Controlling drives with 4-20mA

[hurk27]

The VFD he's using comes with a PID controller built in: http://static.schneider-electric.us/docs/Machine Control/OEM Solutions/8000DB0701.pdf

Ok I didn't look it up, wonder if hes applying any derivative? for most simple level control like this I would leave the derivative set to off and only use proportional and integral to tune the tank, wonder how much the outflow from the tank varies, if it is widely it could just be sending the controller into Integral windup if the integral is set to high, wide disturbance's can make tuning a nightmare but can be done, most of the auto tunes will not even get it close in these cases.

If I had to pick a different controller to use, it'd probably be an Omron E5EK-AA2 since it does self-tuning.

I have found that auto tune is great for slow changing processes but if you have a process that changes often and widely it can't over come it.

Where we're at now: Full should read as 20mA but only seeing 17mA. There's 19 volts across the transmitter, and presumably 5 volts across the drive's input (0.02A x 250 ohms) which equals the 24 volts from the power supply. The transmitter should function down to 11 volts, so that's not the issue. The transmitter specs say it's good to 650 ohms on the loop, minus the 250 ohms on the drive means we've got 400 ohms to work with. That's 8000 feet of 24-ga cable.

His last post stated that after working with a factory rep on the phone he was able to get the zero and span to 4-20 on the probe, not sure about what he has on the drive, but my guess that is his next objective

I'm stumped...

I guess you could program AI2 Max Value as 17mA but that doesn't really fix the root cause.

Or use a 500 ohm resistor to convert the signal to 2-10 volts and connect it to AI1

See above

 

[Stus Dad]

How are you setting up your 4-20ma?

How are you setting up your 4-20ma?

Could be the drive input has a 250 ohm resistor and is actually getting a 1-5V -- and the drive internal brains is interperting the input to show the miliamp signal. Might want to check that.

I've seen more than one with a 240 ohm radioshack resistor* - that is an automatic 4% drop in the signal the VFD will see.

ice
* Interesting note: When I have seen these installed, no one has ever admitted to changing out the resistor :roll:

For example if you range 4-20 ma as 0-4095 counts you should be using 819 as 4 milliamps and 4095 as full on your tank level. If you set the counts to 0-4095, you will not have the same relationship in your fill ratios. You may have done this, but I never saw anyone mention it in their threads. 12 ma would be half full based on one calculation and would not be on the other. 819 represents bottom of tank 4095 represents top. If you use 0, you are treating it like the scale as 0-20ma. My first post here, so cut me some slack!