Another VFD question

Hi
I have a Woods 7.5 Hp 220V 3ph input VFD. In my a shop I have several machines some 220V 3ph others 575V 3ph. At the moment I run them with a
rotary phase convertor linked to a 3ph 220V to 575V transformer.
I am interested in using the VFD to power all my machines (I only use one at a time) by feeding it with single phase 240V and running the output through a 3ph transformer and then to all the 575V machines. The 220V 3ph machines would be wired directly to the output of the VFD.
The largest motor I have to run is a 5hp 575V motor on my lathe that has a clutch.
Is this setup feasible? Can I switch off the three phase while without switching off the VFD first? Would having a small 220V 3ph idler motor always attached to the vfd output aleviate any problems with switching off the working machines? Has anyone else tried this?
If this would work I wouldn't have to listen to the phase convertor all the time.
Stan
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ATP 2 things: 1) Ask the manufacturer of the VFD about this. 2) If the rotary converter noise is objectionable, place it away (outside?) from the immediate shop vicinity.
I know from nothing about VFDs but I would not think the load of a smaller idler motor would have any adverse effect. Did you mean to say your idler motor was "a small 220V 3ph

Bob Swinney

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Correct. And you could turn off the rotary converter as well.
Bob Swinney

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All the VFD manufacturers I have asked say not to disconnect the load from the VFD. For example, one told me you can do that exactly ONCE without blowing the semiconductors. Going from a large motor plus small motor to small motor only is the same kind of stress, just a smaller magnitude. The TB Woods drive also has an output phase loss trip function that would probably cause trouble.
You can download manuals or contact the factory at http://www.tbwoods.com
The output of the VFD is a chopped waveform that is probably not going to be good for the transformer, which is built to handle a single frequency sine wave.

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Charles sez: "The output of the VFD is a chopped waveform that is probably not going to be good for the transformer, which is built to handle a single frequency sine wave."
Not quite so. It can certainly be agreed that a VFD can be harmed by operating it into zero load - like any transmitter. But a transformer stands no more chance of being harmed by the VFDs output than does an induction motor, which BTW looks almost exactly like a tranaformer to the VFD.
Bob (gimme a rotary anytime) Swinney

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The chopped waveform contains harmonics (odd order), which can increase heating of a transformer's core, but as Bob says, no more so than the heating it causes in the motor. The difference is that the motor has cooling air flowing through it, the transformer doesn't. So a marginally sized transformer will get awfully hot.
Again as Bob said, the real hazard to the VFD is suddenly switching off a large load it is driving. The current pulse in the wiring at the moment of interruption will "ring", possibly generating a very high voltage transient which may pop the output transistors of the VFD, depending on the particular operating frequency, and the inductance of the wiring.
OTOH, just operating the VFD into an open circuit is usually not harmful. Unlike a conventional matched impedance transmitter, it is designed to be a low impedance source driving a higher impedance load, and it is operating Class D (switch mode). Negligible current flows when the output is unloaded, just enough to charge the output wiring capacitance, so there isn't a large inductive kick when it switches.
Note that driving a transformer with an unloaded secondary changes things. There will be an inductive kick as the magnetization current of the transformer reverses. If I were contemplating operating that way, I'd want to strap suitably rated MOVs across the transformer primaries to snub any inductive spikes, or always maintain some rotating load on the transformer secondary to serve the same purpose.
Gary
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Hi
Thanks for the input. What are MOVs and how do I size them?
stan
Gary Coffman wrote:

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MOV, Metal Oxide Varistor. It is a component which has a very high resistance until the voltage exceeds a certain threshold, then it becomes highly conductive. Its purpose is to snub voltage transients. A MOV is rated for the voltage threshold, which must be higher than normal *peak* operating voltage, and for joule (watt-second) capacity to absorb surge.
For your purposes, a voltage threshold of 340 to 370 volts would be about right.
The stronger the expected surge, the larger joule rating the MOV requires. If you choose too small a joule rating, the MOV will simply explode when it encounters a transient. In your case, you'll need a fairly high joule rating because it'll have to conduct for up to a half cycle at the ringing frequency. I'm guessing something in the 40 to 100 joule range, though the latter might be a little high.
Gary
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Thanks fo the tutorial, I have never heard of these.
stan
Gary Coffman wrote:

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    Hmm ... I see problems here -- assuming that you want to use the variable speed functions of the VFD. The primary problem is that the transformer is sized for 60 Hz (or in the UK, probably for 50 Hz.) The size of the wires determine the current capability, but the size of the iron determines how low a frequency it will accept. As far as I know, nobody makes general use transformers rated down to 20 Hz or lower (probably 20 Hz is the lowest you want to run unless you add an external booster fan for keeping the motor cool while its own fan is running a lot slower than designed.)
    Running a transformer at too low a frequency for the iron core's design causes it to draw excessive current, possibly damaging either the VFD or the transformer (whichever is weaker).
    And remember that the VFD starts a motor out by producing something like 0.5 Hz, and sweeping it up to running frequency, so even if you don't want to run it at lower frequencies, it *will* do so for a few seconds whenever starting a motor.
    So -- you *really* don't want the transformer after the VFD.

    Aside from the problems with the transformer (which could be solved by getting a second VFD for the higher voltage devices, and running *that* from a transformer), most VFDs warn you to not switch the lines between the VFD and the load (motor). It is far better to remote the buttons on the VFD to control it with the machine's own switch. (This is what I'm planning to do when I shovel a three-phase motor into my 12x24" Clausing.) This solves the problem of instinct having you reaching for the drum switch or buttons on the machine by *using* those to control the VFD, which then controls the machine.

    I don't know. I can tell you that I'm currently running a 1HP milling machine from a 7.5 HP VFD and using the machine's drum switch to interrupt the power to the motor -- but I'm doing this because the machine is too far away from the VFD to properly remote the switching, and because the VFD is so greatly over required capability for that machine. If I were running a machine with a 6 HP motor or so, I would not be switching it's power from the VFD as I am with the little machine.

    Your proposed idler motor would still be making noise -- and I'm not at all sure that it would protect your VFD from the switching of the heavier machines. (You don't say what the draw of the larger machines happens to be.) I would say that you want, at a minimum, two VFDs -- one for the higher voltage machines, and another for the lower voltage machines. Are you going to use variable speed on all of the machines? If so, I would suggest a VFD per machine -- that is my eventual target. I have all the VFDs -- all I need is the time. (And I'm not even dealing with any high-voltage machines.
    Good Luck,         DoN.
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