Arboga mill converted to VFD control (pictures)

Did you mean double throw?

Got a source for those?

i

The switch 's resistance should be much less than the input impedance, but big enough so as not to exceed the limit on current produced by the CC terminal.

5k should most likely work and those are very common pots.

I put a 1k pot in my drive the other day and it works great.

i

Thanks Gunner, Iggy, Don I scanned through the manual looking for the wiring diagram and next to the potentiometer it says "10k ?" I"m not sure why the "?" is printed in the book but that's what it says. I looked at the on line listing at Radio Shack - not too pricey, about $3. The only problem is, they only list 0.5W potentiometers and Don had mentioned it ought to be 2 Watts.

I looked in the Mouser catalog and I spot two that would probably do the job. One's a high precision single turn pot EUP-1400-10k for $4.15. The other is listed as a 53 series industrial pot with a 25k rotational life expectancy. 785-531C10k for $6.76 Both of these handle 2 watts.

It looks like the control switches for my VFD should be momentary switches.

RWL

If the control voltage is 10 volts, then a 10k potentiometer would dissipate 10v*10v/10000ohm = 0.01 watts. You do not need a 2 watt 10k pot.

You can always test that with a wire that has two bare ends. Just touch two appropriate control contacts, while staying away from R,S,T and U,V,W and other big screw terminals.

i

[ ... ]

Yes, I did.

For the usual ones -- try Mouser, Digi-Key, and the other electronic suppliers.

For the locking handle ones -- those I stumble across at hamfests -- occasionally.

O.K. Mouser Electronics has them -- but not cheap. Look at:

assuming that still reaches it. If not. look for Mouser part number

785-2TL1-1A (A Honeywell switch -- about $40.00 in unit quantities.

The one shown has to have the handle pulled to switch from any position to any position. There are some which will switch to one side only without pulling, and the other side requires pulling. Others can be switched from either "ON" position to the center "OFF" position, but must be pulled to switch to either "ON".

You deal with enough surplus stuff so once you know what they look like, you can keep your eyes open for them. They are normally used for things like activating munitions in an aircraft, where you don't want someone to accidentally switch something explosive on. :-)

On eBay -- these look like possibilities:

300333989154

This one is lacking the center off, but it has a close-up which shows how the locking works.

330259446730

But if you have to *buy* switches *new*, look for large pushbuttons, with the locking mushroom switch for the panic stop function. You can get those at places like Grainger, if you have the ability to buy there.

Enjoy, DoN.

Don, I thought that you initially discussed a pot/switch combination?

i

That will be at the lower limit, but should be just fine.

VFD makers specify 2 watt pots for mechanical robustness, not electrical load. Nor do you want machine vibration to cause the pot to walk.

One thing about the Forward-Off-Reverse switch: Do not make it too easy to operate, or the machine may start when you accidentally brush the switch. The classic scenario is that one is setting things up, and gets chopped up when the machine starts unexpectedly. So use nice big and clunky switches, even though the current is maybe 10 milliamps.

Joe Gwinn

Are you sure that is not an Omega with part of the printing missing? I can't get my editor/system to create one, but you can use wikipedia to display it:

Anyway -- the upper case Omega symbol is commonly used as shorthand for "Ohms", so that is saying "10 K Ohms".

Some of the makers suggest 2 Watt pots. One reason is that there is more resistance element area so it will run longer before it gets noisy. Even at worst case (the DC motor controller which puts 20 V across the pot, and using a 1K pot instead of a 5K pot, we are still really under 1/2 Watt.

But -- the larger 2W pots generally have a better feel and greater ease setting a particular value, so I would use them anyway.

O.K. The first one looks nice. 270 degree rotation is normal, but when it said "single turn" and "precision" I was afraid that it had no stops so it would keep going from 0 up to full and then back to zero. These are used for some instrumentation type tasks, but would be poor for controls of this sort. However, what you have found should be a very nice one. The wirewound element will work well for a long time. And they claim 100K rotational life for the first one.

I can't seem to find the second one by that part number.

That depends. Usually there are usually wiring configurations for momentary pushbuttons (with a NC (Normally Closed) pushbutton for the stop), or for rotary or toggle switches). (Different pages in the manual, usually.)

If retrofitting a machine which had a drum switch, I would use the continuous contact wiring pattern, selecting the proper contacts out of the drum switch, so I could continue to use the same control that I was accustomed to.

Enjoy, DoN.

[ ... ]

No -- that was someone else. The combination would limit the ability to get really slow running for positioning a chuck or something similar. (You don't want to run the motor for any significant time at that low a speed unless you have made provisions for an external fan to force air through the motor.)

Also -- the switch in the pot knob is a bit harder to shut off in a hurry. The red mushroom panic switch is best, next is the usual drum switch wired to control the VFD.

I actually have a switch which I intend to use when I put three phase on my Clausing. It is a lever in a pleated rubber boot which rotates a shaft with cams operating three switches -- one at the zero center position, and one switch for each direction of travel. But the cam also has a 1/4" hex socket which can accept the shaft of a potentiometer and which rotates through the standard 270 degrees. So what I want is a 10K pot with a center tap so I can wire it for half on each side of the center off position, using the cam switches to select the direction, and the half of the pot operated by the lever to select the speed. (The assembly came from a hamfest, and I have no idea where to get more of them, but I knew what I wanted it for as soon as I saw it. :-)

Enjoy, DoN.

VFDs supply the speed pot from a low impedance DC source typically 10V or less and the pot slider is almost completely unloaded. This makes the choice of pot value very non critical - any pot resistance between 3 and 10kohm is typically recommended as suitable and an even wider range would make little difference.

Power dissipation is also very low. Even in the worst case scenario, with the lowest value pot, dissipation is much less than 1/4W so even small radio type pots are perfectly usable. However you may prefer the mechanical ruggedness of a pot with higher power rating.

What is seldom mentioned, but important in practice, is the resistance law. Most wirewound and many film pots are linear law which is not very suitable for a machine tool working over a wide range of work/cutter diameter and speeds. The useful low speed end is limited to narrow region near anticlockwise. What is preferable is is a log law pot as this opens up the low speed end and gives a close approximation to equal % speed change for equal pot angle change over the whole speed range.

Log law radio type pots are easy to find but heavy duty types are much less common. A solution that I find preferable is switched speed setting. The 12 step series resistor sequence

100 Ohm 120 150 180 270 330 390 470 560 680 820 Gives a constant 20% speed change per step which is about as close as is ever needed in typical machine tool use For more VFD information try "Electric Motors"- second edition- Jim Cox Pentagrid

Interesting commentary. Thanks!!

Gunner

"Somewhere a True Believer is training to kill you. He is training with minimum food or water,in austere conditions, day and night. The only thing clean on him is his weapon. He doesn't worry about what workout to do--- his rucksack weighs what it weighs, and he runs until the enemy stops chasing him. The True Believer doesn't care 'how hard it is'; he knows he either wins or he dies. He doesn't go home at 1700; he is home. He knows only the 'Cause.' Now, who wants to quit?"

NCOIC of the Special Forces Assessment and Selection Course in a welcome speech to new SF candidates

That was a good thought. I ran upstairs and looked in the printed manual. There it is printed as the ohm sign, so it is 10,000 ohms. When I made my previous post, I had been looking in the manual that I had downloaded as a pdf file and that's where the omega appears as the "?". I tried using copy and paste from the windows character map to paste an ohm symbol into this reply, and it came out as a question mark rather than an omega, so on an electronic file, the omega is being misinterpreted by the computer as a question mark.

RWL

Does ALT234 (?)work?

--Winston

Well ... it shows up as a '?' here, which means that it is using the section of the extended ASCII characterset with represents control characters with the parity bit set instead of clear. Sun refuses to display these -- at least with the ISO-8859-15 characterset which I use. Perhaps it is characterset dependent. And assuming that the 234 in ALT234 is an octal number, it does land in the parity-bit-set control character range (0x9c, with 0x80 through 0x9f being the extended control characters.

Enjoy, DoN.

Yes, that was Unicode UTF-8.

Just checking. Thanks DoN.

--Winston

Radio Shack pots are garbage. Spend a few bucks and buy something like this:

then it will last for years. I might even have one laying around that would work. They are mil spec, and availible surplus. They are sealed, so you don;t have to wory about dust or metal chips damaging them.

The two watt pots are made for long life, and have lower wiper noise that the cheap crap. They were used as volume controls in military radios and other electronic equipment in WW-II, and are still good.

Buy cheap crap if you want to, but cheap pots are a foolish investment.

[ ... ]

Yes -- so that is who took over the Allen Bradley line of mil-spec pots. Glad to know that quality pots are still made.

Enjoy, DoN.

Buy good pots once, or crap every six months. I prefer quality over quantity. :)

They are the only used pots worth salvaging from scrap industrial and Military electronics.