We recently reviewed a set of power monitor data from an MR (Magnetic Resonance) site. The facility was plagued by severe voltage sags; we ended up with a rather copious collection of classic but nonetheless ugly event waveforms. And in the course of analysis, we noticed that some sags caused system shut-down, some rode right through, and some perhaps caused an error or lock-up which the customer attempted to reset by powering down the system.
Reviewing equipment response to severe events in this way can help to calibrate system sensitivity when manufacturer or factory data about sag susceptibility is not available.
Example #1: System Rides Through Voltage Sag
Despite a fairly serious sag, no sign of direct impact on the imaging system. Current levels shift during the sag event itself, but remain at about the same level before and after the sag.
Example #2: System Shuts Down During Voltage Sag
At the time of a severe voltage sag, load current drops to a lower, standby or system-off level, and remains there.
Example #3: System Shuts Down During Second Voltage Sag
During these sag events, the system appears to ride through a severe voltage sag; but shuts down during a subsequent sag 15 seconds or so after the initial sag event.
Example #4: System Current Drops Following a Voltage Sag; Customer Shuts System Down
Following a severe sag event, current drops partially. Suspect that one or more subsystems shut-down and resulting system alarm or errors results in customer shutting down the system, 30 seconds after the sag event. Note drop in current not directly related to a voltage event.
I’ve recently taken the dive into some stage lighting – investing in a low cost controller, some light stands, and some inexpensive ($30 / per) PAR cans.
Once upon a time, such stage lighting was out of reach for many reasons. Cost, of course, but also infrastructure – with a typical PAR can running 300-500W, each lighting pole would require a full 15-20A circuit. And dimmer technology (often phase-controlled SCR’s or triacs) was notorious for inserting hum and noise into audio feeds – requiring careful measures to separate lighting power and cables from audio equipment.
Modern LED technology, although not as aesthetically or artistically pleasing, has brought such lighting into the hands of mere mortals. At 20W a can, the entire lighting package here draws about the same power as 2 x 60 W incandescent bulbs. Not much to worry about in terms of interference (although I ran a separate power feed because I had the extension cord, and you never know)
Add to that reliability – incandescent bulbs might be good for 1000-2000 hours, the MTBF for these units is upwards of 50,000 hours. And at $30 a can, replacing a light is not much more expensive than replacing an incandescent bulb.
It was far from a “rock show” in terms of lighting effects (I was single-handedly setting up and running both sound and lights) but it really set the mood / tone for the evening. And because LED’s are RGB controlled (each fixture can run Red, Green, and Blue colors, from 0-100%, as compared to incandescent lamps which require fixed gels to control color) I was free to monkey around with lighting when I got the time or the mood of the evening dictated.
So for about $300 – I have a nice little lighting package at my disposal for music, meetings, productions, and fun.
It’s a different world from when I was young, to be sure!
Every so often I get supporting info with a power audit that reads something like this:
“Inordinate number of issues occurring with equipment as compared to other sites and systems. Everyone is in agreement the power sags, but questions if this has an adverse affect on the equipment.”
True Confession: It gives me a little bit of pleasure to find not a single facility or utility voltage sag in the resulting data. Power quality becomes, at times, the convenient excuse for equipment problems that are actually rooted in operator error, inadequate design, improper installation, inadequate servicing, environmental conditions, etc. When people confidently sling around power quality as the one known reason for problems, it’s almost always a good sign there’s something else going on.
Facility / utility voltage. We see a local outage, a small drop in voltage during emergency power system testing, and voltage flicker. But no serious voltage sags.
In this case, associated monitoring on the output of a UPS system showed perhaps the real problem – severe voltage sags associated with load switch-on. The UPS is either undersized for the applied load or in need of adjustment or maintenance. Yet another case of the “solution” being part of the problem.
UPS Output. Minor voltage drop during equipment operation, and severe voltage sag related to switch-on / inrush, points to a UPS that is undersized or in need of adjustment or maintenance.
Load inrush current, with visible collapse of UPS output.