On Thu, 14 Apr 2005 18:20:28 -0700, Eric R Snow
wrote:

I've asked one person who should know, and Automation Direct, and
haven't gotten the answer I want. What I wanna do is run a single
phase motor, without a centrifigul starting switch, with a VFD. I'm
told it can't be done. You can't even hook up 3 single phase motors
to one VFD. Oh well. But is there some way to fool a VFD into thinking
it's controlliong a three phase motor so the speed can be varied on a
single phase motor?
ERS

In principle, once a single phase motor has been
electrically or mechnically run up to its normal operating
speed, it can thereafter be speed controlled by a VFD in
exactly the same way as a 3 phase machine. However the
practical difficulties are pretty horrendous.

Firstly the VFD needs to be rated for at least 3x the
motor power AND of a type that is not discommoded by a 100%
unbalanced load. Although the semiconductors are not
necessarily overloaded by this condition, the control
circuits may interpret this as a load fault and shut down
the VFD.

The second problem is starting - the usual single
phase starting arrangements are optimised for operation at
normal power supply frequencies and will fail miserably at
widely different VFD frequencies. +/_ 10% is probably
possible but much outside this gets pretty doubtful. You'll
probably be forced to start the motor at normal supply
frequency and only then adjust to the required operating
speed.

Jim