Safety contractors and drives.

For those of you using safety contactors do you put the contactor on the primary side of the drive or on the secondary between the drive and the motor. I have heard pros and cons of both ways. One of the cons is cycling the drive each time a estop is hit shortens the life of the drive. I was told that if its on the secondary, it is best practice to hardwire the enable signal. Also if the safety contactor ever welds shut this could be another potential problem. The current drives I am using are Ethernet drives. Next time I will go with safe-off drives but that's another's story.

I have no hard evidence to prove it, but it seems to me that we've had to replace more drives (servo and VFD) in cases where the contactor is located on the line side, and drops with an E-stop. Most manuals I've read warn against cycling the line power too often. When I have brought the issue up with OEMs that provide us with machinery containing drives in this configuration, they argue that an E-stop should be used as exactly that--an emergency stop--and thus should not be activated very often. Unfortunately that just isn't reality at our facility.

My usual practice is to put the contactor on the load side of the drive. The contactor's coil is driven by an off-delay safety relay. When an E-stop is triggered, the drive enable is dropped immediately (or whatever is required to provide a controlled stop), then the contactor opens about 0.5 seconds later. Depending on the application, the drive enable signal might be a PLC output, or simply tied to the E-stop circuit. If a PLC output is used (so that the drive can be disabled during normal operation, if necessary) then I would typically power the output module common from the E-stop circuit, so that safety isn't dependent on the PLC function.

I've been doing it this way for 10+ years with very few failures.

Here are some threads with similar discussion:
http://www.plctalk.net/qanda/showthread.php?t=38141
http://www.plctalk.net/qanda/showthread.php?t=44595
http://www.plctalk.net/qanda/showthread.php?t=72286

The safety scheme I design depends on the safety assessment of the machine.

If I absolutely have to kill the power to a motor "now" than I'll put the safety contactor in front of the motor and make sure there is a mechanical brake. see NFPA 79 category 0 Stop. Can be kind of rough on the electronics though. If "quick" allows, I like to send a stop signal to the drive before disconnecting its load.

If the time allows, and I'm not using a safe enabled drive, I'll drop the power to the drive. With some time to allow the drive to fully turn off before bringing power back on. see NFPA 79 category 1 stop.

The drives with the safe enable input are the easiest to work with. You can satisfy cat 0 without being to rough on the electronics but small drives are not usually available with this feature.

Again, the safety assessment can save a lot of money, in electronics and attorney fees if it ever comes down to that. Its always nice for the machine builder to have that document if OSHA ever comes knocking because of a problem at the customer's site. But we've never seen a customer disable a safety feature, have we?

I have several thousand drives here and I find the same data as Kolyur that I have more drive failures in general when done from the line side.

On the load side you just have to make sure the drive output is not longer conducting before you break it's path to load to prevent spiking your IGBT's.

I use the method of dropping the drive enable on estop and delaying the contactor dropout then signal when the contactors have dropped out with a light and by releasing the guarding gate.

Here is a link to what I use http://www.ab.com/en/epub/catalogs/...28112/Bulletin-100S-C-104S-C-Accessories.html

I agrewe with the above posts on risk assessment !st. Iusually find that I can use an immediate enable via hard wiring from the E-Stop relay , but I use an AB Safety delay timer (0.5 to 3.0 Sec) to drop the safety contactors out.
Paul

The best practice is to use contactors on both the line and load side -- but that's expensive.

We have always put a single large safety contactor in front of all drives in our control panel. When an e-stop is pressed, they all shut off. There is also circuitry to prevent the contactors from pulling back in until all of the drives have been off long enough for them to full power down.

They don't seem to like powering up if they aren't full powered down first.

Bit_Bucket_07 said:

The best practice is to use contactors on both the line and load side -- but that's expensive.

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What is your reasoning for calling this "best practice"? I realize that having contactors in series can provide redundancy for safety, but what's the logic behind putting one on each side of the drive? Why not both on the line side or both on the load side?

kolyur said:

What is your reasoning for calling this "best practice"? I realize that having contactors in series can provide redundancy for safety, but what's the logic behind putting one on each side of the drive? Why not both on the line side or both on the load side?

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Opening the load side circuit of a drive while it is running will damage the drive. A single instance may not destroy the drive, but the effects of contact arcing on the drive's output circuits are cumulative. Ideally, one should use an early switching auxiliary contact on the safety contactor to disable the drive output prior to opening the main contacts on a load side safety contactor.

Opening the line side as well provides redundancy and removes power from the drive. However, placing a contactor only on the line side will not effectively remove power from the motor circuit on a typical VFD and will therefore not satisfy the E-Stop requirement.

kolyur said:

What is your reasoning for calling this "best practice"? I realize that having contactors in series can provide redundancy for safety, but what's the logic behind putting one on each side of the drive? Why not both on the line side or both on the load side?

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If the risk requires it, I would still put both contactors on the output side with reset monitoring...I use safety contactors (force guided) like ThePLCKid. I have not seen a failure yet, but again, we disable the drive before or at the same time that the contactor opens, so the contactor is not suffering from abuse unless something breaks...

With two safety contactors (output side) in series and both monitored by the reset, it would take a major miracle for the circuit to fail in an unsafe state and I see no possible way to improve upon that by killing drive power...killing drive power may be cheaper out of the gate, if you can knock out ten of them with one set of contactors, but cheaper and smaller ain't always better.

I have yet to see a safety circuit which had both upstream and downstream isolation contactors. There are plenty of things I am sure I have not seen, mind you, but I would expect that type of arrangement to be very rare.

OkiePC said:

If the risk requires it, I would still put both contactors on the output side with reset monitoring...I use safety contactors (force guided) like ThePLCKid. I have not seen a failure yet, but again, we disable the drive before or at the same time that the contactor opens, so the contactor is not suffering from abuse unless something breaks...

With two safety contactors (output side) in series and both monitored by the reset, it would take a major miracle for the circuit to fail in an unsafe state and I see no possible way to improve upon that by killing drive power...killing drive power may be cheaper out of the gate, if you can knock out ten of them with one set of contactors, but cheaper and smaller ain't always better.

I have yet to see a safety circuit which had both upstream and downstream isolation contactors. There are plenty of things I am sure I have not seen, mind you, but I would expect that type of arrangement to be very rare.

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I have seen them in applications in which the VFD also has a bypass contactor option. The primary purpose of the load side contactor in these applications is to isolate the drive when operating on the bypass contactor, but it is also dropped out for E-Stops.

When using drives with built in STO like the new 525's what is the best way to disable 20 drives as it requires 2 contacts per drive ch1, ch2 so it would require a safety relay expandable upto 40 safety contacts, so am I right in thinking you can common these up? And would this de-rate the safety category?

There are safety relays with expandable contacts but with that many do it over CIP safety and avoid all the hard wiring.

You could also gang multiple smaller relays in zones or for each drive.