Controlling current from the motor on PC directly from VFD? possible? 2

[spaaw]

Hi,
I have a Variable Frequency Drive that will control a motor. I also have LabJack automation device connected to the VFD and im controlling the motor using LabView Software. My question - do i need a CT to measure the current or can the current be displayed on the PC from the VFD without a CT?

I know i can see the current value on the VFD display, but i want to have it on my PC.

Thanks
/Peter

 

[danw2]

For those who don't know (I didn't), a LabJack is a USB/Ethernet based measurement and automation device which provides analog inputs/outputs and digital inputs/outputs.

Using a Labjack to control the VFD infers that you are sending the VFD an analog output from the LabJack to an analog input on the VFD.

If your VFD has an analog output and that AO can be assigned to represent the speed variable; and you have a LabJack with an available analog input, then you could get the speed variable from the VFD into the PC via analog.

 

[danw2]

No edit function on this forum . . .

The request was for current, not speed.

Same deal, if the VFD offers a current value and it can be assigned to an analog output, then run with it.

 

[Skogsgurra]

danw2 is correct, use the analogue output. In most VFDs the frequency and motor current (sometimes torque) are pre-assigned to AOs (default). LabJack has limited bandwidth. How fast do you need to record current changes?

Also, be aware that AOs are not updated very quickly. That is illustrated in the attached recording.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[spaaw]

thanks for the fast replies!

@Skogsgurra ->

The motor will be controlling a machine that will separate polymers (plastics). IF the machine jams = the current will go up. When that happens, I will set my program to start the motor in reverse and then forward again. The reason to monitor the current is for my jam solution. You think the bandwidth limit will be an issue? (thanks for the picture)

 

[Skogsgurra]

No. No probs.



Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[automatic2]

check your VFD docs to see if there isn't an programmable alarm output which could be put to use.

Personally, first thing i'd check to see if your drive had a comm port, and if you could get onto that for parameter I/O.

 

[winflex]

Spaaw,

I'm in the process of setting up a system much like yours:
A 4-20mA signal from a flow transducer will be fed in to LabView.
In LabView, a flow setting will be input by the user and a PID loop will send a 4-20mA signal to the analog current input on the VFD to set VFD speed.

Does anyone have any experience / tips with this?

 

[danw2]

I haven't used Labview, but they've been around for a long time, so they probably use control terminology.

The "flow setting" is called a setpoint.

The 4-20mA flow rate signal from the flow transducer is called a "process variable".

A PID loop is 'tuned' with 3 parameters or 'terms'

gain or proportional band (same function but values are reciprocals of one another)
reset or integral
rate or derivative

If the flow rate is measured close to where the VFD-pump is then the flow rate response to changes will be quick. In such a case the gain term is typically a very low value, or the PB term is a very large value.

I'd start off with no rate/derivative action; zero or as close to zero as you can make the value.

 

[jraef]

You may want to consider looking into directly communicating with the VFD. Most offer direct serial communications now, typically with Modbus RTU protocol over an RS485 comm port. By using serial comms, anything the VFD CPU can see can be read by the PC in real time. What brand of VFD do you have?

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
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[sajjadtts]

Hi,
I want to connect PC as HMI with Mitshubishi PLC. Please help and inform suitable Softwatre and communication procedure.

Thanks,
Sajjad Hyder

 

[jraef]

That would need to come from the VFD manufacturer. Most PLCs will offer adaptation to the various standard protocols, but only the VFD mfr knows what protocol they use.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[ozmosis]

spaaw
if your VFD is capable, it might be better to measure torque for this application than just current.