VFD enable signal, parallel connection

orionk · Dec 22, 2011

Need some opinions on the following:
I have 4 VFDs controlling different motors. All communication, including on/off commands go through Ethernet. For safety purposes I want to disable drives by physically breaking connection between COM and ENABLE terminals when e-stop is pressed. Question is - can I use a single circuit and connect all coms and enables in parallel? For sure all coms can be interconnected but I am not sure if connecting digital inputs would not do any harm. VFDs are Emerson Commander SK series ranging from 1 to 3HP. All located inside the same enclosure. Thanks!

bernie_carlton · Dec 22, 2011

I would keep them seperate. Use small 'terminal type' relays for isolation and drive their coils from the one signal.

brucechase · Dec 22, 2011

Actually since this is an estop, you really need to do a risk assessment and verify what you want to happen and how it is supposed to happen. Breaking a control signal for an estop is not the best method of estopping a machine.

I would consider installing a safety contactor in the motor circuit that also disables the control. This can be accomplished by using a safety controller (I use a Banner SC22). Any other method opens the possibility of not properly stopping the motor and causing a serious injury or death (especially if you use the small terminal type relays).

If you are unsure of what to do, I would seriously suggest that you get someone experienced to help you.

DickDV · Dec 22, 2011

Aside from the safety relay integrity level questions, this comes down to whether the digital inputs on the drive are optically isolated or not.

That would be a question for Emerson Tech Service.

orionk · Dec 22, 2011

brucechase : Thanks for your opinion. I know perfectly what I do and actually in this particular field there is no need for safety devices involved. I have been working in other industry where stopping the machine is an extremely important feature and I know quite well what regulations say. Once again - question is not about how to implement safety, question is weather it is safe or not to connect digital inputs in parallel on 4 VFDs.

Brijm · Dec 22, 2011

I would recommend the relays as well to isolate the inputs (safety relays or controller if personel safety is applicable). As to the safety, it depends on the application. I'm not sure about the Commander SK's, but the Unidrives have a dedicated safety enable input.

I'm going to disagree, with the contactors in line with the motor circuit. If safety is an issue, often it is safer to ramp to a quick stop, rather than remove power and coast to a stop. But you will need to do a risk assessment to determine which is proper. Safety can get very complicated, it depends on the proablility and serverity of an incident occuring.

dmargineau · Dec 22, 2011

Ensuring you are not using the VFD's internal control supply power within your circuitry, you should be able to "send" the same output to be read as an input by several VFDs.
We have a few setups where the same 2Amp relay output is being read as a Digital Input configured as "Enable" by ten AB PF70.
Just to make sure, verify that the "parallel sum" of the currents drawn by each digital interface doesn't exceed the physical output's allowable load.
You could get the information from the manufacturer's spec sheet.

TurpoUrpo · Dec 22, 2011

Quick skim trough SK's manual did not imply that external voltage could be used.

OkiePC · Dec 22, 2011

orionk; said:
For safety purposes I want to disable drives by physically breaking connection between COM and ENABLE terminals when e-stop is pressed.

orionk; said:
Thanks for your opinion. I know perfectly what I do and actually in this particular field there is no need for safety devices involved.

So, which is it? For safety reasons, or other?

brucechase · Dec 22, 2011

For safety purposes I want to disable drives by physically breaking connection between COM and ENABLE terminals when e-stop is pressed.

e-stop = emergency stop

cycle stop = controlled stop

If this is NOT safety, then just use the PLC to stop the drive.

dmargineau · Dec 22, 2011

TurpoUrpo said:
Quick skim trough SK's manual did not imply that external voltage could be used.

In this case I am afraid each drive's internal control power "hot" has to be routed through a separate relay contact and "returned" as an input.
I would not recommend "parallel-ing" power supplies, not even "identical" ones...

orionk · Dec 22, 2011

Probably I should not have said "for safety reasons". In this case safety is not a big issue. I just want to do what I used to do - cut off all the motors not by just sending a command from PLC but from removing the enable signal from the drives. Motors will stop anyhow by a command from PLC, I just want to "duplicate" the stop command in case something happens with PLC command. For those who is interested in what the system is and how it operates - I can provide electrical diagrams with field interconnects and panel layout, our process engineers would also have process description if you want. I just want to know if I can do what I want without using four relays. Maybe someone have already had this experience.
Thanks for all of you, any information is appreciated.

The Plc Kid · Dec 22, 2011

orionk said:
I just want to know if I can do what I want without using four relays. Maybe someone have already had this experience.

This was answered in post # 11. If you are using a single external powersource then there is no need for relays.

Most drives use there own internal power supplies for this and there is no option for a external power supply because of the circuit design and the way it is referenced.

You don't want to common drives together that have or are using internal power supplies as you can create differences in potential and damage drive components.

DickDV · Dec 22, 2011

I don't see the problem with using one of the drive's internal power supply as long as all the COM's are strapped together and as long as the power supply can supply enough current.

I have done jobs where we wanted this kind of arrangement to power the control circuitry and we put all four drive power supplies in parallel except that each had a series diode in the + lead. That way, the power supply with the highest voltage powered the controls and, if any one of them failed, the other three would be there to carry on.

VFD enable signal, parallel connection

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