VFD motor controllers

I have a lathe that runs faster than what I would like on the slowest speed. Two answers seem to be available. Replace the motor with 3-phase
motor and a VFD controller, or replace the motor with a DC motor and PWM controller.
The DC version, I think I understand electronically, although maybe not effective powerly (I've heard lower DC power motors effectively look like higher power AC motors -- ?).
The 3-phase VFD (Variable Frequency Drive) seems simple enough in principle, but I'm wondering what's in the best cost effective versions.
The one I want will take single phase 230V AC and convert it to 3-phase motor drive output that is variable in freq.
First question: what happens to the input? If it is single phase, I assume it goes through a full wave rectifier to get it as smooth as possible. Then what? Caps wouldn't help much at this kind of power I'd guess, and commercial units are small. Just ignore the bumps?
I think the drive to the three phases is a form of PWM, probably microprocessor generated, and done by IGBTs.
So, It's obvious I don't know a lot about this except my first-level assumptions. Can anyone provide a basic description of what is happening in these VFDs and what might make better or worser implementations?
Thanks. Hope it generates some interesting observations. I know this can get deep, but at a first level I'd like to hear the the basic theory about how the input power might be adapted and controlled.
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VFDs are generally more cost effective. If you really want the DC route, some good deals can be found at Surplus Center.
I've converted several machines to three phase/VFD and been really happy. If you have room in your machine, increase the motor horsepower a bit. First you never can have too much power, and motors only develop full power at top RPM. I think they are all constant torque. Horsepower is torque*RPM, so you have 1/2 the power at half speed.
As far as how they work internally, there's a little box that contains magic smoke tightly packed in there. You don't have to worry about anything, as long as you don't let the smoke out. <VBG> You want a 230 volt VFD and a 230 3 phase motor. Connect single phase 230 VAC to L1 and L2. Connect your motor to T1 T2 T3. You can use the VFD keyboard for for all functions but I find it far handier to connect switches for forward/stop/reverse and a pot for speed.
Here's the best place to buy your VFD http://www.driveswarehouse.com You probably want the L200 series drive.
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xray wrote:

bridge rectifier + big cap.
for a single-phase input, the crest factor will be fairly nasty. Better drives have a small (3-5%) line choke to increase the conduction angle.
A fairly hefty DC bus cap is used, typically sized for lifetime - it has to deal with the extremely high RMS input current, as well as the high-frequency output current.

almost exclusively digital PWM nowadays. it used to be done with 3 reference sinewaves, a triangular carrier and 3 comparators. And a whole bunch of other hardware, to deal with the ratshit waveform quality caused by very low switching frequencies.

2 types of drive: scalar (V/F) & vector control.
Scalar control maintains constant machine flux by keeping the ratio of output voltage to frequency constant. At half speed, the drive outputs half voltage (done by halving the PWM duty cycle).
Vout = Vrated*Fout/Frated
Vrated, Frated are nameplate voltage, frequency of machine.
at very low speeds, the stator IR drop becomes significant, so some form of "boost" is added - the output voltage is Vboost + Vrated*F/Frated
dynamic response isnt too great, but can be improved with speed feedback (shaft encoder).
Scalar control is based on a steady-state model of the AC machine, and as such ignores dynamic effects. For what you want, scalar control is just fine.
Vector control is based on a full-order dynamic model of the machine, and requires some form of speed feedback - shaft encoder or speed observer (so-called sensorless operation). Sensorless drives may or may not work well at zero speed (observers get tricky here, and all drives are not created equal), but with a shaft encoder, a vector drive can easily do shaft position control, and provide full torque at zero speed - its actually kinda neat to set the shaft speed to zero, and try and turn it by hand (for a small machine). A well-implemented vector controller will *not* allow the shaft to move.
Vector-controlled induction machines routinely replace DC drives nowadays, even in demanding applications like steel rolling mills.

HTH
Cheers Terry
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wrote:

Thanks, Terry. Exactly the sort of explanation I was looking for.
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wrote:

My father has been at me for years to make him a single phase to 3 phase VFD for his lathe. I have finally got around to it, just tweaking power supplies ATM (never use a voltage mode smps for industrial control) and writing the code.
The unit I have designed is based on a microchip design. Single phase rectified and filtered. Microcontroller using sine look-up tables to generate PWM. An IR IGBT module that takes care of all the high side switching stuff. My estimate is that it will cost under AU$100 for a hobbiest to make (no certifictions) providing that an aussie company will stock the IR modules. So far it is working quite well, cept the damn smps to drive the uP and other stuff.
I wish someone would design the switcher for me (hint hint), i just cant seem to get a smps to work reliably for me. It would be nice to have it working with a UC3842 with a non isoloated output of +5 and +15dc :))))) (hint hint again) :))) I will get there...
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Real Andy do you post to cnczone.com? They have a bunch of forums, and the electronics section is pretty active.
This is very interesting to me, but my skill level is about the magic smoke level.
Rod
The Real Andy wrote:

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xray writes:

A typical VFD is just a switching mode power supply feeding a programmable signal generator. The hard part is doing it reliably at high voltage and current.
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Richard J Kinch wrote:

Oh yes. the word "just" involves a whole lot of grief :)
We once ran some tests on a tiny little 1/2hp toshiba inverter. After slamming it from regen voltage limit into motoring current limit, it locked up at about twice rated current. for a short time :)
we then ripped it apart, and found the DC bus cap was about the same size as my little finger. what the ?! buried deep in the manual was a requirement to replace it every *year*
Cheers Terry
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wrote:

Makes one wonder WTF they were thinking (or not thinking.)
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On Tue, 27 Sep 2005 21:12:25 GMT, the renowned xray

VFDs, like industrial temperature controls, are an insanely cutthroat business.
Best regards, Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
snipped-for-privacy@interlog.com Info for manufacturers: http://www.trexon.com
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On Tue, 27 Sep 2005 17:34:14 -0400, Spehro Pefhany

Scary -- being a cost-consious buyer who is about to shop for one.
Always planned to buy one, but started this thread because I wanted to know what is in them.
If I get one, should I immediately crack it open and see if it should be repopulated with better parts?
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On Wed, 28 Sep 2005 03:14:00 GMT, the renowned xray

;-) I guess that's one approach. Or get a heavier duty one to begin with. Watch the derating factors for single-phase power input and temperature.
Best regards, Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
snipped-for-privacy@interlog.com Info for manufacturers: http://www.trexon.com
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Spehro Pefhany wrote:

yep. Yaskawa made more drives per month than we had done in 30 years....
Cheers Terry
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xray writes:

They were thinking, "must get edge of probability distribution just past the warranty period".
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Richard J Kinch wrote:

LOL. but yes.
Cheers Terry
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controllers', on Tue, 27 Sep 2005:

The PHB is not extinct in Japan. Someone presumably specified the outside dimensions of the product before it was designed and remained deaf to objections.
--
Regards, John Woodgate, OOO - Own Opinions Only.
If everything has been designed, a god designed evolution by natural selection.
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