PowerFlex 525 and Safe Torque-Off Circuits

Cydog

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Good Morning ,

I am looking into a project to replace 17 drives with 17 Powerflex 525 drives. I would like to use the Safe Torque-off circuit. I would like to keep the drives powered up , even when a guard is opened , or an Estop is pressed. What is the safest way to do this using the Safe Torque off circuit on the Powerflex 525 drive . I imagine it is quite reliable , correct ? I imagine what shortens the life of a frequency drive the most is powering it up many times , correct ? Do you folks think just using the Safe Torque Off is the best way to go ? I seen some safety circuit examples on the Rockwell Automation website.

Thanks so much for your advice.
 
You should definitely use STO, that's what it's for.

In general, modern drives that have STO should not have the power turned off and on repeatedly, as it will shorten the life. Jraef or one of the other drive gurus can probably describe it more accurately/correctly, but the general gist is that seeing as you have STO and no longer need to switch power to the drive repeatedly, the power components that handle the drive startup come under considerably less strain now that power stays on all the time and you just stop the drive safely with STO. So, the drive manufacturers make those components smaller and cheaper to save cost.

If you choose not to use STO and cycle the power to the drive a lot, you'll probably find that those components are not really designed for it and will likely fail prematurely.
 
If you use an external power supply and have multiple drives you can energize the safe off inputs and the enable for the drive thru your safety circuit and then I always go thru the local disconnect as well. I also set parameter T105=1 to disable the drive from faulting if the safe off is open.
 
I imagine it is quite reliable , correct ?

Yes. The exact technical reliablity level depends on what kind of devices (safety relay or safety controller) you wire into the Safe-Off inputs.

It will make your head spin a little, but Rockwell says the Safe-Off feature is good for Performance Levels PL(d) or PL(e).

The PowerFlex 525 Safe-Torque-Off function is suitable for use in safety applications up to and including Category 3 /PL(d) according to EN ISO 13849-1 and SIL 2 according to EN 62061 / EN 61800-5-2 / IEC 61508.

In addition, the PowerFlex 525 STO function may be used together with other components in a safety application to achieve an overall Category 3 / PL(e) according to EN ISO 13849-1 and SIL 3 according to EN 62061 and IEC 61508.

I have a machine that could tear me in half when I enter the workcell, and I regularly padlock over the e-stop button because I know it's connected properly to the STO inputs. Other folks might go for the additional degree of safety that comes with an output disconnect and a second lock.
 
You cannot get PLe with the STO input to a 525 alone, you would need for example a contactor in series and enough diagnostic coverage so that the probability of both means failing at the same time is very low.

Other VFDs have STO that can be used as PLe.

Oh, and both PLd and PLe is more than just component usage, you need documentation and testing and sign offs and what have you.

I think you need to do a risk assessment and decide what safety performance level you need for your guards.
 
Thank you all for your help . AustralIan , would do you mean by PLd and PLe ?
I think before you undertake a safety plan implementation, it would be a good idea to understand the safety standards you need to adhere to. SIL refers to Safety Integration Level, PLa, b, c, d, e is the Performance Level of a safety system. They are from different safety standards and you should determine which one (or both) that you need to satisfy in your application and location.

Here is a good place to start, TUV is one of the premier safety testing agencies throughout the world.

https://www.tuv-sud.co.uk/uploads/images/1397220180236544250395/sil-or-pl.pdf
 
The existing control safety circuit is Category 3. I may go ahead and purchase and place contactors between the PowerFlex 525 output and the motors. I will also run the S+ , S1 and S2 thru the contactor , along with breaking the wires going to the motor. I guess to is alright to use a regular contactor , correct ?

Thanks for the link to TUV . Safety evaluations are more complicated than I thought.

Thanks again for your expert advice.
 
I'm off home now and while I don't have near enough time to give to this thread as I'd like; still, having read the mention of using contactors after drives, while also using Safe Torque-Off (STO) on the same drives, I just want to throw this into the mix...

This is straight from my copy of Rockwell's "MACHINERY SAFEBOOK 5" here on my desk...

Rockwell said:
Drives and Servos

Safety rated drives and servos can be used to prevent rotational energy from being delivered to achieve a safety stop as well as an emergency stop.

AC drives achieve the safety rating with redundant channels to remove power to the gate control circuitry. The redundant channels are monitored by either external or integral logic depending on the type of drive. This redundant approach allows the safety rated drive to be applied in emergency stop circuits without the need for a contactor.

The Servo achieves a result in a manner similar to the AC drives by using redundant safety signals which are used to achieve the safety function “safe torque-off”.

Also, the traditional Safety contactors are classed as "Safety actuators", of which, drives and servos that support embedded or add-on STO are also included i.e. either/or may provide the necessary Safety actuation for a Safety Function...

Rockwell said:
Safety actuators

Safety contactors and Starters

PowerFlex® AC drives
PowerFlex drives are available with safety features. The PowerFlex 525 AC drives include embedded Safe Torque-Off as a standard feature. Safe Torque-Off is an optional feature for the PowerFlex 40P, 70, 700H, 700S, and 750-Series AC drives, which also support Safe Speed Monitor functionality.

Kinetix® integrated motion
Kinetix 300, 6000, 6200, 6500 and 7000 servo drives all feature built-in safety functionality. With Safe Torque-Off, a drive output is disabled to remove motor torque without removing power from the entire machine. Safe Speed Monitoring permits users to reduce and monitor the speed of the application to help an operator safely perform some types of work without completely stopping the machine.

On your mention of also wiring the drive STO signals through the contactors - this should not be necessary as these signals would be interlocked through the Safety relay so you should not have to use contactor auxiliaries to double up here. If the Safety relay has de-energized then its output contacts will have opened, which will turn off the STO channels.

I haven't time to get into the Performance Level (PL) side of things too much, but essentially, and in keeping with Ken's quoted information, the PowerFlex 525, using Safe Torque-Off, can achieve SIL 2 Category 3 /PLd alone. To achieve SIL 3 Category 3 / PLe, STO would have to be used in conjunction with other Safety measures to achieve this higher Performance Level.

You need to know which Performance Level (PL) you require here before deciding whether or not it is permissible to use STO only on these drives.

That is, you may or may not need Safety contactors here. PLd, for instance, may permit the use of no contactors here.

You also have to consider the cascading arrangement for the STO on this many drives (17). Can you load one Safety relay with this many STO connections in parallel or must you split them over several Safety relay contacts, or must you use more than one Safety Function, etc.? Is there Zoned Safety implemented or now required?

Using STO you will achieve a Category 0 uncontrolled stop for each driven motor. Is this acceptable from a risk or hazard point of view?

These, and other questions, are really only best answered by a thorough Risk Assessment. Just because the current installation is Category 3 / PL(?), does not mean your changes do not warrant a re-assessment. It may still end up being that Category 3 / PL(?) is sufficient. But you must determine this once more and not just assume that the current PL will still be what is required after the change. It "probably" will be, but you should not just assume it.

Cydog said:
...Safety evaluations are more complicated than I thought...

Unfortunately, this is an all too often response in these situations.

Regards,
George
 
Last edited:
The existing control safety circuit is Category 3. I may go ahead and purchase and place contactors between the PowerFlex 525 output and the motors. I will also run the S+ , S1 and S2 thru the contactor , along with breaking the wires going to the motor. I guess to is alright to use a regular contactor , correct ?

No offence intended. You need to stop immediately and get a professional in. You are out of your depth.
 
Since STO circuit on the powerflex 525 is only PLd, maybe you should use another drive instead.

I know ABB have PLe on their STO circuits and I'm sure others as well. That's as good as it can get. No additional components needed.
 
Pete.S. said:
Since STO circuit on the powerflex 525 is only PLd, maybe you should use another drive instead...

Hi Pete,

"only PLd" would suggest that this Performance Level is both generally inadequate in most cases, and/or specifically inadequate in our friend's case here.

With regard to PLd in general terms of adequacy - PLd is considered a "control reliable" Performance Level, as is PLe. PLd is only one degree of assessed possibilty of, or probability of avoiding or limiting exposure to harm, away from being PLe. PLd is widely used in Safety Function implementations and is considered a highly reliable level of risk reduction, where a Risk Assessment has determined that this Performance Level is required.

With regard to PLd, or even PLe in specific terms of adequacy to our friend's scenario - we do not know what Performance Level is required here. So we cannot say that using PowerFlex 525 drives with STO, which may "only" be used up to PLd, is inadequate here and that other options, which may be used up to PLe, should be considered.

Until they/we know what PLr is required here, we are all just explaining the options, I suppose.

Regards,
George
 
Powerflex 527 has built in PLe STO, both hardware or networked.

The powerflex 750's have option cards for PLe STO either network or hardwire.
The PowerFlex 750's also have an option for PLe Safe Speed, which may be a requirement for different applications.

A risk assessment will determine the Performance Level Required, but there is nothing stopping you from using the highest performance level of component.
 
PF525 Pld...maybe PLe...?

Hi guys...

FYI you can get PLe from Cat 3 depending on the DC (Med) and MTTFd (High) as shown in the 13849=1 standard - Annex K and Figure 5. There’s a little overlap.

Also if you look at the PF525 safety data - it has a DC value of Medium, and an MTTF of 3593 and if you use MTTFd = MTTF x 2 / 10 you can get a High value (over 100 years)...so if you also apply that to the Figure 5 and Annex K table of 13849-1 it can get PLe as well? Could be an argument as well...?? Just saying lol.
 
One thing to bear in mind which I've recently discovered when using STO with the PowerFlex 525 is that the drive goes into fault when the safety circuit fails. Not only does it require a reset of the safety circuit but also a fault clear command to the drive before it is ready for use again. I have automated the fault reset in my code when the safety circuit becomes healthy but I find this a bit of a pain. I'm not sure if there's another parameter in the drive that can be set to prevent this?
 

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