I'm looking at a simple system consisting of a VFD with safe torque off (probably going to be a Nidec Control Techniques Unidrive M300) and one e-stop button, both running dual channels. As I understand it, standard practice would be to connect the e-stop button to an e-stop relay, and the outputs of the e-stop relay to the STO inputs of the drive.
Assuming I'm using a basic safety relay, not one of the fancy ones with cross circuit detection, pulsed outputs, or sequence monitoring, what does the e-stop relay add safety-wise?
With a safety relay, I hit the e-stop button. No single failure prevents it from cutting power to the safety relay on at least one channel. When that happens, power to the STO inputs is cut and we get a category 0 stop. To restart, release the e-stop button and press reset.
Without a safety relay, I hit the e-stop button and it cuts power to the STO inputs. To restart, release the e-stop button and press reset.
The main difference I see is whether the reset button is connected to the safety relay or to the drive, both of these would appear to meet the requirement that releasing the e-stop button doesn't restart the motion, but merely permits the motion to restart. Is there a significant difference between the reset circuits behind the scenes?
Assuming I'm using a basic safety relay, not one of the fancy ones with cross circuit detection, pulsed outputs, or sequence monitoring, what does the e-stop relay add safety-wise?
With a safety relay, I hit the e-stop button. No single failure prevents it from cutting power to the safety relay on at least one channel. When that happens, power to the STO inputs is cut and we get a category 0 stop. To restart, release the e-stop button and press reset.
Without a safety relay, I hit the e-stop button and it cuts power to the STO inputs. To restart, release the e-stop button and press reset.
The main difference I see is whether the reset button is connected to the safety relay or to the drive, both of these would appear to meet the requirement that releasing the e-stop button doesn't restart the motion, but merely permits the motion to restart. Is there a significant difference between the reset circuits behind the scenes?