STO (Safe Torque Off) and motors installed in Explosive Atmospheres 1

[JuanBC]

Hello,

I would appreciate your experience regarding the following situation

Context:
We have a 260 kW (380V) motor which is fed from a VFD. Motor moves a pump for gasoline export through a 50 km pipeline.

The motor has 2 PTCs that according to the VFD's manufacturer should be wired to an "additional board" and not to "normal analog inputs" because the motor is ATEX (IEC Ex "nA" - to be used in Zone 2)

Reading an "ATEX manual" (attached) that the manufacturer sent, it seems that this "additional board" is intended to trigger the STO function of the VFD.

For which I ask myself: is this function necessary? Or can the motor stop freewheel?

Is there another reason why you think that an additional board would be needed?

Thanks in advance,
Juan

JBC
.......
"The more I read, the more I acquire, the more certain I am that I know nothing"

 

[BrianPetersen]

"Safe torque off" is just that; when the STO enabling inputs go down, the drive can no longer generate alternating current, thus the motor can not generate torque (this is different from saying that it will "stop", although perhaps it will stop, eventually, after freewheeling to a stop).

STO inputs are normally provided by a safety component (safety relay, safety PLC, safety-rated remote I/O card, etc) which will be expecting redundant dual-channel input signals and provides redundant output signals. I am thinking that the "additional board" that you are talking about, is a device which takes in the signals from BOTH temperature sensors, compares them for plausibility and self-checks as needed, and then provides a suitable safety-rated interface signal to the drive's STO inputs.

They will not want you bringing those signals to a "normal analog input" because normal analog inputs carry no safety ratings - there is no "PL" or "SIL" rating. There is no fault protection, no protection against tampering, etc.

Do it the way the drive manufacturer wants you to.

ISO 13849-1 (where the PL ratings for safety-related parts of control systems is coming from) is not at all easy to read or understand.

 

[JuanBC]

Hi Brian,

I was definitely wrong about STO (I thought it was a "controlled stop")

Thanks for the clarification.

This drive has two STO inputs (STOA and STOB). I understand that if any of them goes to "0", "STO function" will be activated and thus drive will no longer generate alternating current, am I right?

Regards,

JBC
.......
"The more I read, the more I acquire, the more certain I am that I know nothing"

 

[BrianPetersen]

Yes, those are dual-channel safety inputs that are meant to be switched simultaneously by the upstream control circuit, in order to provide redundancy in the circuit.