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On 4/22/14, 10:51 AM, David Boyle wrote:
I have a chance to get a motor and vfd drive for a nice price for my
srsg. My only concern is would a 3 phase synchronous motor powered by a
variable frequency drive running at 1800rpm be in sync with the line
voltage?
There's a couple issues here (and I'm sure you'll get other responses
with other issues)
1) For a VFD driving an induction motor (the most common type), the
speed of the motor will NOT be the speed corresponding to the line
frequency being fed to it. There's always some "slip" between the motor
speed and the synchronous speed with the amount of slip depending on the
mechanical load on the motor. More load, more slip. Some clever
drives sense the phase of the current and can automatically adjust the
voltage and frequency to hold a (reasonably) constant shaft speed under
slowly varying load.
A VFD driving a synchronous motor, on the other hand, will work as you
expect. There's been plenty of material on this list about how to go
about modifying an induction motor to make a synchronous motor (at
substantially reduced torque output), and that would work with a VFD.
2) The bigger challenge is that most VFDs don't have a fixed
relationship between the line phase and the output phase (how could it,
if the frequency is "variable"...)
That said, if you had a way to measure the phasing of the spark gap
rotor and you have one of the VFDs with analog speed control input, you
could set up some sort of phase locked loop to set the phase whereever
you want. I would think that a rotary gap would actually be fairly easy
to stabilize the loop, since the mechanical load is pretty constant.
3) You could conceivably drive your HV transformer from the VFD as well
as driving the rotary gap motor. IN that case, the HV would be properly
phased relative to the rotor in a reasonably consistent way (depending
on motor design, etc.)