I am tasked with setting up the STO function on 2 PF525 drives for a client. While I have limited experience with this feature, I am determined to ensure its successful implementation. From my understanding, the STO inputs on the drives are connected to an external safety relay using the NO contacts. Initially, I thought of placing the E-Stop on the coil side of the safety relay, but this doesn't seem like the most efficient approach. Why introduce an additional relay when the E-Stop could directly control the function? It also worth noting that the drives will be operated via ethernet connection.
Safety relays are equipped with intelligence to conduct internal tests for short circuits and other potential hazards. This feature is often described in the product manual, recommended by the manufacturer to ensure optimal safety measures. Adhering to the manufacturer's guidelines can protect against unforeseen problems in a worst-case scenario.
Ensuring the functional safety of machinery goes beyond just the wiring of e-stop pushbuttons to motor drives. In the past, the standard practice was to connect an e-stop button with a pull or push-to-reset mechanism to an Enable pin on a drive logic board. However, issues such as shorts in the wiring or a malfunctioning circuit could prevent the e-stop button from effectively stopping the machinery. To address these concerns, safety relays with dual channels, short-circuit and cross-circuit detection, feedback monitoring, and manual reset inputs are employed. It's essential to have a thorough discussion about safety relay options, such as the Guardmaster 440R, safety contactors, or force-guided relays like the 700S-CF contactor or 700-HPS ice cube style, when implementing Safe Torque Off (STO) for the first time. This proactive approach ensures that the safety aspects of the application are properly addressed during the overall risk analysis of the machine system.
Ken provided excellent insight into the topic. I want to elaborate on a few key points that are important when discussing anything beyond a basic device with inputs, logic, and outputs, as mentioned by Ken. These include channel-to-channel shorts, cross wiring, and shorts to 24VDC. These are crucial considerations to keep in mind when working with such devices. The Safety Torque Off (STO) function is designed to cut off the firing signal to the drive output, allowing the drive to smoothly come to a stop. However, this feature can pose a potential issue when dealing with machinery like saws or other equipment with hazardous motions, where a sudden stop could be problematic. In some of my previous applications, I used a stop command to bring the motor to a standstill before activating the STO function. By prioritizing safety features like STO, you can position yourself to tap into a new market segment focused on safety-conscious customers. This proactive approach can open up opportunities for growth and expansion in your business.
Jeff Kiper discussed the importance of the STO, or Safe Torque Off, feature in drive systems. The STO removes the firing signal to the drive output, allowing the drive to free wheel to a stop. However, this could pose a safety issue in situations involving hazardous equipment like saws. In some applications, a stop command is used to bring the motor to a standstill before activating the STO for added safety measures. The concept of safety in drive systems can sometimes be unclear. While Siemens documentation states that STO is the main safety function, there are drives with SS1 (Safe Stop 1) functionality that stop the drive before engaging STO. Yet, according to manuals, SS1 is not considered a safety function until STO is activated. In practical terms, this distinction may not be noticeable in most machines, but in cases of equipment like fans or centrifuges, the time it takes for natural stopping can be prolonged.
Do not disregard the importance of VFD STO, as it offers the same functionality as using contactors to shut off power to the motor. In both scenarios, the motor will come to a stop on its own due to inertia. Whether this is a viable option should be determined in the risk assessment process. A VFD STO can provide a high level of performance, such as achieving PL=d or PL=e with Siemens G120 and its integrated STO. Implementing redundant contactors may be necessary to reach these safety levels. Refer to the VFD documentation for information on what can be accomplished and how to set it up. Additionally, take into account the advice from Ken regarding the use of a safety relay for added protection.
I understand your point about keeping the setup direct and eliminating unneeded components. However, the safety relay acts as a guarding component in most STO, or Safe Torque Off, setups. Should the E-Stop be triggered, the safety relay ensures that the shutdown is executed within acceptable safety measures. Also, keep in mind that while the PF525s can technically be operated directly via Ethernet, the STO function necessitates some form of hardwired safety input. This helps to ensure redundancy and robustness in your safety system. Such crafting in the safety functions ensures that unexpected failures are dealt with in the fastest and most reliable manner.
I see where you're coming from, it indeed seems like adding an extra step. However, there's a good reason these systems are set up this way. The safety relay acts as a layer of protection not just for the system but more importantly, for the people interacting with the system. If something fails with the E-Stop and it's wired directly to the STO, then you lose that safety feature entirely, increasing risks. Using a safety relay gives yet another opportunity for the system to catch a failure and respond appropriately. Additionally, safety relays can usually handle more current than E-Stops, which can further protect the circuit and prevent potential faults. Ethernet operation should hardly affect this setup. Simply ensure the setup is compliant with safety regulations.
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Answer: - To set up the STO function on PF525 drives, connect the STO inputs on the drives to an external safety relay using the NO contacts. Consider directly connecting the E-Stop to control the function without introducing an additional relay for more efficiency.
Answer: - Yes, instead of using an additional safety relay, you can connect the E-Stop directly to control the STO function on PF525 drives for a more streamlined approach.
Answer: - The STO inputs on PF525 drives should be connected to an external safety relay using the NO contacts. Consider optimizing the setup by directly connecting the E-Stop to control the STO function.
Answer: - Operating PF525 drives via an ethernet connection does not directly impact safety integration; however, it is essential to ensure that the safety features, including STO and E-Stop, are properly configured and integrated into the system for safe operation.
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