It may have unexpected, unintended, and dangerous consequences
During the past several months, we at Delta have noticed a huge increase of tech support inquiries and field service calls regarding what would normally be simple discreet I/O problems. The issue has been that suddenly certain outputs within a PLC system are “staying on” or “not turning off” when commanded. All of these have been 110 VAC outputs. All have been in service for years and have functioned flawlessly, for the most part, until now. The PLC indicates in logic that the point in question is “off”. However, on output cards with field side indicators, they denote that the output is “on”. Usually this indicates a defective or shorted output point.
After doing some research into the issue, we found that this is not the case here. We have discovered that a contactor or motor starter had recently been replaced. This may have been due to a failure or in some cases, an upgrade to the clients’ entire motor control center. In an effort to reduce energy costs, the manufacturers of these products have made them more energy efficient, i.e., more “green”. In doing so, the power that these devices consume is reduced—seemingly, a good thing to accomplish. To reduce the energy consumption of any given electrical device, when the voltage is constant, if the current flow is reduced, the power consumed is reduced. These newer devices, contactors and motor starters, now draw less current from the 110 VAC source. As in our clients’ systems, this source is a 110 VAC PLC output card. The original design of these cards allowed some “leakage” current to flow, even when the card is commanded to an off position. This “leakage” current is typically in the low milliamp (mA) range. With the old contactors and motor starters, this was insignificant. However, using the newer more efficient devices, this leakage is occasionally enough current flow to “hold-in” or keep the device energized. This is an unsafe and potentially dangerous situation which may cause equipment damage or injury to personnel.
In some applications, even the E-stop may not cause the device to shut off!
After we discovered this issue, we suggested that some clients test their systems and applications where other device replacements have been made recently. In two cases, on different systems, it was found that the new devices did not shut off when commanded.
The remedy? Fundamentally, all that is needed is an increase in current flow. This may be accomplished by “paralleling” another device with the one in question, but sometimes this is not possible. An appropriately sized capacitor may be used, but typically when these components fail, they do so in a shorted mode and may cause other electrical equipment to be damaged causing unwanted downtime. The capacitor solution should be avoided.
Another remedy would be to modify the I/O card itself. In fact, some manufacturers have “removable” jumpers to disengage the protective circuitry that causes the leakage current. I strongly discourage this practice, for two reasons: First, this disengages the circuitry that also protects the output portion of the card. This will shorten its life. Secondly, months or years from now should the card fail, it will be replaced and most likely the jumpers or modifications will not be made and the dangerous situation will again arise.
However, the easiest, safest, and most economical solution is a resistor. The resistor must be sized accurately to prevent it from burning up when full voltage is applied in the “on” condition, and it will generate heat as a by-product.
If necessary, Delta Automation can review your specific application needs and provide the properly sized device for your application.