Reading the Tea Leaves

Sometimes when reviewing power monitoring data, key information is left out of the problem statement. But the astute power quality engineer can “read the tea leaves” and pick up information about the installation, equipment, and technical issues.

A set of data from a Magnetic Resonance Imaging system was presented for analysis with the following notes:

System has had several intermittent issue that have caused system to be down and functional upon arrival. System issues have been in RF section. No issues have been reported since system since installation of power recorder.

RMS Voltage and Current

Chiller RMS Logs

First clues come from looking at the RMS logs of the voltage and current. The voltage is suspiciously well regulated – and is probably a UPS or power conditioner rather than a normal utility source (which will tend to fluctuate over a 24 hour period). A second clue is the small voltage increase or swell related to load switch-off – typical of an active source, not typical of a passive source.

Second, this appears to be a regularly cycling load – a pump or compressor. MRI systems typically have a chiller or cryogen cooler associated with them – so odds are good this was monitored on this load, and not on the MRI system itself.

Chiller or Cryogen Cooler Load

Chiller Cycling RMS
More evidence supporting the chiller or cryogen cooler load – a regular (practically like clock-work) cycling load, with a marginally higher operating current (~30 Amps), but a very high inrush current (~180 Amps)

Chiller Transient

Normal chiller or cryogen cooler inrush is seen here. A minor (~5%) voltage sag was captured during each inrush current, as well as minor associated transients (probably relay or contactor switch bounce)

Abnormally High Inrush Currents

Chiller Sag

In addition to the regular inrush currents associated with chiller cycling, six instances of very high inrush current were captured. These were seen both as voltage sag events as well as current triggered events. We’re concerned that this high inrush current may be causing an overcurrent condition on the UPS / power conditioner – which may be throwing a fault or error, or perhaps switching to Bypass.

Chiller Swell RMS

Looking at the RMS logs of the high current swell / voltage sag event, we see that it precedes a period of extended chiller / compressor operation. Unknown if this is normal operation for the system / device or indicates an error or fault of some sort.

Summary

Although the accompanying technical information was thin, we’ve “read the tea leaves” and provided the following analysis bullets

  • System appears to be powered by a UPS or power conditioner. Service personnel may not have known this.
  • System appears to be a chiller, compressor, or similar device (not the medical imaging system itself)
  • Occasional high current swells were seen; these may be normal or may point to system issues.
  • Voltage sags and collapse during these high current swells may indicate that the UPS or power conditioner is overloaded, and may be experiencing faults or alarms that may be impacting system operation / uptime