4-20mA

Hi there,
I just want to ask a simple question.
can I attach my pressure transmitter (4-20mA) to Inverter and PLC Analog Input. If I can so what should I use : series or parallel connection? and what Is the maximum distance for 4-20mA transmitter so It won't lose signal.

Thank you

Yes you can use the 4-20ma signal to both devices. You will have to tie them in series since it is current.

The short answers:
1. Maybe.
Provided that:
a) You don't have more than one non-isolated device (ie. input card with common signal return)
b) you connect the wire properly, in a loop, with current leaving the + terminal of all power sourcing devices and entering the + terminal of all power receiving devices (If you don't understand this, I'm sure someone can elaborate).
c) You ensure that the total loop resistance (devices and wire) is not greater than the rated load of the transmitter.

2. You should use a series connection, but the polarity of your devices must be correct. See b) above.

3) The wire distance doesn't matter, provided that c) from above is satisfied.

4-20 mA part 2

OK thank you for you both
Q for Jimbo : So I must wiring like this + terminal (from transmiter)--> + terminal (from inverter), then - terminal(from inverter) --> + terminal (from PLC), then - terminal (from PLC) --> back to - terminal(from transmitter).
It make a loop right.

Is it true that 4-20mA signal can stand from EMC or noise?
If I have more than one transmitter, can I use this method. Example : Transmitter1 used in inverter1 and channel 1 of PLC, then transmitter 2 used in inverter2 and channel 2 of PLC. Which is - terminal from PLC both channel is connected. If wrong can give any suggest? How to do this.

Actually my project is to control 4 pump RPM(use inverter) with 4 pressure transmitter (because of different valve) and monitoring it with PLC to show them in HMI.
Thank you

Jitu Automation said:

OK thank you for you both
Q for Jimbo : So I must wiring like this + terminal (from transmiter)--> + terminal (from inverter), then - terminal(from inverter) --> + terminal (from PLC), then - terminal (from PLC) --> back to - terminal(from transmitter).

Click to expand...

You forgot the power supply. Most pressure transmitters are 2 wire loop powered. Not all, but most. So if yours is loop powered (it doesn't have terminals for a separate power supply) then you need to stick a 24Vdc power supply in the loop.

revised:
+ terminal (from power supply)--> + terminal (on transmitter)
- terminal of transmitter ---> + terminal (on inverter),
- terminal(on inverter) --> + terminal (on PLC),
- terminal (on PLC) ---> back to - terminal power supply.

Jitu Automation said:

Is it true that 4-20mA signal can stand from EMC or noise?

Click to expand...

Generally accepted as true, but debateable in certain circumstances.

Dan

I forgot to mention that a single DC power supply is normally shared among multiple loop powered transmitters.

Jitu Automation said:

Actually my project is to control 4 pump RPM(use inverter) with 4 pressure transmitter (because of different valve) and monitoring it with PLC to show them in HMI.

Click to expand...

Usually inverters are used to control motor/pumps in order to eliminate valves. It isn't clear to me what controls the valve in your scheme.

A strategy where PID control is done in the inverter to regulate motor/pump speed and the PLC is used as a front end for the HMI is reasonable if the inverter has PID capability.

Like Jimbo3321 says, if either the PLC or inverter analog inputs are not isolated, then there's a good chance that you'll have ground loop or common mode problems running the pressure 4-20 in series, particularly when more than one loop is involved.

Dan

I hope, Jitu has got his answer..., If not, try this :use a signal multiplexer, One Input (4-20mA), two output 4-20mA, external power supply: 230vAC, 50Hz/60Hz. this will surely make your life easy, most of the PLCs available are common shorted, (i.e., all negative are shorted) you will loose your signal / resolution in doing multiple inverter & PLC input. Hope this helps.

Thanks Dawn for the answer, but still confuse

I need to control 4 motor/pump speed with 4 inverter, I used pressure transmitter for inverter analog input, so the motor speed can vary depend on pressure of water.I will use V/F control, because the inverter didn't PID function.But for my HMI I need to show what is the pressure for each pipe(not valve that i mention yesterday).So I want to connect the transmitter with PLC and Inverter, the problem now is that I have 4 similar control. maybe I will try to describe to you the connection I think could work :

+ terminal (24 VDC supply)----->+ terminal (transmiter A)
- terminal (transmiter A) ----->+ terminal (inverter A)
- terminal (inverter A)-------->+ terminal (PLC ch 1 analog in)
- terminal (PLC ch 1 analog in) ----> - terminal (24V DC supply)

and another circuit

+ terminal (24 VDC supply)----->+ terminal (transmiter B)
- terminal (transmiter B) ----->+ terminal (inverter B)
- terminal (inverter B)-------->+ terminal (PLC ch 2 analog in)
- terminal (PLC ch 2 analog in) ----> - terminal (24V DC supply)

and repeat this until C and D unit.But still have the same power supply 24V for each circuit.

is this connection will cause problem ground loop?because the PLC common terminal (-) is usually connected together in internal.Still get confused about this?

Thanks

Jitu Automation said:

Thanks Dawn for the answer, but still confuse

I need to control 4 motor/pump speed with 4 inverter, I used pressure transmitter for inverter analog input, so the motor speed can vary depend on pressure of water.I will use V/F control, because the inverter didn't PID function.

Click to expand...

I doubt that this configuration can be made to work to control pressure. If your VFD (Inverter) does not have a PID function, then the analog input can probably only be configured as a speed command. In that case, connecting a pressure transmitter to the input will not allow the drive to control to a set pressure.

The easiest solution (If possible) is to connect your pressure transmitters to PLC analog inputs and connect the VFDs to PLC analog outputs and use the PLC to run a PID for each motor.

for Jimbo :
Correct me If I'm wrong :
If pump/motor is spinning faster, when the pipe and the other is constant or near constant, then the pressure of the water is rising right? but when the pump is slower then the pressure is lower. (because of water) I used this for something like water distribution.
I use pressure transmitter, when the pressure is high then the motor get slower, but when the pressure is low then the motor get faster. and when the pressure is too high the pump will stop.

Any something wrong?your advise is good,at least I have another option for working on my project (must add analog output then)
Thanks...

We still don't know. What is controlling the valves now?

Jimbo3123 is correct. Connecting the pressure transmitter to the VFD, when the VFD has no PID capability is useless.

The relationship you describe between pressure and VFD/motor speed is not likely to work for a single pressure, to say nothing of varyuing pressures with varying loads.

Without PID, you have no means of correlating pressure to VFD output. PID compares a pressure setpoint to the actual pressure and calls for motor speed that will drive the error to zero.

Yes, you can put the pressure signal into the VFD, but so what? You need an inverse relationship with speed (low pressure requires higher speed) not a direct relationship.

And the likelihood that pressure varies on a curve matching required speed is close to zero.

Connecting the pressure transmitters directly to the PLC for both PLC PID control and SCADA access is an accepted means of accomplishing pressure control.

Jitu Automation said:

for Jimbo :
Correct me If I'm wrong :
I use pressure transmitter, when the pressure is high then the motor get slower, but when the pressure is low then the motor get faster. and when the pressure is too high the pump will stop.

Any something wrong?your advise is good,at least I have another option for working on my project (must add analog output then)
Thanks...

Click to expand...

How do you plan on inverting the pressure signal so that a increasing analog signal from the pressure transmitter will trigger a slow-down of the motor?

How do you plan to introduce a pressure setpoint (or error value between PV and SP) to your configuration?

If you can overcome both of those problems, you can possibly setup a rudimentary proportional controller. Proportional only controllers (The P in PID) are prone to oscillation.

I can't spend any more time trying to convince you not to try it this way. All I can do is tell you that you are asking for trouble.

jimbo3123 said:

How do you plan on inverting the pressure signal so that a increasing analog signal from the pressure transmitter will trigger a slow-down of the motor?

How do you plan to introduce a pressure setpoint (or error value between PV and SP) to your configuration?

.

Click to expand...

Sorry for late reply, so much thing to do.
I usually change the X and Y parameter in VFD.like if Input 4mA then the freq is 50 HZ and if the Input is 20mA the freq is 0 Hz. With that I can inverse the motor speed.

I calculate the transmitter resolution, example the set point is between 2-4 bar (below 2 bar motor must full speed, if higher than 4 bar motor must stop), then I have transmitter for 1-5 bar for 4-20mA.So from calculation I will have the bottom and top limit for analog input for working at zero speed to full speed.

like this (2 bar /5 bar x 16(range between 4-20mA)) +4mA = 10,4mA
then ( 4 bar/5 bar x 16) + 4 mA = 16,8 mA
So I set If the Input at X0 is 16,8 mA then the speed y0 is 0 Hz. And If Input at X1 is 10,4mA then the speed at Y1 is 50 Hz.

Thanks

jimbo3123 said:

I can't spend any more time trying to convince you not to try it this way. All I can do is tell you that you are asking for trouble.

Click to expand...

Please don't have hard feeling,I just want to know the reason why I can't use that.The reason that will be understood so I will get enlightment. I just want to know why?why and why?Sorry if I chase for the answer and not only advise. I think an engineer like us must always think about what,why,how?not always agree about someone said?right?
Please don't be emotional if read this. And sorry if there is any wrong words.

What Jimbo and I are saying is that we don't think it will work, but you don't want to hear our recommended solution because you're not familiar with PID. It's too much work for us to prove your negative.

PID was invented to do what you're describing, but you believe you have some alternative scheme that will work. Maybe you do. My experience says your scheme won't work, but if you believe it will, I suggest that you go plumb and wire up your scheme for one pump loop and try it.

When you find that the pressure signal doesn't match the motor speed needed to maintain pressure, and you try jiggering the ranges this way and then that way to try to make pressure match the required speed and then determine that it's a whole lot of work trying to match things and it still just doesn't quite work right, then you can re-consider doing a PID loop.

Dan