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