On 2008-10-06, Pete C. wrote:

David Lesher wrote:

[ ... ]

Get a pushbutton, a big industrial one. Allen Bradley comes to mind. They
are basically Legos; you start with the basic button and add contacts of
the type needed.

It should have 3 contacts rated for the full current of the motor. They
will be closed in OUT mode, but open when pushed in. Wire in series
with all three legs going to the motor..

Have a low current 4th contact going to the VFD shutdown connection.

This will break the power feed to your mill when hit, AND send a shutdown
to the VFD. [It may take a while to turn off, however, as compared to the
button. Maybe even as much a second or two; see what the VFD Fine Manual
says.]

This way, even if the shutdown command does not work, the power *is* off.

The shutdown pushbutton will be expensive new. There are millions surplus
but it will be tricky to mail-order same. If you have a surplus place
nearby....

The E-stop switch belongs on the power feed *to* the VFD, *not* on the
motor leads from the VFD, otherwise you just have a "VFD destruct"
button. The diagrams in the VFD manual should show the proper wiring for
an emergency stop.

Except that the VFD has a set of big capacitors in there, which
could keep it running for a second or so after the power line is cut.

As long as this is only to be used in a true emergency, what I
would do is set it up to drop power to the VFD (two NC contacts), and to
*short* the power from the VFD to the motor (three NO contacts) so not
only will the power be removed from the motor, but you will also have
regenerative braking.

This is likely to not damage the VFD -- because they have
circuitry to sense excess current and to shut themselves down under
those conditions -- but it is not *certain* that it would survive this.
More likely than with the high voltage spikes from opening the circuit
to the motor windings which can fry the output transistors.

Remember that it is truly an *emergency* stop button only. When
life and limb are in danger, is the time to hit this -- not just when
you want to walk away from the machine for a while.

Enjoy,
DoN.

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