On Tue, 15 Jul 2008 07:11:26 -0500, Ignoramus19502
wrote:

So, everyone is saying that it is bad to place a switch between a
motor and a VFD (if it is switched when running, of course).

I am sure that it is true, but I am not sure why. Also, is there
something that can be done to mitigate a switch between VFD and
motor being switched when running.

The question is not entirely idle in nature and pertains to my lathe.


I have a VFD that currently supplies the three motors on my surface grinder.
Within the year, it will be supplying the two motors on my HLV lathe, the
three motors on my mill and the motor on my small drill press. All switched
after the VFD. There is an idler motor to help damp out spikes and to avoid
the possibility of the VFD seeing a leading power factor load. Id copes fine
with this treatment. Having said that, it is rated at 23kVA and 650V and
running at no more than 7kVA and 300V.

Modern VFD's can cope with overload, they either limit the load or trip. They
can't cope with large voltage or current spikes. Current spikes are caused by
having a capacitive load (usually overly long cables) and can be eliminated by
fitting line reactors sold for the purpose. Voltage spikes can be caused by
switching off or reversing a load at the end of the long cables. Mitigate this
with an idler motor near the VFD.

Works for me and my VFD anyway.

Mark Rand
RTFM