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# Can VFD force vector control be used to stop?

I have an interesting (yet frustrating) project I am trying to get to function accurately. My customer is a machine tool shop that refurbishes well casing for oil and water drilling. We installed a VFD driven system to move the pipes from area to area. The reason for the VFD is for the high torque start and stop and also the fact that not all the pipes are the same size and weight. The problem I am trying to overcome is when they are driven into the CNC lathes that clean up the threads on the end. Previously, they would pass a sensor that started a time out sequence to stop. Not only is this a violation of NFPA 79 code, but the heavier pipes would end up crashing into the back wall of the CNC lathe. I figured that it must be possible to use the force vector calc's in the VFD on start up to determine the stop point, thereby achieving the same stop point for any size or weight of pipe.

Currently, a timer is responsible for stopping. As such, the larger pipes have crashed into the far wall of the CNC lathes. I considered a resistor break, but I do not believe it will actually control to the final stop point. Although, it probably would prevent crashing the lathe. Total travel of the load is approx 5-7 feet linearly so it never needs to accelerate very fast. But if the force required for different masses could be seen by the VFD, it could use that as a reference to stop within a specified tolerance window. We have been lucky so far not having DC overvoltage. I know it's easy to do.

A: Using a so-called "db" resistor does not mean you have to brake to stop. All it does is absorb the energy when you change speeds. So you can go at some higher speed until you reach a certain point and then brake to a lower/creep speed to go home. You can probably do this all with logic built into the VFD; that is if you bought a good one. A two steps approach can work well. A first limit switch would change the speed to a low, fixed speed, say 5 or 10 Hz. Then a second limit switch would do a fixed deceleration from that known fixed speed ending up at the desired stopping point. This kind of logic is built into drives designed for motion control.

The VFD must be extending the ramp time with the heavier loads to prevent DC bus over voltage trips. There are probably programmable settings that can disable that protective function. But when you do disable that protective function there will be overvoltage trips. I'm sure it will need a dynamic braking setup to stop this. Work with the VFD manufacturer in order to get the correct braking setup.

VFDs , AC Drives , FAQ