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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