Need to change ohm value of pot.

Here's the set-up. I am in the process of installing a new DC drive on my EE1000. One of the pots I want to use is for the CSS(constant surface speed) This control pot has a rating of 50k, but the DC drive has a setting of only a 10k pot for speed control. So, is there any way to reduce the ohm value of the 50k pot so that the DC control thinks its a 10k pot?

Another question is the switching between the main speed control pot and this CSS pot. Would it be better to be using a switch that drops out one pot before the other replaces it electrically, or one that doesn't drop circuit A until circuit B is introduced? Seem like either way has its own problems, but I am no electrical engineer. I should mention that the DC control unit is almost exactly like a VFD except it only converts AC to DC, but not the next step of changing the current back to a frequency controlled AC The control is a Eurotherm brand, regenerative model.

Dale

Many VFD's have a scaling function to support variable resistance differences. Perhaps your DC drive does as well. 50K to 10K may be a stretch, even if it's only 5:1.

If I were running CSS, I would want a system that made me switch into the appropriate mode along with any electrical hardware. Having the spindle speed up "automagic like" could throw metal around.

Valley City, 'lived there as a boy. 340 6th street NW. I got caught out on the highliner a time or two. ;-)

Most VFDs use 0-10V control. The pot acts as a voltage divider to give the variable voltage.

Have you tried wiring up the 50K pot? I would be surprised if it didn't work just fine.

No, I haven't tried wiring the 50k pot in yet. My cousin, who is a Master electrician with decades of industrial work with this kind of stuff, didn't think it would be a good idea, but I suppose the worst that would happen is getting an error message. So will finish all the parameters and see what happens with this 50k pot.

Dale

I'm not sure what your cousin is thinking.

If it was a 1K pot you were going to use, it might draw more current than the control circuitry could safely supply, but using a higher-value pot should not pose any danger. At worst, it might not work to control the speed.

If it was me, I wouldn;t hesitate to try it.

BTW< why do you need to use a 50K pot? Why not just get a 10K pot, as specified?

I knew I would forget to mention why the 50k pot. Where this pot is positioned, has if I remember right has 2 or 3 gears mounted on the shaft, making it have a much longer shaft than an average pot has. While with this new drive I only need to switch between the main speed pot and the CSS one, there is gearing between the cross feed shaft and this pot and I don't know which gear matches up with the gearing from the cross feed. So, unless forced to, will stay with the original.

Dale

A 50k pot instead of a 10k pot should work just fine and may not make much difference.

The input impedance or sensitivity is such that a 10k works fine and as mentioned but described differently it becomes 2 resistors in series across the source voltage with the center tap providing some voltage between the 2 ends.

So it most likely will work just as designed if is linear and not audio taper.

It may not be real linear near one end or the other but it should not cause any risk to try.

Basic ohms law stuff...is K ohms current is in mili - amps, and increasing resistance by factor of 5 reduces any current by same so carefully connect it and see what it does.

What can happen with a higher value pot is that the input resistance of the control circuit may be low enough that the "effective taper" of the pot will not be correct. This may, or may not, cause an operational problem.

Typically, a pot has the highest internal "effective resistance" at a setting that puts the slider at the half-way point of the total resistance value. A 50K pot will have an effective series resistance of 12.5K at that point*. A 10K pot will have only 2.5K effective series resistance.

If the load imposed by the control input is too low, it will cause the "halfway point" of the pot to actually present less than half voltage to the control circuit. That could foul up some types of constant speed circuits a bit.

* the value is due to there being 25K to ground, and 25K to the supply from the "middle" of a 50K pot. The value of those two in parallel is 12.5k. So the voltage produced is half the supply volts, presented through 12.5K. If for instance, the control circuit has a load resistance of 25k ohms (as some do), then for the 10K pot, 50% actual would read out as about 46%, but for the 50K pot, 50% actual would read out as only 33%.

I understand the pot and its mounting details make it difficult to just change it out for a 10k pot... but consider these two possibilities:

1: If the pot is a common manufacturer (post a pic) then it's possible to disassemble it and put a new element in it.

2: Make a new bracket that holds a 10K part with a mating gear up against the gear train at an appropriate point. (Gear can probably be plastic.)

Chip

IF (and only if) all else fails, there is a solution involving a small bit of circuitry. Assuming 1) the VFD uses a DC voltage input as most do and 2) You are willing to do some circuit construction THEN you could build a unity gain buffer circuit that would go between the pot and the VFD. That should solve the problem. We can give you a design for the circuit if it becomes necessary. I also think the 50k pot will work.

Thanks to all who have had excellent advice to my question. I took the lathe control box apart today and found out a few things that I had forgotten. This 50k pot was being used as a rheostat(only the wiper and one terminal being used). Also my recollection of the rheostat being mechanically operated was faulty. There is nothing connecting this 50k rheostat to the cross feed, meaning the CSS is done electrically. Long and short of this is that it will be much more difficult(if not impossible) to re-hook up the CSS. When it was done with tubes and rectifiers(as this 1957 lathe was done) was one thing, but making a solid state DC drive do the same thing doesn't look possible

There was a pot and meter that was used to pre-set the surface feet per minute, then a knob was set to the starting diameter of the piece to be turned. Beyond that, how it was done has not been fully discovered. I will contact Monarch to get the proper operator manual, just to fill in some of the details on how this was set up originally, but mostly will forego my dream of CSS

But, thanks again for the advice

Dale