The process of finding, selecting and qualifying commercial off-the-shelf Uninterruptible Power Supplies (UPSs) for military applications can be an exercise in frustration. Manufacturers' specifications and datasheets are written primarily for commercial business markets. They often do not reflect the wider spectrum of information needed to properly determine their applicability for use in a specific military application.

There are three industry standard UPS design topologies: off-line, line-interactive and on-line. For most military applications, the off-line and line-interactive designs are low-cost designs and are not acceptable. They were designed to provide basic battery backup. These types of units simply pass the utility power through the UPS until utility power is lost at which time they turn on and switch to battery-powered inverter operation.

In contrast, the advantages of a good on-line UPS are many. They provide a continuous duty inverter with a low distortion true sinewave output (3% THD typical). Their wide input voltage range marries well with a tight output voltage regulation--around ±2-3% typical. They also provide wide input frequency range while providing tight output frequency regulation. As the inverter is continuously providing the UPS output, there is no disruption of output power when the utility voltage is lost or returns. Meanwhile, battery mode operation time can be lengthened by adding more battery capacity. In some cases, the UPS may also be used as a frequency converter providing a 50, 60 or 400 Hz output independent from a 50, 60 or 400 Hz input. Online UPSs also eliminate the widest range of utility and generator source power problems. Figure 1 shows an on-line UPS and the sinewave input and output waveforms.

Construction Concerns

Most off-the-shelf UPS models available are not designed to meet the rigors of full Mil-Spec requirements. With modifications, some models can be made fairly rugged. As part of the engineer's initial investigations, care should be taken to verify the basics. The UPS market is very competitive and as such, some products available are not constructed using double-sided FR4 type laminated circuit boards. The manufacturer has cut costs by using inferior "chopper glass" board material. Even worse from a shock and vibration standpoint, they have used single-sided circuit boards without being plated through.

Circuit board, transformer, battery and heatsink mounting practices could be other areas of concern. Most good quality off-the-shelf on-line UPS products have been designed to withstand the normal shake and vibration sustained during shipment, which can be substantial. Continued trips across the desert in a Humvee (Figure 2) is another matter. The unit may require substantial modifications or even a mechanical redesign to meet more stringent military specifications. If the required quantities are large enough, the manufacturer may be willing to make the changes providing a timely and cost-effective UPS solution to the engineer's requirement. If not, there are other UPS companies offering build-to-order pre-ruggedized off-the-shelf products.

Environmental Concerns

Most domestic off-the-shelf UPS products available are tested to meet or exceed the UL1778 safety standard. As such, they are designed and intended for use in a protected environment. This means that they are not weatherproof, they do not come equipped with protective air filters, and importantly, they have not been tested by UL for operation in a wide temperature range--”only within 32° to 104°F (0° to 40°C) with a non-condensing humidity of 10% to 95%. Should the military application be inside a temperature-controlled shelter, this may not be a problem. The fact is that some well designed on-line UPS electronics, if provided enough cooling, would operate over a temperature range of 14° to 122°F or (-10° to 50°C).

For the standard Valve Regulated Lead Acid (VRLA) batteries typically used in off-the-shelf UPS units, it is another issue. Battery manufacturers state that if these batteries are operated or stored in temperatures above 90°F, they will have a noticeably shortened life expectancy. If operated in a continuous 122°F environment, the battery life could be shortened to less than one year, while at 75°F they would be expected to last three to five years. Battery charging above 104°F (40°C) also becomes a problem as the batteries should not be charged above this temperature. Some off-the-shelf UPS manufacturers offer wide temperature range battery options. The wide temperature range VRLA battery has been designed to operate over a -54° to +176°F (-65° to +80°C) range, but will still have a reduced life at temperature extremes. Charging is also not an issue at the elevated temperatures.

Operational Concerns

Proper qualification testing of any off-the-shelf UPS selected is imperative. Verifying the manufacturer's specifications is a good place to start, but must be performed in some areas with the worst-case scenario in mind. For instance, take the simple input voltage specification. It might be 120 Vac ±20% or 96 Vac to 144 Vac and one might assume that verifying operation between these voltages would suffice. The specification should not be taken by itself, but in the entire context of the other specifications that may be affected over the entire input voltage range. Beyond the battery mode testing, proper load testing of the UPS should be conducted. Proper load testing can be a real indicator of the quality and performance capabilities of a UPS. Load testing must also include overload testing to verify the proper protection capabilities have been incorporated and functional.

Overload testing also provides a good insight as to how the UPS handles differing overload conditions. For instance, a given UPS application may include powering a pump motor that has a very high inrush current demand. This demand may exceed the rating of a 3kVA UPS for a short period of time (less than 200 milliseconds). A 3kVA UPS from one manufacturer may have enough overload capacity to support the load without a problem. A UPS from another manufacturer may only sound an audible warning during the short overload period and recover. A UPS from a third manufacturer might sound a continuous alarm and turn off its output, requiring it to be totally shut down and restarted.

For load testing, both a linear and non-linear load with a 0.7pf should be used during the testing and the test results recorded with both loads. Much of the electronic equipment available today incorporates switch mode power supplies that typically have a non-linear input stage consisting of a rectifier and filter capacitors, giving the device an input power factor of about 0.7pf. These types of loads tend to gulp current toward the peaks of the sinewave. When several pieces of this type of equipment are connected to a UPS, the high non-linear current demand may cause problems for a poorly designed UPS.

DC Offset Measurement

Beyond the specifications, there are a couple of key parameters to look at. The first is the output DC offset. Is the zero crossing point of the output sinewave at zero volts or has it shifted at some DC level? DC offset is a very important consideration, especially if the UPS is to have a transformer connected to its output. DC offset should be measured with both no load and full load connected to the UPS output. A good UPS will have a DC offset of less than 50mv. DC offsets of more than a volt should be a matter of concern. Figure 3 shows a DC offset measurement circuit for an on-line UPS.

The second parameter to be verified is the input inrush current when the UPS is initially turned on. On-line UPS units typically have an input stage similar to the switching power supply previously discussed. When the UPS is initially turned on, large input filter capacitors charge, which causes an initial short duration current spike that can be quite high (over 75A) if not properly addressed in the UPS design. If a toridal input transformer is incorporated into the UPS design, the inrush current can be much higher. When installed in the field, this can result in nuisance circuit breaker tripping when the UPS is turned on. Figure 4 compares the waveforms of controlled inrush current versus an uncontrolled one.

In conclusion, verifying the individual specifications without a clear understanding of how the specifications interrelate will not give a clear picture of the UPS unit's true overall performance, thermal margins, problems, reliability or suitability for a specific application. The evaluation criteria stated only covers some of the key areas of concern, but should give a clear indication if the off-the-shelf UPS is worthy of more investment of time and further qualification testing.

Falcon Electric
Irwindale, CA.
(626) 962-7770.
[www.falconups.com]