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The capacitor will shift by almost 90 degrees but I need a shift of 120.
The motor only pulls 2.5 amps per leg at 230volts. So what if I build 2
precision phase shifters from op amps and then drove the motor through
some beefy analog devices like igfets? Sure they would work hard
throughout the cycle and I would have to heat sink them but if I got
some oversized bruisers would they not accept their hard life in a stoic
fashion?
On 01/26/15 20:29, Jim Lux wrote:
> 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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