VFD Safety Circuit

Vager

Member
Join Date
May 2018
Location
NY
Posts
8
Hello everyone,

Attempting to figure out the simplest way to add a safety circuit to shut off conveyor motors when the covers are opened during operation. Am I on the right track using a safety relay to drop power via safety contactors or is there a way to take the contactors out of the equation and interface with the power flex VFD's?
 
It depends on the model of PowerFlex VFD and what degree of risk / safety standard you are trying to meet.

Reliably preventing the output transistors from running is the heart of the "Safe Torque Off" technology that Rockwell (and other vendors) have added to VFDs in recent years.

Without those technologies, contactors were the only way to be reliably stop a VFD in a way that was robust enough to meet various safety standards.

But with "Safe Torque Off", whether hard-wired or done over a safety network, you can do it without contactors or other isolation devices.
 
Thanks Ken!! Thats exactly what i was looking for.

Gomez,
I planned on using somekind of non-contact switch so i’ll be sure to check those out.
 
Powerflex 700 to the best of my knowledge but i will have to verify that tomorrow.
 
You need to know exactly what drive you are working with to know if it has a "Safe Off" option module available. I know the 70EC and 700S did, but I don't think the ordinary PowerFlex 700 drives had one.

Here's a good RA document discussing various methods for interfacing safety relays with drives, to meet various degrees of safety operation:

https://rockwellautomation.custhelp.com/ci/fattach/get/1174886/0/filename/drives-ap003_-en-p.pdf

Safety circuit design is never a one-size-fits-all matter. You absolutely need to analyze the whole automated system and do a proper risk assessment.
 
Do a risk assessment to determine how reliable your safety circuit needs to be.

Document your risk assessment.

Document your analysis of your controls design and how it meets your required safety performance.

Test it on site and document if it performed as required.
 
These days, unless you're talking a very simple safety circuit, I usually go for a programmable safety controller over a hard wired controller, that way when someone wants to change something, it's easy. My controller of choice is the Omron G9SP (20 inputs, 10 outputs) however sadly not expandable for safety IO.

As others have said, you need to know if you have Safe Torque Off or not, that'll change everything.
 
If you do have a PowerFlex 700, there is no STO option for it, you will have to use contactors with a safety relay. Just understand all the risks with using contactors on VFDs. If you put them on the line side and open them every time you power down, you will stress the pre-charge resistor and shorten it’s life, which leads to drive failure. If you put the contactors on the load side, you must interlock them with the Disable circuit of the drive to turn off the transistors before the main contacts open, otherwise you risk damaging the output transistors. I haven’t read it recently, but I believe the document that Ken Roach linked to explains how to accomplish this.
 
If you do have a PowerFlex 700, there is no STO option for it, you will have to use contactors with a safety relay. Just understand all the risks with using contactors on VFDs. If you put them on the line side and open them every time you power down, you will stress the pre-charge resistor and shorten it’s life, which leads to drive failure. If you put the contactors on the load side, you must interlock them with the Disable circuit of the drive to turn off the transistors before the main contacts open, otherwise you risk damaging the output transistors. I haven’t read it recently, but I believe the document that Ken Roach linked to explains how to accomplish this.

Jraef, thanks for providing a very clear and succinct answer to this question. You seem to make a habit of doing this.
 
To help clarify it a little the drive safe in a BFD is safe Torque only. Meaning the VFD output transistors are turned off and blocked off.
But keep in mind all solid sate devices leak even when they are turned off the motor can not move but their will still be power on the motor leads (motor lead to ground). You could still get a bad shock if you come in contact with the motor leads.

There is no substitute for a good air gap disconnect switch.
 
Unfortunately I do not have the STO option on these drives so it looks like I'm stuck with the contactor and programmable safety relay strategy. Currently the circuit already has a disconnect with early break contacts which operate the coil of another contactor. Both of these are located between the VFD and the motor. As of now since I have to drop power to multiple VFDs at once I plan on using a safety contactor in series with several of the early break contacts to disable the drive circuits and at the same time de-energizing the contactors located between the disconnects and the VFDs. If I'm interpreting this thread correctly that configuration should stop the motors and not damage the VFDs.
 
To help clarify it a little the drive safe in a BFD is safe Torque only. Meaning the VFD output transistors are turned off and blocked off.
But keep in mind all solid sate devices leak even when they are turned off the motor can not move but their will still be power on the motor leads (motor lead to ground). You could still get a bad shock if you come in contact with the motor leads.

There is no substitute for a good air gap disconnect switch.

Right when you need to replace a motor or make a scheduled maintenance with a machine already stopped, locking the feeder switch is the way to go.

But when you need to stop a loaded and running machine for emergency, a disconnect switch could be dangerous to open and cannot be connected to a safety circuit. A contactor will make it, but as described above, cost and space is wasted as long as the need to be sync with the VFD .....
The STO take care of that without extra cost....

Most of the time, the safety circuit isn't for peoples that work safe but against peoples that try to save time and by-pass safety rules to do dangerous things. A disconnect switch won't work with them.
This is why we also need devices that cannot be easily defeated.
 
Right when you need to replace a motor or make a scheduled maintenance with a machine already stopped, locking the feeder switch is the way to go.

But when you need to stop a loaded and running machine for emergency, a disconnect switch could be dangerous to open and cannot be connected to a safety circuit. A contactor will make it, but as described above, cost and space is wasted as long as the need to be sync with the VFD .....
The STO take care of that without extra cost....

Most of the time, the safety circuit isn't for peoples that work safe but against peoples that try to save time and by-pass safety rules to do dangerous things. A disconnect switch won't work with them.
This is why we also need devices that cannot be easily defeated.

Many disconnects can be outfitted with aux contacts that are “early opening”. Sometimes these contacts can be used with VFD’s that have the safe-off modules.
 

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