Electric motor on KBC mill

Buy a VFD to run the mill. Three phase works very well. Many VFDs can be driven from 220 volt single phase, with a derating.

Joe Gwinn

Correct. I've never heard of a 1 or 3 phase motor; it would be ...interesting....

Choices:

a) New motor

b) VFD or similar

c) There's a hack solution for running 3Ph motors on single phase, at reduced power and way reduced starting torque.

Hey Vernon,

Comments on Rotary Phase Converter (RPC). Is there anything "better"? Yes. Anything cheaper? Probably no. Anything easier? Probably yes, but not a whole lot. Anything that even a less than fully equipped shop can not do? No, it can be REALLY simple. Smallest foot-print? No, by far the biggest, but it can be at some distance, and easily "tucked out of the way". Anything sophisticated? Not necessarily. Easy to find parts locally? Probably, but depends on where you are located and considering "locally" to be within an hours drive.

Probably a bunch more questions too, but my "first" RPC was just another 3Phase motor the same size as the one I wanted to drive, and coupled directly to a 1/2 HP 1750RPM 120volt single phase from a furnace fan that I had laying around. The direct coupling in my case meant V-belt and pulleys. There are certainly a few more sophisticated options, but this method does away with all the "electronics". The smaller motor is first turned on and allowed to get the larger 3phase up to speed, and then the 220 is applied to the RPC generator motor, and voila...3 phase is available. Kinda crappy

3phase, but good enough to drive your mill. To me, one, if not the only, non-replaceable feature of 3phase is the instant reversibility of the mill or lathe spindle.

I note (after I started to type this) that another poster replied with a webpage for an RPC circuit. That's the "sophisticated" stuff, and has to do with self-starting, versus my suggestion of "manual starting". Nothing wrong with that at all if you have the skill/talent and time and inclination. It IS what I have now of course, but I first wanted to see mine run!

Take care.

Brian Laws>At an internet auction a few days ago I bought a KBC bench mill. The >model is Grip-16.

To everybody who answered. Thanks! Upon reflection a rotary phase converter will suit us to a tee. I have already started googling and quickly found some "how to" instructions. About a year ago, when I went to pick up an auction item I bought, the auctioneer gave me a huge plenum box that sat atop a multi-story commercial building. It was the cooler / blower for the entire building! It is a large, well insulated box, containing a large blower driven by a 3 phase motor. I don't know the horse power of the motor. But I'm guessing about 15 HP.

I also don't know the voltage. But I will find out tomorrow when we go to our farm. For purposes of discussion and assuming that it IS a

15 HP, 240V 3 phase motor, would this be overkill as an idler motor? I actually have a couple of other 3 phase tools I've had forever that I haven't been able to use for the same reason. One of these is a very large band saw!

It sounds like with a properly designed and built unit I can power all of this stuff simultaneously. These are: a 48" bandsaw, a Delta Unisaw, and the newly acquired mill. The band saw probably has the biggest motor. But I'm pretty sure all of these motors are smaller than the motor in the air conditioner blower box.

Tomorrow I will get the technical specs off the motor data plate.

Thanks to all.

Vernon

Vernon,

Kudos on your enthusiasm for the machine arts. While you're gathering information on phase converters check into Metalweb News. FWIW, a 15 HP 3-phase motor would make an excellent rotary phase converter. The only consideration might be the amount of power consumption while running idle. That is typically about 1/6 or so of full-load rated HP. Suggest you consult an electrician for wire and breaker sizes.

The pictures neglected to show a view of the mill that is clearly stamped "3 phase". I thought the motor was single phase because the auction listed it as "230v". Until today, apparently in error, I thought that 3 phase motors were 208V.

However, the KBC tools web site for this mill almost leads me to believe that the motor can be wired for single or 3 phase.

Will somebody help me out here? A motor is either single or 3 phase but can't be changed. Yes?

I already understand that a 3 phase machine can be operated from single phase with a phase converter. And I may have to go in that direction unless I can get a single phase replacement motor.

This mill was built in 1989. Since even when new it wasn't a high end precision machine would it be prudent to check the accuracy of the X Y table? As we get deeper into machining I'm sure I will join the quest for split hair precision. But for now, we just wanna play.

On Monday I will call KBC and look into my options. But meanwhile, any advice on how to logically approach the aftermath of a possibly illogical purchase will be appreciated.

Regards and thanks,

Vernon

Depending on the building size it could be anywhere from 10HP to

50HP - and anything over 15 would most likely be 480V or 240/480V. Unless you are talking Canada, then it could be 575V.

Be careful to provide cooling if needed - some of those motors are rated "Air Over" on the nameplate, and have to have external forced air cooling flow. The motor mounted inside the fan plenum would have cooled itself, but not if you reuse it as a rotary converter - you will need a fan.

Even if you get a small propeller style fan for a condensing unit and put it on the output shaft - they make "Universal" blades and hubs that mix and match, because you will have a Big Shaft and only need a little blade.

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Ouch! To run that from a 240 V line to full horsepower, you would need 46.88 Amps single phase -- not counting the surge needed to start it. At 480 V it would draw half the current -- but you are not likely to have 480 V around the shop if you don't have three phase -- both would be considered "industrial" power levels.

And it would draw quite a bit more than that to spin it up at first -- unless you used a "pony" motor to get it up to something like half speed or so before you switched power to it.

If you have a 50 A 240 V breaker in your box, and an appropriate outlet for that (or direct wiring to the motor and electronics to make it auto-start) you will want to take the time to "tune" this idler -- first adjusting capacitance between the generated phase and the two line feeds, and once that is balanced, then another set of capacitors across the incoming line to tune for minimum current unloaded. This will at least keep the reactive current from cooking off the breaker every so often.

Even if it is dual voltage, it is probably wired for 480 V to keep the current down.

In that case, I would suggest picking up a three phase breaker box, and providing an individual breaker for each machine tool.

That would help greatly -- but only as long as the idler motor is small enough so that you *can* start it on the power available from your breaker box.

Good Luck, DoN.

ing that it IS a

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Joseph,

I'm still trying to wrap my electrically challenged brain around the differences / advantages / disadvantages between rotary converters and VFDs. As soon as I'm able to articulate an intelligent question I'll ask one!

V

A VFD is a big magic solid state box that delivers AC of from {say} 10Hz to 100Hz. It USUALLY will be one able to deliver 3 phase power. [But there may well be smaller single phase ones, so cabbage emperor...]

How it does that.... Well think of it as an audio oscillator and a BIG amplifier for the speakers err motor.

Its major plus is the variable frequency means whatever motor it feeds will be variable speed. It's almost a waste to buy one just for a single speed task, but a friend did to drive an air compressor.

A rotary converter is a semi-kludgey way of getting 3 phase from a single speed source, at the line freq. [60 Hz]. Ditto the capacitor-based schemes. They may work well enough for your case. But neither one will deliver the large starting torque a true 3ph supply will offer.

Does this help?

Hi David,

It does help. But I don't understand this part:

"They may work well enough for your case. But neither one will deliver the large starting torque a true 3ph supply will offer."

Are you saying that a VFD is "a true 3ph supply..."? Or are you saying that "nothing's as good as having 3 phase and not NEEDING a box"?

V

A 3ph VFD will deliver real, balanced, 3-phase. The other two hacks do not; you may or may not be able to live with their limitations.

Utility 3phase has one major plus -- a LOT of starting capacity courtesy of the grid. Well, two -- the cost! But the minus is -- single speed.

I've not taken one apart, but I assume a VFD is really a switcher supply [thread tie-in!] within. In short, it takes single phase 240VAC, makes DC, has an oscillator and inverter and makes AC with it.

That's basically how the VFD built into a rail transit car such as Metro here works; except it starts with 700 volt DC from the third rail. You can hear some of the cars sing as they start up; I think the upgraded

300 series are the most obvious. Before we had VFD's, we needed DC motors to have speed control; I think all the DC-motor cars here are upgraded by now.

Here's cheap: a starting cap & a push button. Wire the cap & PB in series between either line leg & the generated leg. To operate: push the button, turn on the power, when the motor is up to speed release the button. That's how I use an RPC on my mill & it works fine. It is not balanced, but balance is not something I need, nor do you, in all likelihood.

HTH, Bob

You can get a VFD for less than 200 - 250.

Bob

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O.K. 20A from a real three phase line -- but about 24A from single phase.

And this is in the range of what can sometimes be found at semi-reasonable prices. I have a 30A VFD which can and does run from a

30A single phase 240V line to power thing rather smaller than 7.5 HP.

I got it (by luck) at a hamfest some ten years ago or so for $115.00 IIRC. Quite reasonable, anyway.

But I've acquired more (more reasonably sized) VFDs to have a separate one on each machine, because you really don't want to have switches between the VFD and the motor -- though that big 30A one is surviving a switch between it and the 1HP motor on my Nichols horizontal mill.

And the current necessary to spin it up at the start.

And bigger than you could really run from your breaker box perhaps?

If it is designed for a motor of your 7.5 HP range, why not. You may still want to add capacitors for tuning and for equalizing the power factor to keep the current through your circuit breaker panel down to reasonable levels and avoid nuisance trips.

Good Luck, DoN.

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Quick and dirty:

VFD Pros: Gives you the ability to vary speed while running over a wide range.

Very efficient compared to a rotary converter.

Can be set up to accelerate the motor over a programmable time, so you don't draw as heavy a current from the wall to get the motor up to speed.

Quiet.

Cons: Does not like having switching between the motor and the VFD, this generates high voltage spikes during the switching, and can blow the output transistors.

So -- rewire the switch on the machine to command start/stop/reverse to the VFD's control terminals and you are fine for driving a *single* machine at a time. Best to have a separate VFD per machine -- among other things so you can adjust the speed of one without affecting the others.

ROTARY Converter Pros: Fairly simple to make.

Can be tuned to minimize current draw and maximize three phase quality.

Can easily run multiple machines (if large enough) because it does not care whether you have switches between it and the load motors. The other machines running actually add to the capacity of the RPC so you can handle a larger motor on the next machine turned on.

If the idler motor in the RPC is enough larger than the motor in the machine, it can handle "plug" reversing (switching to reverse while at full speed forward). If the idler motor is not large enough (say the same HP as the machine's motor), attempts to plug reverse sometimes result in the RPC reversing instead of the machine. This can get exciting if your machine is about to hit something (more likely on a lathe than a mill).

Cons: Wastes power idling when there is no load turned on, so you probably want to remember to turn it off.

Some of that waste power is turned to noise which can be annoying (but which will remind you to shut it off when you are done -- if you don't banish it to a box outside the house to keep the shop quieter. :-)

Draws a heavy current spike when first starting before you ever turn on your machine.

Depending on the nature of your particular motor, you may need to provide additional fan cooling.

There are probably other things which I have not thought of, but others may add to the list.

Enjoy, DoN.

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There are smaller ones designed to *run* only from single phase, and power a 1/8 HP motor (I have one -- about the volume of a brick and a lot lighter).

But not for driving normal single phase motors at variable speeds. The tricks which start single phase motors are rather frequency sensitive. Most capacitor start ones will need a different value of capacitor for each frequency range -- rather impractical.

Shaded-pole induction motors (such as old phonograph motors or little fan motors (pre muffin fan style) use a trick consisting of a heavy shorted winding around only part of each pole, causing a phase shift at that end of the pole to nudge the motor to a start. This is rather frequency sensitive as well. I've played with such a motor and a VFD -- and you get some range of variable speed, but for below say 60% speed, or above perhaps 120% speed, the motor will simply stall. So it is not practical.

Except that it is normally done by PWM (Pulse Width Modulation) so the transistors driving it can simply switch fully on or off (both low power conditions), and avoid the half-on state where the maximum power is dissipated in the transistor -- generating heat and leading to failure.

Whatever *three* *phase* motor will be. Single phase will not be happy. :-)

Air compressors can draw a *lot* of power, and running one from a VFD (if you run it a lot) will probably cover the cost of the VFD in power bill savings over two or three years. They are close to 100% efficient because of the switching mode output, while a RPC is rather inefficient.

Actually -- a *larger* idler motor than the load motor (1.5x or

2x) will probably provide all the torque you need -- especially if you take time to balance the converter by adding capacitors between the generated phase and the other two legs to produce approximately equal current in each leg.

The capacitor start only certainly throws away motor horsepower, at least for full-time operation. Someone here has measured produced horsepower from the cap start trick and found that the motor will produce that full horsepower -- but the current through the only powered winding is much higher than that when it is driven from true three phase, so long term operation can wind up burning out the motor's windings.

Enjoy, DoN.

buy one just for a single

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