Allen Bradley VFD's

Can you send a 4/20 mAMP speed signal to one Allen Bradley VFD, IE: powerflex 40, 70, or 700 and have a second identical drive run the same speed as the first. Is this possible? If so what is the best way to do this.

Thanks
Bob

Are the drives networked in any way? (i.e. Ethernet, devicenet, etc)

Perhaps this thread will help:
http://www.plctalk.net/qanda/showthread.php?t=646

Thank you very much. I think I will connect the current loop to one drive and use its speed output to control the second drive.

Again Thanks.
Bob

bobwithdana said:

Thank you very much. I think I will connect the current loop to one drive and use its speed output to control the second drive.

Again Thanks.
Bob

Click to expand...

Look at the drive parameters, you can bias the speed output also.

bobwithdana said:

Thank you very much. I think I will connect the current loop to one drive and use its speed output to control the second drive.

Click to expand...

If you do it THAT way you won't have true redundancy. That is, if the FIRST VFD fails, the second may not be able to take over the load. Depending on your process this may not be an issue, but if for example you are running pumps, a more desirable design plan would allow either VFD to work independently of the other.

An analog loop splitter or two analog channels from the PLC with the same value in their respective registers, and therefore the same analog value out, would provide two SEPARATE analog signals, one to each VFD, regardless if both are functional, powered up, or even if one is out for repair.

Keeping a spare splitter, or a spare analog output card on hand would be much cheaper than keeping a spare VFD on the shelf.

Just some food for thought.

Stationmaster

I think you can use an analog output signal from the first drive and connect it to the analog input of the second drive. Then you can adjust your scaling to match the speeds. I have seen it done before when a customer wanted to track one conveyor to another.

can't you just wire the vfd 4-20's in series?

Lacking any kind of fieldbus linkage, I would convert the 4-20ma to 2-10V with a 500 ohm resistor and then use the voltage signal to both drive's speed input in parallel.

That way, you can loose one of the drives, even remove it, and the remaining one keeps running. That may be an advantage. If not, the daisy chain arrangement would be ok.

If both drives have to run together, you should think about cross-linking the fault relays as well so, if one drive faults, the remaining one can't run.