I think that I've got a pretty good handle on the relative advantages of
static converters, rotary converters and VFDs for operating three phase motors
on
single phase current. With the VFDs coming down in price and their ability to
control motor speed and provide a bunch of other programmable features they
are pretty attractive. The VFD manufacturers warn of two potential problems,
though:

1. They recommend that the VFD be the last thing between the power supply
and the machine - no on/off or reversing switches between the VFD and the
motor.
They warn that use of such a switch may smoke the VFD.

Although it's probably more applicable to metalworking than woodworking
machines, there are times where I'd like to be able to instantly reverse a
motor,
or to bump it on and off.

I've heard that if you use a VFD that is well oversized for the motor in
question, you can get away with the on/off and reversing switches downstream
from
the VFD. Anyone have any experience with this?

2. The VFDs - or at least some of them - are reputed not to produce a smooth
output current, but rather one with harmonics or noise that can fry some
older motors. Almost all newer motors are supposed to have insulation that is
rated for inverter use.

How old does a motor have to be before it is cause for worry? How reliable
are older motors when used with VFDs? I'm talking about normal use, and not
using the VFD to run the motor at 30% or 300% of its rated speed.

Thanks,

John Martin

I posted this message a while back on the OWWM group and got one relevant
response. Responder said he had no problems with plug reversing or with old
motors used with VFDs. Anyone have any other experience?

John