A good example of an automated safety innovation that can save users time and money is the 24 V safety circuit system introduced in 2002 by SEW-Eurodrive (www.seweurodrive.com).
This system provides a 24 VDC circuit fed to a motor drive from a safety relay. The safety 24 VDC signal is used for two purposes. First, it feeds the portion of the processor that generates the pulse width modulation switching signal controlling the firing of the power transistors. Second, it develops the gate voltage that actually fires the transistors.
Cutting power to this 24 VDC safety circuit is a much cheaper way of disabling motor drives than the traditional method of cutting 480 VAC power with a contactor. It also meets the EN954-1 safety standard.
In SEW-Eurodrive’s and some other motor drives, the 24 VDC control power is fed from an external source. But, care must be taken when implementing this solution in other cases because, in some drives, the DC control voltage is developed inside the drive instead of being fed from a separate 24 VDC power source. For internally generated DC control, it’s necessary to interrupt DC control voltage connection at each drive with a safety device.
This solution saves money because it is much cheaper to interrupt 24 VDC power as opposed to 480 VAC power. Another advantage is quicker recovery because 480 VAC power to the drive is not lost. Cutting 480 VAC power forces users to wait and let the DC bus discharge before power is restored. Quickly turning 480 VAC power to a drive off and back on can be dangerous, and also shortens drive life.
“In a typical industrial installation like an automotive plant, there are two safety contactors used for every variable-frequency drive,” says Tom Curtin, automotive industry account manager at SEW-Eurodrive.
“This safety architecture presents an opportunity for huge savings — an estimated $650 for each 5 hp drive in components alone — because you’re able to use one safety relay instead of one safety relay and two contactors for each drive.”
General Motors has implemented 24 VDC safety circuits for shutoff of variable-speed drives at many of its plants worldwide. “One of our typical plants uses a thousand or more safety contactors,” according to Hans Rodgers, senior manufacturing project engineer for GM’s Controls, Conveyors, Robotics and Welding group. “We’ve been able to cut costs by at least 60% by using this simpler and less expensive 24 VDC safety circuit design.”