Relays might seem to be in retreat for all but the most basic control applications in the face of seemingly relentless PLC advances. Instead, relays actually are adding capabilities and encroaching into control territory formerly occupied by PLCs.
Figure 1: This automated storage & retrieval system from Daifuku America handles materials and is controlled by a combination of relays and programmable controllers.
Today’s relays and programmable relays are better than PLCs in two main categories. Traditional relay functions such as voltage conversion, current amplification and electrical isolation fall into the first category.
“We generally use power relays for high power or large inductive loads,” says Carl Van Tilburg, PE, electrical engineer in the controls department of Daifuku America (www.daifukuamerica.com). “The nature of these loads demands a hardened component with load-breaking capabilities.” Daifuku is located in Reynoldsburg, Ohio, and makes a variety of material handling and assembly line equipment (Figure 1).
The second category includes control functions for which relays are a better alternative in terms of cost, performance and ease of use. “For our retrofit control application, we needed a simple-to-install, low-cost, add-on solution, and programmable relays filled the bill,” explains Jeffrey Smith, product support manager at MEC Aerial Work Platforms (www.mecawp.com) in Selma, Calif. MEC manufactures a variety of electric and internal-combustion-powered self-propelled aerial-work platforms for the construction industry (Figure 2). For MEC’s application, a PLC was too complex and expensive, so it instead used a type of pre-programmed relay.
Let’s look in more detail at instances and applications where relays and programmable relays are a better solution than low-end PLCs.
PLCs Can’t Do This
PLCs continually add capabilities, but these additional functions usually improve control, connectivity and performance. This leaves the field wide open for relays in traditional application areas. Some of these tasks can be performed by PLCs, but not practically from a cost/performance perspective.
“Solid-state and electromechanical relays excel at conversion, amplification and isolation,” observes Mike Garrick, product marketing lead specialist for Phoenix Contact (www.phoenixcon.com). “Relays often are used to convert 24 Vdc to 120 Vac so machines can use safe, low-voltage control, with relays switching to higher voltages as required. For example, a 24 Vdc PLC output might control a 24 Vdc relay coil that would toggle a 120 V solenoid on and off.”
Relays commonly are used to amplify current so low-cost, high-density and low-current PLC outputs can drive high-current loads. For example, a 24 Vdc/500 mA output point from a PLC could connect directly to the coil of a relay, and the relay contacts could drive a high current load.
As isolators, relays protect critical control components from dangerous loads and electrical noise. “When power is removed, inductive dc load devices collapse the magnetic field in an attempt to sustain current flow,” explains Garrick. “As a result, a voltage spike is generated and sent to whatever source is turning the device off and on. A relay contact can absorb the kickback voltage spike.”
Jim McConahay is a senior field applications engineer with Moore Industries (www.miinet.com), and he details some other electrical advantages of relays and programmable relays over PLCs. “Programmable relays have much higher inherent RFI/EMI protection than PLCs, often as high as 50 V/m,” notes McConahay. “Robots and machinery often are positioned by or controlled by VFDs, which produce a tremendous amount of electrical interference, so noise immunity is important.”
Replacing a damaged programmable relay requires a fraction of the time and cost of replacing a PLC; it takes up much-less cabinet space than a PLC does and generates much less heat than a PLC does, adds McConahay.