once again resulting in still increased use of VFD's with our screw presses. Improvements in screw design in the last two years have resulted
in presses which squeeze tighter than was previously possible. A consequence of this change is that during upset conditions these screw presses might jam and trip out on
overload.

A solution to this problem is to program the VFD so that motor load is monitored. When the load approaches something like 90% of the motor capacity, the VFD automatically
goes into reverse and the press runs backwards for a few turns of the screw. This almost always relieves the jamming condition. Normal operation resumes when the press
resumes its forward motion.

We have found this operating mode to be invaluable in certain manure and paper mill installations. The screw press can be left running un-attended, without fear of a shut-
down due to overload.