Greetings, I have a query regarding the design of a safety circuit's output side. I have a setup consisting of 7 VFDs, each with 2 safety contactors connected in series on the load side. All VFDs' output contactors will be controlled by the same safety events. I plan to use separate output channels from my Safety PLC to switch each safety contactor per VFD. My question is: Is it advisable to use the same 2 outputs for all VFD contactor pairs (total of 2 outputs), or should each VFD contactor pair have its own 2 distinct outputs (total of 14 outputs)? I hope this explanation is clear. Thank you.
When setting up your system, it is essential to check the rating of the output points and the inrush/holding current of the contactors. Additionally, it is recommended to incorporate a feedback loop for safety integrity monitoring with each set of safety contactors to detect welded contactors. If all contactors are driven from one pair of outputs, the feedback loop must pass through all 14 contactors. In this setup, a single stuck contactor can prevent a reset for all 14 contactors. Alternatively, by wiring each drive to individual pairs of safety outputs, each contactor pair's feedback loop can be connected to its own safety input, ensuring that a single contactor fault only impacts that specific motor. While safety I/O can be costly, it is worth considering this approach for improved safety monitoring. Typically, a single pair of safety contactors is used for a single safety function, with Safe Torque Off wires paralleled off the coils for all VSD's. However, this setup may not be sufficient if your drives lack STO capabilities or if the required Performance Level exceeds the drives' STO rating.
When setting up safety contactors, it is essential to check the output points' ratings and the contactors' inrush/holding current. It is also advisable to incorporate a feedback loop for safety integrity monitoring, such as welded contactor detection. If all contactors are controlled by a single pair of outputs, a loop through all contactors may be necessary, causing a single stuck contactor to affect all 14 contactors. Alternatively, wiring each drive to individual pairs of safety outputs allows for each contactor pair to have its own feedback loop, ensuring that a single contactor fault only affects that particular motor. While safety I/O can be costly, considering this approach is worthwhile. Typically, a single pair of safety contactors feeding the DOLs with Safe Torque Off wires for VSDs is seen for a single safety function. However, depending on the PL rating desired, verifying the safe state before reset is crucial. This means ensuring that each drive is off and not running before resetting. Implementing individual safety output resets for each drive or a collective reset for all 14 drives is essential. If cost and rack space are not constraints, opting for individual safety output resets is recommended. This setup allows for potential future zoning adjustments and ensures that a single safety contactor failure does not impact all drives simultaneously, providing more flexibility and reliability in the system.
Based on your requirements, it seems more precise and safer to allocate unique outputs for each pair of VFD contactor. This means assigning a total of 14 outputs. This configuration would offer you granular control: you can oversee individual safety incidents, enhance diagnostic capabilities, and permit a more structured troubleshooting process should any issues arise. A fault in a single VFD can thus be addressed without disrupting the entire system operation. The initial setup might take more effort but it can potentially save you time and resources in the long-run.
In terms of safety circuit design, it's critical that any failure is localized to ensure safe operation. So, if you use the same two outputs for all contactor pairs and one pair malfunctions, the rest of your system would be affected, potentially increasing risk. On the other hand, allocating distinct outputs (14 in your case) to each VFD contactor pair creates an individually controllable setup, which could isolate issues and improve overall safety. Yet, it also depends on your controller’s capabilities and your system's complexity. Plus, more wiring may be involved, which could influence costs and maintenance. Therefore, while it seems beneficial to have distinct outputs for each pair, you might want to balance these factors to suit your specific system requirements.
From a safety perspective, having each VFD contactor pair managed by its own distinct output channels is the more reliable solution. If something goes wrong with one VFD's safety contactors, you will be able to isolate the problem without affecting all the other units. This design offers improved fault isolation and potentially minimizes downtime across your entire system. However, it will consume additional outputs from your Safety PLC and possibly increase your system's complexity. Balancing safety priorities with system efficiency and cost is key in making such decisions.
Hi there! Your topic is quite interesting. From a safety perspective, I would suggest having separate outputs for each VFD contactor pair. This allows for individual control and isolation, which can be beneficial in troubleshooting and potential safety events. It may take up more outputs and complicate the wiring side of things, but from a control and safety viewpoint, individual separation seems more advisable. Although, this largely depends on the specific needs of your system and other factors like local regulation standards and company safety protocols. Ultimately, you should weigh the advantages and disadvantages and decide which option fits better with your setup. Hope this helps!
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Answer: - It is advisable to use separate output channels for each safety contactor per VFD to ensure independent control over each VFD contactor pair. This approach would require a total of 14 outputs for all 7 VFDs.
Answer: - Using distinct outputs for each VFD contactor pair provides individual control and monitoring capability, allowing for better safety and flexibility in managing the system.
Answer: - When designing safety outputs for VFD contactors, factors such as redundancy, independence, and fault detection should be taken into account to ensure the overall safety and reliability of the system.
Answer: - When connecting safety contactors in series on the load side of VFDs, ensure proper wiring, coordination with the Safety PLC, and adherence to safety standards to maintain the integrity of the safety circuit.
Answer: - The priority should be on ensuring the safe operation of the system by implementing a robust safety circuit design that offers redundancy and individual control over each VFD contactor pair
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