Safety stops - a materials handling feature
When it comes to safety in manufacturing operations, the theory tends to be "better the devil you know". This can sometimes spawn over-engineered solutions, based on older, but proven technologies.
In electrically-driven industrial machines this has long been the case, where the essential 'emergency stop' function has traditionally comprised an external hardwired control circuit, incorporating two series-connected interposing contactors in the power circuit.
According to Darren Klonowski, applications engineer with drive solutions provider SEW-Eurodrive, these emergency-stop contactors can be expensive and cumbersome. "The cost of contactors for a high power frequency inverter can become fairly significant, not to mention the large amount of real estate required in the cabinet," said Klonowski.
Another pitfall of the traditional design is the risk of contact failure. "That's why you need the second contactor," said Klonowski. "Without it, if the contact fails, the current will continue to flow no matter how hard you push the stop-button."
Australian Standard AS4024.1 'Safeguarding of machinery' and European standards EN954-1 and EN60204-1 'Safety of machinery', are the safety standards often applied to Australian manufacturing industry machinery. These standards require that the emergency-stop circuit immobilise and make safe all dynamic workings of the machine. This doesn't necessarily imply that the power supply needs to be cut off from the machine.
In most cases in Australia, the materials handling machines are driven by electrical motors; increasingly, these are controlled by variable speed inverters. With this in mind, the manufacturing industry initiated the development of new drive safety technologies, integrating the safety stop function into the frequency inverter itself.
"The integrated safety solution has all the functionality of the old hardwired system, but it's in one inclusive package," said Klonowski. The benefits of this new design fall into two groups: those to the system integrator in terms of design and installation, and operational benefits to the plant floor personnel.
First and foremost, eliminating the need for two contactors by substituting the traditional safety circuit, initial component costs are reduced. "Fewer components means less expense," said Klonowski. Although a drive with integrated safety features can be more costly than a standard drive, the scale tips back the other way when factoring in the additional contactors."
The other area where system integrators really appreciate the difference is in installation time. "The integrated safety stop solution means the engineer spends less time planning in the design stages, but it also means that the installation happens a lot quicker, without compromising the system safety characteristics," said Klonowski.
"Besides the time and cost savings, the integrated safety solution also frees up valuable space in electrical cabinets, providing an important future-proofing element. That extra room could mean the difference between fitting in more drives at a later date and needing a new cabinet," said Klonowski.
Another major benefit of the frequency inverter safety solution is in the restart time after an emergency stop has been initiated. The legacy hardwired safety solution used the contactors to disable the drive by removing the power. Consequently, it also de-energised the drive's electronics. For a restart, the drive needed first to be fully re-energised, which created an inherent time lag. "That restart can take a second or two, but in sectors where there's a lot of moving machinery that all needs to be restarted in sequence, this 'second-or-two' per drive accumulates to represent a significant amount of down time," said Klonowski. In effect, an emerg-ency stop executed by the integrated safety solution maintains the three-phase supply to the drive and only interrupts the designated safety stop control input.
Drives with integrated drive safety stop functionality are already proving popular in many Australian industrial sectors employing electric motors as the prime motive force for machines. Safety circuits incorporated into these frequency inverters must be certified in accordance with Australian and global safety standards to levels prescribed by each industry. "It really is a matter of breaking from tradition and looking at the facts," said Klonowski. "It's about safe machine control instead of automatic power disconnection."
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