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Metalworking (rec.crafts.metalworking) Discuss various aspects of working with metal, such as machining, welding, metal joining, screwing, casting, hardening/tempering, blacksmithing/forging, spinning and hammer work, sheet metal work. |
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#81
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VFD as 3Ph shop supply
In article ,
Ned Simmons wrote: On Sun, 22 Feb 2009 11:48:46 -0500, Joseph Gwinn wrote: In article , Jim Wilkins wrote: On Feb 11, 9:10*am, Joseph Gwinn wrote: Doesn't the back EMF of the idler motor contribute power to the motor being started? Yes, it does. This back EMF is precisely the motor acting as a generator. I've been thinking about the issue, and there are a number of datapoints to consider: VFDs will trip (complaining of overvoltage on the DC filter capacitor) if trying to stop a motor driving too much rotating mass. The classic solution is to provide a braking resistor to absorb the energy causing the overvoltage. The VFD manuals all say that the energy comes from the kinetic energy of that spinning mass. Right, and it occurs because at any point in time (while the VFD is decelerating the motor) the motor's speed is greater than its synchronous speed, as determined by the VFD's output frequency at that instant. Yes. The VFD provides the current to maintain the field the motor uses to be a generator. If one instead has a resistor across the motor and abruptly disconnects the power, leaving resistor and connected motor to their own devices, the field is maintained by regeneration, and too much load on the "generator" will cause the regeneration effect to collapse. Pentagrid has mentioned this regeneration and collapse in past postings, and I've also seen it discussed in textbooks. With a VFD driving multiple parallel motors, some of which are more heavily loaded than others, all these effects will play out simultaneously, and the net result will depend on the details. I have VFDs for my tools, so no RPC to play with. I may have to get a pair of three phase motors just to experiment upon. My local source of cheap motors vanished a few years ago, so it may be a while. Joe Gwinn |
#82
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VFD as 3Ph shop supply
In article ,
David Lesher wrote: On Sun, 22 Feb 2009 11:48:46 -0500, Joseph Gwinn VFDs will trip (complaining of overvoltage on the DC filter capacitor) if trying to stop a motor driving too much rotating mass. The classic solution is to provide a braking resistor to absorb the energy causing the overvoltage. The VFD manuals all say that the energy comes from the kinetic energy of that spinning mass. Do the VFD's switch in the resistor when braking; or leave it on all the time keeping the room warm? They all switch. Joe Gwinn |
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