By Dan Hebert, Senior Technical Editor
any process industry control systems require uninterrupted power for safety shutdown, ride-through during brief power outages and the orderly retention of process data. Until recently, this need could only be met with expensive, bulky, and non-industrially-rated commercial Uninterruptible Power Supplies (UPSs).
Until now, the typical commercial UPS was primarily designed for business office or control room applications and required a conditioned environments free from shock, vibration and conducted interference. Because of these requirements, UPSs have not exactly been control panel friendly. Not helping either is the fact that they can get quite expensive and normal output voltage for the typical commercial-grade UPS is 120 VAC. Recently introduced products from Phoenix Contact (www.phoenixcon.com) and Siemens (www.siemens.com) have addressed this problem by providing DIN-rail mount industrial UPSs for 24-VDC applications.
Both vendors have designed their UPS/battery systems for use with their 24-VDC power supplies, although other vendor’s 24-VDC power supplies will also work if properly designed and installed.
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“Until now, the typical commercial UPS was primarily designed for business office or control room applications and required a conditioned environment free from shock, vibration and conducted interference.”
A typical process control system would use 120 VAC as the input to a 24-VDC power supply. Phoenix Contact recommends surge protection if the 120-VAC input is subject to power spikes. “Power supply input range is typically 85264 VAC, single-phase; or 90350 VDC,” says Bruce Desmond, the UPS product manager for Phoenix Contact. “Surge protection is recommended if transient voltage spikes are expected.”
The power supply’s 24-VDC output is connected to the UPS input. The UPS supplies uninterrupted 24-VDC power to loads such as PLCs, I/O, operator interface devices, sensors, and 24-VDC instrument loops.
In a typical installation, the UPS is connected to a 24-VDC battery. The battery allows the UPS to sustain 24-VDC power for extended periods of time ranging from fractions of a second to a few hours depending on battery size and power draw from connected devices. Both Phoenix Contact and Siemens make a wide range of power supplies, UPS modules and battery modules. All components are suitable for DIN-rail or panel mounting and can operate at extended temperature ranges except for the batteries. Battery modules are typically rated for operation at temperatures up to 40° C. This means that control cabinets with battery modules must be air-conditioned, ventilated or over-sized. Because heat rises, mounting battery modules in the bottom section of a control cabinet also alleviate the effects of high cabinet temperatures.
Phoenix Contact’s battery modules can be DIN-rail mounted or panel-mounted. Siemens’ battery modules cannot be DIN-rail mounted.
For applications with reduced power requirements, Phoenix Contact offers a buffer module instead of the UPS/battery combination. The buffer module input accepts the 24-VDC power supply output. The buffer module’s 24-VDC output can be used to drive 20-amp loads in the same manner as the UPS/battery combination.
The buffer module does not contain a battery. Instead , it stores power with capacitors, thus limiting power storage capability. “Our buffer module can be used for very short power ride-through of 200 milliseconds to 4 seconds depending on load. Because there are no batteries in the module, it is maintenance free and rated at for operation at temperatures up to 60° C,” according to Desmond.
As Desmond alluded, batteries require maintenance, and they also have other operational issues. Unlike solid-state components, rechargeable batteries have a memory and lose capability with each discharge and recharge cycle. This can render a UPS/battery set inoperable over time.
Both Phoenix Contact and Siemens provided diagnostics for their UPS/battery sets. “Battery diagnostics include automatic presence and quality check, local diagnostic feedback via built-in LED indicators, and alarm outputs,” says Desmond.
For its 6-amp and 15-amp UPS modules, Siemens offers an optional serial interface or USB interface, as does Phoenix Contact. Relevant messages concerning the status of the DC UPS can then be sent to a Windows 2000 or Windows XP PC. Siemens software shows the status of the UPS. It is also possible to freely define the responses to the various operating states of the DC UPS module.
The price for these UPS/battery modules are much less than commercial UPS equivalents, especially considering the special mounting and environmental conditioning required when installing them in a commercial UPS/battery in an industrial control cabinet.