Over what distance is it reliable/advisable to use a potentiometer signal as a speed reference for a drive ? I've considered converting it to a 4-20mA, is this necessary ?
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1\$\begingroup\$ There is no "potentiometer signal". Potentiometer is a resistor. You can use it in a voltage divider to get a specific voltage relative to the resistance. The distance you can pull the wires is dependent on how high the voltage, how thick the wires, how noise-protected they are and how much noise you can tolerate. \$\endgroup\$– Eugene Sh.Commented Jun 27, 2017 at 13:55
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1\$\begingroup\$ What are your Specs for distance, cable type ( shielded twisted pair, STP preferred) , Pot resistance, and input instrument impedance. There are 3 issues, DC drop from load, Common mode noise CM ( induced from solenoids, DC motors etc) and differential noise (DM ), such as Radar or SW Radio noise, lightning noise then how much delay is acceptable in Pot change. If we know anything about these influences in your environment, then we can define specs, then the solution is trivial. Until then it is guesswork for even experts. \$\endgroup\$– D.A.S.Commented Jun 27, 2017 at 14:09
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\$\begingroup\$ Sounds like you are already using it. If you are using it and you are happy, why change it? But if you are having problems that you think might be fixed by going to current loop, you should say what those problems are in your question. Edit the original question, I mean. \$\endgroup\$– user57037Commented Jun 27, 2017 at 15:14
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2 Answers
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You can use a potentiometer for a speed setpoint. The details depend on the drive setpoint input options. If the drive takes, say, 0-10V and has a 10V output you can connect a pot directly (and the drive manual will show you how exactly to do it, and suggest the range of possible pot element resistances).
If it accepts only 4-20mA you would be best to use an appropriate transmitter, and refer to the transmitter manual for the appropriate pot resistances.
A pot is best for rough settings and fast easy changes, maybe for something like a conveyor. If you need precise settings that are rarely changed, digital input (like from the drive keypad) might be better.
As to the distance, voltage signals are best over relatively short distances, preferably within a cabinet or machine. Say up to a few meters. In ideal conditions you can extend it much, much further, with a shielded cable, but in a typical high EMI environment it's best not to. Current signals can be sent much further- hundreds of meters- with little trouble, however you may still have to take precautions if lightning and similar issues can occur nearby.
answered Jun 27, 2017 at 14:01
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\$\begingroup\$ Do you think buffering the voltage with an op-amp would extend the useable range much, by lowering the source impedance? \$\endgroup\$ Commented Jun 27, 2017 at 15:10
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Generally the reference signal should be over a very short distance due to the high di/dt in the application. While there is no hard rule, the further you extend the signal, the greater chance of picking up interference.
A 4-20 mA control loop is a much better choice due to its inherent noise immunity.
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\$\begingroup\$ Wow. Why the downvote? I wish downvoters would explain themselves. I cancelled it out with an upvote. \$\endgroup\$ Commented Jun 27, 2017 at 15:12
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\$\begingroup\$ @mkeith Thanks for the upvote and I agree with you on down voting without comment. But in fairness, I edited my answer because I originally thought the OP was proposing to put the pot on the shaft as a speed indicator. After rereading because I thought that was too far out there, I realized what the OP meant. \$\endgroup\$ Commented Jun 27, 2017 at 15:16
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\$\begingroup\$ Oh, I didn't check the edit history. Makes sense! \$\endgroup\$ Commented Jun 27, 2017 at 15:23