No power interrupts, please
Martin Rowe, Senior Technical Editor -- Test & Measurement World, 6/1/2008
PROJECT DESCRIPTION
Uninterruptible power supplies (UPSs) deliver power to computers and communications equipment when AC mains line voltages are too low or nonexistent. The equipment must switch to battery power, and its UPS must supply uninterrupted power to the critical load until utility power is restored or until the batteries are depleted and the UPS shuts down. UPS manufacturer Falcon Electric (www.falconups.com) has been providing low-power, high-frequency online UPSs for more than 18 years.
| More about the device under test, the challenge, and the tools. |
Mike Stout, VP of engineering at Falcon, explained that engineers perform production verification by adjusting the input voltage using a variable AC power source. Using a digital multimeter (DMM), they verify that the output voltage is 122 VAC 0.5 V under no load and the output frequency is 60 Hz 0.1 Hz. Using trim pots, they adjust the output frequency to 60.0 Hz. Using the power analyzer, engineers verify that the total harmonic distortion (THD) of the output voltage is less than 3%.
 UPS tests require resistive and nonlinear (capacitive) loads. |
Next, engineers set the load so the UPS runs at full output power. They measure voltage and current and verify that the “full load” LED illuminates. Then, they reduce the load to the midpoint and then to the one-third power level and verify the load level display. They again measure the UPS output voltage and verify that it is within the 3% range. They then increase the load to 25% above rated power and verify that the overload LED turns on and the UPS output overcurrent protection asserts.
Military-grade UPSs typically start drawing current from their batteries at about 60-VAC input but provide a warning at 84 VAC. Commercial-grade UPSs draw from their batteries at voltages between 80 VAC and 84 VAC. Engineers monitor the UPS output with DMMs and oscilloscopes to verify that it remains at 120 VAC 3%.
A UPS will also switch to battery power should the AC line voltage get too high. For 220-VAC models, the UPS will typically switch from AC mains power to battery power when line voltage reaches 260 VAC. For 120-V units, a UPS must hold its output voltage to 120 VAC 3% when the input voltage ranges from 90 VAC to 138 VAC (120 VAC –25%, +15%).
Falcon’s engineers use power analyzers to verify the output current, power, and harmonics of the company’s UPSs. “The highest levels of harmonic distortion typically occur at the third and fifth harmonics,” said Stout. “We see very little distortion beyond the seventh harmonic.” Engineers also use a spectrum analyzer to measure electromagnetic emissions.
UPSs require testing at full load, and Falcon engineers have designed their own linear and nonlinear loads for design-verification testing. The linear loads provide up to 40 kVA in 500-VA increments. Nonlinear loads simulate switching power supplies, the most common load that customers power with a UPS (see figure). Engineers also connect thermocouples to some components to verify that they remain within specified limits.
LESSONS LEARNED“When it comes to UPS power products,” said Stout, “you can’t over design.” When the company first introduced its online UPS, the design was so robust that the engineers were able to increase the power output without increasing heat sink sizes in many cases. “A good solid design is essential as there is no money in returns,” Stout said.
DEVICE UNDER TEST
Uninterruptible power supplies used in commercial and military applications. The supplies deliver up to 10 kVA at 0.7 power factor, or 7000 W.
Perform design verifications and production testing of uninterruptible power supplies. Verify their voltage, their frequency, and their ability to deliver the specified power to linear and nonlinear loads. Perform emissions testing. Monitor temperature of internal components. Verify the communications interface operates from battery power when expected.
- Agilent Technologies: spectrum analyzer, harmonic-distortion meter. www.tm.agilent.com.
- California Instruments: 50-Hz AC power source, 400-Hz AC power source. www.calinst.com.
- Extech Instruments: sound level meter. www.extech.com.
- Fluke: handheld DMM, power-quality analyzer. www.fluke.com.
- Stanford Research Systems: two 16-channel temperature-monitoring systems. www.thinksrs.com.
- Tektronix: oscilloscope. www.tektronix.com.
- Valhalla Scientific: three-phase power and current meter. www.valhallascientific.com.
