Brad (Customer) asked a question.
How to replace a potentiometer on an output circuit.
I have a vibratory feeder supplied at 120VAC, and the speed is adjusted with a traditional dial potentiometer. I would like to replace the potentiometer and control the feeder with a PLC/analog output. How do I replace the potentiometer? I see analog to potentiometer signal conditioners. I see Pulse Width Modulation as an option. Could I use 1 leg of a VFD, and bring the PLC analog output into the VFD to set the frequency and hence the output voltage?
Can you list the complete VFD model number?
The details of the speed controller will be important. Some SCR controllers have the pot at a point in the gate circuit were voltages are nothing like what a PLC analog output can hande and thus would need a special isolator.
I was going to send an analog out put to the VFD, setting a frequency. I would use a single leg of the VFD output to get single phase, 120V. The analog would not have to be in the voltage or current range of the vibrator. The vibrator motor is small, and pulls maybe 1-2A. So a very small GS20 VFD, single phase in.
I have a feeling that only pulling power from a single phase of a VFD output may not end well for the VFD. It will probably even fault out immediately.
You would want to go phase to phase (as opposed to phase to ground) for single phase, but, as long as the particular model allows you to ignore the single phase operation fault, it shouldn't be a problem.
If a parameter can be set to not fault on single phase loading, it would still be up at 240v from phase-phase and the OP needs 120v on his single phase which would come from phase-ground
the Max output voltage from the drive should be adjustable as well.
Yup, that is what I am seeing when I test with a drive. I get DC bus voltage faults, and now a ground fault. OK, so back up this general question... If you were converting an old machine to PLC control, and you had a component on the machine that was controlled via a potentiometer, how would you go about replacing the potentiometer to allow for PLC/ analog control? That would seem like a common issue when retrofitting to new control system... There must be a simple answer...
Did you try wiring the output phase-phase instead of phase-ground (or neutral)?
You didn't say anything about what the load is? Is it a DC motor or a single phase AC motor or what. If its a single phase motor the easiest is to replace it with a 3 phase motor. If you incoming power is 120 VAC use a GS20 115 vac input model and put a 230V 3 phase motor on it. Then you can control the speed with the 4-20 ma input on the drive. If the load is a DC motor then use one of the DC controllers that accept a 4-20 ma input but these are expensive. As for controlling a single phase motor with a drive I do it all the time by putting a 230/115 vac transformer across 2 output phases of the drive and step the voltage down. But you must make sure that the motor your using does not use a capacitor start/run setup. The capacitor is switched in and out of the motor circuit based on motor speed.