TIG helmet

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Re: TIG helmet

Postby Peter Schmelcher » Mon Apr 17, 2017 3:38 am

A log splitter that goes bang, what's not to like.

Yesterday I tested the gear motor with the VFD.

The VFD output is square wave so no capacitor loads are allowed. The scope capture is 2 of the phases to ground and the math channel is the difference between the phases or what the motor winding has across it. I didn’t measure the electrical noise but wouldn’t expect it to be quiet. The VFD ramps up fast (adjustable), however, you can’t use relay contacts or switches to engage a stationary motor to the VFD output, motor inrush can be 10x current of full load. They have parameters for riding through short power bumps or waiting for a coast to stop before attempting a spin up.
TEK00003.JPG


Single phase motors usually have a capacitor, starter winding, and centrifugal switch to get the motor turning correctly and part way to synchronized speed. From my old memory I think all induction motors can be speed controlled the catch is provided they are turning in the correct direction. I will give it some more thought but probably not a fit.

The gear motor has 1/2 a degree of transmission backlash so fine for a welding table but not 4th axis machining.
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Re: TIG helmet

Postby Doug Coulter » Mon Apr 17, 2017 10:42 am

Yeah, EE Smith's "blinding flash and deafening report". (old SciFi) I just want my yard back and to get it done before nature overgrows stuff and makes it really hard. Not to mention...thermal security for next year.

I'm not sure the motor type in my lathe (240v) or the mill (120v), probably they have a capacitor in there (but in series with some winding) - both are reversible and ~ 5 Hp (Chinese rating). I know if you switch directions on the lathe while it's spinning at all, it just keeps going the same way.

Given the way I use either one, I've thought that if I get that round-tuit, I might adapt a DC treadmill motor to either one, which would make speed control a snap - those readily available boxes are killer-good. You can run a nominal 4k rpm motor at 100 rpm and have full torque and speed regulation vs load. The reduced available full load duty might not bother me, as it's rare that I use the full power or either one for any length of time.

It would take some machining to do...but hey. On the other hand, it might be like This Old Tony's cheap drill press refurb. An interesting detour (de force?) that wound up in nearly the same place anyway. I don't need to move belts that often. My shop is more to enable me to do things at all, vs a production or job shop, I'm usually not in a hurry. It's just a nice capability to have.

Seems like anything without some kind of feedback right at the "output" has backlash issues. My lathe, which I got new, has enough slop in the drives that threading fine threads takes some hand-pulling on things to force the backlash to always be in the same state at the start of thread, for example.
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
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Re: TIG helmet

Postby Peter Schmelcher » Tue Apr 18, 2017 4:34 am

A quick search turned up several youtube videos of single phase motor experiments using a VFD with varying success.
https://www.youtube.com/watch?v=NuZsyDMP9os

For giggles I tested a single phase motor with my VFD. I disconnected the starter capacitor and feed the third phase via a series 60W light bulb (a 1000W bulb would have been a better) to the capacitor winding. My thinking was that a capacitor shifts current 90 degrees so 120 should be close enough but no joy. Not a definitive experiment mostly to protect my new VFD.

My drill press and metal band saw would benefit from additional speed control. Both are good enough for most jobs and then when not I make do.
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Re: TIG helmet

Postby Peter Schmelcher » Thu Apr 20, 2017 5:07 pm

2nd VFD experiment was successful I guess. I removed the light bulb (grew a pair) and monitored the 3 currents with a clip on amp meter. The motor turned the speed was settable and the direction could be reversed. The phase currents were not excessive but not balanced (no surprise). At low speeds I could see the motor rotate from pole to pole and though cool, however with the wrong speed that would be a nasty mechanical resonance. The starter winding is made as inexpensive as possible given the short duty cycle. The starter current is very high for a second and then it is turned off. The wire should be a smaller diameter and have fewer turns. Reducing the starter winding current with an adjustable output transformer might help balance the rotary magnetic field. Another test I did was disconnected the starter winding after the motor was turning. If you intend to always use the motor above say 1/3 of the rated rpm I would actually consider this. This raises the last issue, the centrifugal starter switch. Mechanical parts of the switch drag and wear until my motor speed is above 90% of the normal rpm so the switch would get modified to open at a lower rpm. Some single phase motors have a running capacitor rather than a starting capacitor and are almost 3 phase motors in disguise so worth further investigation if you have one, I don’t.

Conclusion it can be done in a pinch but I would first consider replacing with a three phase motor.
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