The nice thing about a 4-20ma current loop is that it's pretty well regulated. So even if your length of line is 1000' long, you can be assured of getting your 4 to 20ma current. The driver simply overcomes the circuit resistance until the desired current level is reached. What's more, even if your 24volts floats around, the current driver should maintain the correct current level for the setting.
The 250 ohm resistor gives you the right voltage at the 20ma setting (20mA * 250 = 5volts). But at the 4mA setting you get 1 volt. It's pretty close, but may be one source of error.
The real question is how much of an RPM change does 1 volt create (not related to the 4mA case, just a unit of measure). If you know the current regulation spec (say it's +/- 0.1%) then the voltage change will be the same +/- 0.1%. By knowing how much of an RPM variance 1 volt gives, you can estimate the variance in RPM.
I speculate that the speed control is a closed loop anyway, so whatever the speed control signal (voltage or current) might say, the actual RPM signal from the motor makes the controller adjust up or down until the right RPM is achieved. As a result, current regulation differences won't have a significant effect since the closed loop design will offset any errors. What's more, the voltage control signal will be effected equally by changes in current - if the current regulation changes by 1%, the voltage signal will change by 1% also.
Most likely, internal to the engine controller, when you put it in the 4-20ma mode, it's switching a 250 ohm resistor into the circuit and using the 0-5volt signal anyway.
Does that help?
You spotted the bug in the ointment. You know juice. Actually you saw all of them. A+
But they were bugs I knew I would have and we can live with.
The 1 volt at the bottom side does cause a little grief but we can live with it. For 4ma we get 1063 rpm instead of 1000. I program the floor and ceiling into the ECM, where 0 volts is 1000 and 5 volts is 1400. We can hit the ceiling but not the floor(from the PLC).
The other bug is that the PLC is not closed loop in regard to RPM, it is closed loop in regard to system inlet pressure. It is direct acting, if pressure is high then output is max.
It doesn't have to be perfect, all they really need these machines to do is slow down some if gas flow falls off.
Your last paragraph re internal switching and still using 0-5 volts... no sir. The 0-5 volts lands at different terminals than the 4 to 20. So it ignores any 0-5 input and looks at 4 to 20. We want to use either input and I added an external resistor to convert the 4 to 20 into 1 to 5 vdc, added a selector switch(0-5 on top, 1-5 on bottom, and selected output in the middle going to the ECM), and when the operator starts the unit he switches it to local and uses the 0-5 speed pot until everything is happy, then he switches it to the PLC(which I converted to 1 to 5). And I have enabled the 0 to 5 volt speed input, nothing is connected to the 4 to 20 terminals now.
I was just intrigued by what would happen if the 24vdc carrying the 4 to 20 fluctuates by a volt or 2. You have me wondering if I have a gap in my understanding of this 4-20 juice. It is always 24vdc(more or less) I thought, but only capable of supporting a given (ma) load. So when you put a load on it, that exceeds the current driver then the voltage falls off. And intrigued by what math is employed here. It's not OHM's Law as I normally use it. We may have discovered that I don't know what 4 to 20 is...Line me out please.
At 4ma I read 1.1 VDC on a Fluke 87, 1063 rpm
At 8 ma I get 2.1 vdc and 1160 rpm
at 12 ma I get 3.1 vdc and 1260 rpm
At 16 ma I get 4.1 vdc and 1360 rpm
And I hit 1400 rpm before it gets to 20 ma, but also before I should voltage-wise, so the Cat end of it is not perfect.
So it's behaving quite well, just has an errant floor.
Are you telling me that the 24 vdc carrier will be nice and constant? This is in the sticks, and power fluctuations are the norm. I would have thought a line current fluctuation may carry into it.
If it is 24 volts, and altering the ma alters the voltage by having a load to ground(the resistor), then that tells me the voltage at the driver end is maintained constant and the current is increased until it handles the load. Or am I heading the wrong way?
Thanks for your help.