Have you got an oscilloscope? It would be very informative to know the voltage and/or waveform on each terminal of the two pots across the full range of travel.
@Paul_KD7HB , I by no means have adequate knowledge regarding this setup and am regurgitating most of what I've learned over the past few months. My understanding is the pots are adjusting the voltage to both the field and the armature.
This may be from my lack of knowledge and thus we may be speaking at different levels, so bare with me; I'm not wanting to change anything other than the double pot so to speak. have something I can take the 5 wires that go to the current double pot and hook them up to a separate component, which will vary the resistance between two wires (for the one pot), and vary the resistance for the other three (from the second pot). The different resistance in those circuits is what does whatever magic happens in the setup that sends the appropriate voltage/current to the motor which adjusts the spindle speed.
I'm not interested in the magic right now; I'm just trying to see about developing something that will vary the resistance between two circuits. Something similar to this: a controller gives out a reading between 0-100 depending on how much it is turned. When it is turned to a setting between 0-50 it adjusts resistance between 0-50 ohms on circuit A; when the setting is between 51-100 it adjusts the resistance between 0-10 ohms on circuit B. Thus when the knob is turned to 25, it results in a resistance of 25 ohms on circuit A and no resistance on circuit B. If the knob is turned to 75 the result is 50 ohms resistance on Circuit A and 5 ohms resistance on Circuit B.
Does this make sense??
That would ONLY adjust the torque. I assume the motors are 3-phase and the thyratron systems creates a constant voltage, but varying frequency, which would change the lathe speed.
The reason you cannot find something to do what you want is strictly economics. Whith a one-time customer that may or may not appear, there is no reason for someone or company to develop such a device. Sorry.
Paul
@JohnRob @Paul_KD7HB The following is information from a friend of mine who has been helping me with the electronics of the lathe and getting it running. I had asked him if he knew the reference voltages off-hand. This was his response.
Monarch official voltages are below. What you measure may be slightly different. Bear in mind that the first dc voltage is a very poor rectified waveform that will have a different shape depending on the nature of the T4 transformer. So the reading you get will depend on that as well as the characteristics of your particular voltmeter. Pot 1 Circuit 22/29A = 251 vdc; Pot 2 Circuit 36/37 @ 0 rpm = 0 vac; Pot 2 Circuit 36/37 @ 2000 rpm = 34 vac
More information regarding this dual pot spindle control:
What I mean by floating is that in the electrical circuit there is no common voltage reference point that is shared by any of the five leads. In other words, the two pots are electrically isolated from each other and also isolated from other parts of the motor control. This makes designing an electronic replacement for the pot function difficult because the two sections have to work together but cannot be electrically connected as thru a common ground. There are other complications as well. For example, you can construct a device that mimics the behavior of the non-linear pot using networks of non-linear electrical components such as zener diodes, but these typically have non-linearity tied to (for example) a voltage or a current rather than a resistance. Since the waveforms applied to both sections of the pot are not really dc (even though P3B nominally regulates a DC voltage) there is no easy way to transform the non-linear voltage or current behavior of the mimicing components into something that looks like variable resistance when the thing that you are basing control on (voltage or current) is varying but the thing you really want to control (resistance) is not.
The points I am making above and their ramifications would not be easily obvious to someone not accustomed to designing electronic circuits so I don't think you are thick headed. When I first tackled this problem, I also didn't fully understand the nature of some of the issues.
My thought is that the most straight forward way to replicate those pots would be either to duplicate them exactly with non-linear resistive elements as in the original, or to build a mechanical device (for example with intermittent gearing) that operates a pair of standard pots so as the give the correct behavior. This is not to say you couldn't do something with a microprocessor or some other electronic circuit - I just think it would be challenging.
@JohnRob This lathe was designed to operate without a common ground. Another way of doing things in days gone past. I have the casting grounded to the electrical circuit it is connected to, but the electronics are not grounded to the casting. There also is no common or neutral wire run to the lathe, just the two hot wires for the 220v.
**haroldsutton I think you will see above we are discussing having two independent pots and possibly using a micro-controller to move them, possibly with stepper motors or similar. We may end up going this route if unable to perform this differently.
**johnwasser I don't have an oscilloscope, my friend does. He could explain the waveform at different points. He has designed a replacement for 2 of the thyratron tubes the lathe uses. The design doesn't fully rectify the waveform, thus the waveform for the DC isn't as "flat" as he speaks about it. There are variations to the voltages and such due to the inherent tendencies of the vacuum tubes, especially as they age. I freely admit, most of this is way over my head and I don't remember that much about the vacuum tubes and similar electronics from back in the day.
I apologize as well as since I'm new I'm limited to the number of posts I can make and apparently I can only mention 2 users per post. 
@Paul_KD7HB , no this is running on single phase.
I'm not asking someone or company to develop any device, so I'm a bit confused by this comment. I'm asking if anyone here knows or thinks this is possible. You all have used these types of devices way more than I and have way more knowledge of their capabilities and limitations. If this is possible (which was the initial title and question), then I have another person I can work with to develop it and perform troubleshooting. If it is possible and someone here is up to the challenge and wants in on it, great!!! We are not out to do this for money, rather to help save some old equipment and those that love these machines and their designs.
With computers and computer designs, saying it isn't possible is essentially giving up. Back in the 60s, if you asked someone how we could call and talk to someone from a small thin rectangular device we carry in our pockets, they probably would have told us we had been watching too much Star Trek. What wasn't done or possible back then, is possible and has been done. It seems like this should be possible with a combination of hardware parts and software and maybe something similar has already been done. Ideas???
The small thyratron tubes I have has i the past have never been vacuum tubes, they are ALL gas filled tubes, such as argon.
Paul
In do understand this, but what about the actual motors themselves?
@Paul_KD7HB There is no 3-phase power near this lathe; the entire thing is single phase. It can be hoked up to 3-phase, but 3-phase is only used if it has a coolant pump which my lathe does not, so it is strict single phase.
The what do the motor name plates have on them?
I've never seen a DC motor with an AC field. Trying to think back to my motors course in college it doesn't seem it would function normally.
@JohnRob remember these pots don't connect directly to the motor, they connect to various circuits which send the voltage to the motor. Everyone is stuck on trying to figure out how everything is connected versus my original question. I can post the entire schematic if you all would like, but these pots do not connect to the motor directly. They connect to the various circuits which alter the voltage and current sent to the motor.
This is part of the schematics where the pots are wired in. P3A and P3B are the pots for the dual head potentiometer.
People here can get pretty snarky (for lack of a better term)... don't let it get to you.
The only thing I could think of aside from a mechanical piggy-back system to move the pots would be something like a mosfet/opamp voltage controller or source that when fully on, produces the high end of the voltage range, and with a PWM signal generated by the arduino, the voltage can then be varied to its minimum range. This would be a new approach to the old tech. But would require atleast a few cups of coffee for damn sure. You said half is 250vDC. Since we know its 5hp, we can find out what the stall current is. You'd need to ask your electronics buddy if there are any "impedance matching requirements " i highly doubt it, but with old mechanical systems, upgrading to new-age controllers, you never know... If you're interested in seeing if this avenue is possible, I'm around!
BTW I appreciate the time you've spent doing your homework. I have many hours working mills and lathes so these projects are always cool to me to go back to my roots!
Edit: never mind about the stall current. Its not driving the motor
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The way this circuitry and how it controls the spindle speed is as follows, paraphrased obviously. When you start turning the first pot (50 k) from the CCW position 0 ohms. When increasing the resistance it increases the spindle speed from zero to about 1000 rpm, when the 50k pot is maxed out CW. Then the second pot (10K) begins to increase resistance which slowly weakens the field, but in doing so dramatically increases the spindle speed to 4000 rpm when maxed out full CW.
With how I understand this circuit works, I'm not sure how your suggestion would fit in. This is most likely due to my lack of knowledge. Does your suggestion replicate what the first pot does and what the second pot does....essentially increasing the resistance of the two circuits??
I apologize if you mistook my questions and comments. The comment were purely due to my curiosity.
In post # 19 I asked for the voltages of the pots to see if it was possible to use a digipot. I understood a 5hp motor would not be controlled directly from a panel pot.
From the schematic I believe it may not be possible or at least not straight forward to replace the 10k pot. Digipots have limited voltage capabilities that would seem to exceed those I have investigated.
For the 50k pot it might be possible but unlikely. It would depend on the Rect1 voltage between circuit 29A and 22.
For instance the "AD5171 Digital Potentiometer" is limited to 5V. It would appear the 10k pot would exceed 5v relative to a ground.
I know of no solid state design to replace these pots with something controlled by an Arduino. If I missed something please let me know.
John
From what I am seeing, as the potentiometer changes its resistance, the reference voltage changes as well. This reference voltage is what is read by the rest of the circuitry. The resistance is not the parameter used to adjust motor speed, but is used to adjust the output voltage of the stage. This voltage reference is then fed as an input to the next stage (however it works to adjust the motor speed)
I think, and a strong I THINK, what you are trying to do is actually adjust a voltage that is fed to the rest of the circuit. The potentiometer is able to accomplish this by adjusting an internal resistance, which then adjusts the output voltage.
I believe your goal should be to adjust the voltage differential between these two points circled in red for the max speed adjustment
As a separate question when looking at the "Reference voltage circuit" diagram picture shown. The 50K pot labeled P3B, the arrow INTO the pot, comes from where? (Is it from ETI/C3J on the first picture?)
22/29A = 251 vdc
It is line 27 and comes from the compensation circuit.
The control for max spindle speed comes from pot P2, is that what you're talking about??
Below is how my friend explained to me the dilemma about trying to replace these pots with updated electronics.
you can construct a device that mimics the behavior of the non-linear pot using networks of non-linear electrical components such as zener diodes, but these typically have non-linearity tied to (for example) a voltage or a current rather than a resistance. Since the waveforms applied to both sections of the pot are not really dc (even though P3B nominally regulates a DC voltage) there is no easy way to transform the non-linear voltage or current behavior of the mimicing components into something that looks like variable resistance when the thing that you are basing control on (voltage or current) is varying but the thing you really want to control (resistance) is not.
Hopefully that helps.