The Curious Case of the UPS Loading

We recently got to review input and output monitoring data from a UPS system (make and model not specified) feeding a medical imaging system. The monitoring was done as a precaution, but we noticed something unusual.

First, take a look at the RMS voltage and current logging of the UPS input and output. Phase A voltage, Phase B current shown for clarity, but all voltage and current phases are balanced and similar.

UPS Compare Input RMS

UPS Input – Normal facility RMS voltage (daily fluctuations, with occasional sags) and RMS current peaks at approximately 80 Amps.

UPS Compare Output RMS

UPS Output – Highly regulated RMS voltage (with small load related fluctuations) and RMS current peaks at approximately 170 Amps.

The discrepancy between the input current and output current is unusual. It would be typical for input current to be marginally higher than output current (due to device efficiencies) but not lower. Our guess – the UPS DC bus (and probably, the battery string) is being called on to support the peak output load.

UPS Compare Output Highest Load

UPS Output – Step change in load current and nonlinear load is typical of medical imaging system. Very small fluctuation in output voltage related to load changes, and small increase in voltage distortion related to nonlinear load current.

UPS Compare Input Highest Load

UPS Input – Even at highest levels, current is linear, UPS must have a unity power factor front end / rectifier. However, lower current level is unusual, and indicates that UPS battery is probably being called on to supply the peak medical imaging load.

There’s really no immediate problem here – the UPS is doing a great job of correcting input power issues, as well as supplying the complex loads (step change, pulsing currents, nonlinear power factor) of the medical imaging system.

However,it’s pretty clear that the UPS batteries are getting discharged during highest current imaging system operations – not really their intended purpose, which is to ride through far less frequent utility sags and outages. So it’s possible that the UPS batteries are being stressed and may degrade or fail prematurely, and need replacement. We’ve referred this to the UPS manufacturer / supplier for attention.

As a quick “in the field” test (we’re doing this analysis remotely, not on site) we might suggest disconnecting the battery string temporarily, and seeing how the UPS performs without the battery, just relying on the DC bus. We’re guessing the UPS might start to collapse or struggle to supply the medical imaging load – and may be undersized for the application without the battery string supplied.

We’ve seen situations where a UPS that has heretofore worked well for years stops working quite so well, because the batteries started to wear out, and the unit was no longer able to supply the peak loads required by the imaging system.