On 1/26/15 3:29 PM, David Speck wrote:
Have you tried running the motor as a capacitor run motor?
On the net, there are circuits for running three phase motors off a
single phase with a motor run cap connected to the third winding. I
don't know for sure if the motor would run synchronously with this
arrangement or not, but it would be relatively simple to try. Many
amateur machinists hook up one beefy freewheeling motor in this way to
generate three phase power from single phase for metal working
equipment. I don't see why you couldn't just use the freewheeling motor
alone to drive your gap.
Called a "rotary phase converter"..
and as you described later, there's a motor start and a motor run
capacitor.
Will it run synchronous? That's an interesting question. Induction
motors need slip (difference in armature rotation speed vs magnetic
field rotation speed) to develop torque. That's because an induction
motor's armature/rotor is "energized" as if it were a transformer from
the stator field.
Big synchronous AC motors have a DC supply to power the armature, or
they can use a permanent magnet scheme (in smaller sizes); but in
general, I don't know that smaller sync motors (like the induction
motor with flats ground on the rotor) will work.
Dave's idea.. try it and see.
On 1/26/2015 12:07 PM, twoten wrote:
I'm trying to find a 120v 1200rpm synchronous motor to spin my 6
electrode rotating spark gap. I have a 1200 rpm synchronous motor but
it's 3-phase and 230volts. I have been looking into microcontroller
solutions and building my own 3-phase motor driver. I have a vfd that
can spin the motor at the proper rpm but the phase drifts slowly around
the clock as seen by my homemade 120hz led strobe light.. Existing
vfd's
don't have inputs for zero crossing signals and so can't be phase
locked
to the line. If anyone knows where I can find a motor or even if they
exist please let me know. Thanks in advance!
I'm sort of surprised it's not at the right speed. Most VFDs have a
crystal, and they'll be pretty darn close if you program it for a
given speed. Some of the new fancy ones read the back emf from an
induction motor and can run an induction motor at synchronous speed.
Maybe what you have is a induction motor (e.g. really 1150 RPM) and
when you run it at 60 Hz, you're seeing the 50 RPM (one rev/second)
slip... If lightly loaded, it might be running at something like 1170
or 1180, and that's would give you what you're seeing.
So program your VFD for 61 or 62 Hz (or 1220 or 1240 RPM) and see what
happens.
If you have an induction motor, you can either work out a table, or
you can do closed loop control if you have an analog speed input.
Look back in the archives.. maybe 10 years or so.
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