Looking for a Job?

For whatever reason, I’ve been getting a bunch of phone calls from people looking for a job. I think my internet and industry footprint is large enough that people find me pretty quickly via search engines – and are surprised that its for the most part, just me. In any case….here’s a couple of resources for those on the hunt

Lineal Recruiting Services (Trumbull, CT) – www.lineal.com – I’ve never actually met Lisa Lineal but I’ve seen her ads in Power Electronics, Power Quality, EC&M, etc. ever since I’ve been working (1983). Scary huh?

Power Technology Associates (Sharon, MA) – www.powercareers.com – I think I’ve spoken to these folks on occasion, and have probably met some of them at various shows and conferences.

Hope these help – good luck on your job search!

Harmonic Panic

Just looked over some power quality studies and an associated string of emails. The expert commented that the high harmonics on the neutral (49% and 51%, two panels) were a major problem, and required “doubling the neutral”.

Of course, the fact that the actual neutral currents averaged 10-15 amps, and never exceeded 250 Amps maximum (on a service entrance panel rated for, oh, 800 Amps) apparently got by this expert. And the voltage harmonics, 2% maximum on one panel, 3% on the other, were a little elevated, but not in any way problematic.

This, my friends is why I treat every diagnosis of high harmonics as suspect unless I see the data and run the numbers. Because too many people know how to hook up meters, push the “automatic report” button, and jump to conclusions. And not enough people have a clue what it all means.

How Good Is Your Ground?

An article in EE-Evaluation Engineering, as passed along by a client. The article is by Vladimir Kraz, Credence Technologies, and J. E. Patrick Gagnon, Texas Instruments. Good piece.

My comment: the article is written from the perppective of a controlled environment (i.e. – semiconductor test, wafer fab, etc) where there is a lot of thought and investment in a good, controlled, ground plane / grid / system – and the goal is to get connected to this ground plane as solidly as possible. For a lot of the systems that I work with (i.e. – medical imaging) there is no deliberately conceived ground system – so things are a lot murkier. If one is trying to break up problematic ground loops / paths, the last thing one might consider is improving the system coupling to the outside world….

Oh, Just Use the Ground

I’m back from a site visit; an MRI system has been having issues. On site, I discovered that a large UPS, which apparently requires 5 wires (3 phase, neutral, and ground) has been connected with a short jumper or bond between the neutral and ground – since a neutral was not run to the device. This was with the sanction (in fact, at the direction of, via the installation manual) of the manufacturer (not a small or fly-by-night vendor, I might add).

The result: 8 Amps of current (presumably from the input rectifier filters) on the protective ground conductor. At another site, we measured 16 Amps of current on the neutral-ground bond (not all of which flowed to the source on the ground conductor, mechanical mounting and conduit connections presumably taking some of the current)

OK, folks, it’s Power Quality 101. YOU DON’T USE THE GROUND TO CARRY CURRENT.

I guess it shocks me (heh-heh, thats a pun) that in 2006 people are still doing the same sort of things that got the whole power quality mess started in the old days.

I got to drag out my leakage current and ground current meters and put into practice some of the things I wrote about in my last Power Quality paper: Leakage and Ground Currents: Measurement Techniques

Keeping It Clean

I direct your attention to this article, in the May 2006 Issue of Evaluation Engineering.

Robert Close, who works for my client TEAL Electronics Corporation, put together a nice article. But the three waveform graphics embedded therein are whats really important. Those are screenshots from the TEALwatch Viewer – an Excel based data vieweer that I put together for the new TEALwatch power monitor.

Nice to see your stuff in the media….

Online Coupons

Just something I’ve put together that’s kind of interesting. I’ve developed an online coupon for one of my clients.

Some interesting features. The border, graphics and body text are all images, so that they print out properly regardless of browser, printer, or screen resolution. The coupon pulls a date, time, and coupon number from the host via CGI scripts – which reduces the odds of mass reproduction. The bar code is kind of a ruse – it simply says ESSEX STEAM TRAIN but it does add an aura of officialness. And the “valid dates” and the authentification field (below the bar code) can be modified by the client using Blogger technology.

Pretty spiffy….if I do say so myself!

UPS and Emergency Power

Cross-posted from a Bio-Med list. I took the time to write this so why not share with my fans and clients?

We’ve had a couple instances of losing E-power (switching to generator) and many of our UPS units failing to provide any backup. These power losses are for just seconds. Today we had a stack of new APC 750’s that not only failed to back up but when power was restored it still would not power the computer plugged into it! Has anyone else had this type of experience or know what might be causing it? Does anyone have a recommendation for a rock solid UPS or a way to keep them from getting knocked out?

I’m not a huge fan of stand-by (or line interactive, which are mostly the same thing) UPS’s for emergency power system use. I suspect that’s what you have here.

Most of the time, such UPS’s feed regular power to the load. When an outage is sensed, the inverter switches in and feeds the load. There is some dead time (usually in the 8 – 16 msec range, 1/2 to 1 cycle). This works pretty well for normal power – the utility rarely fails abruptly (typically some drop in voltage over several cycles, as motor loads backfeed the line) and the UPS transfers over cleanly with a minimal impact on the load.

Emergency power systems provide a lot of complications. Most of the time, facilities persons fire up the generator, bring it up to speed / sync, then exercise the transfers switches to “test” emergency power (which is not the most realistic test of the emergency power system, but has minimal impact on the facility). As a result, there is often a short transfer time (from 10 – 50 msec) and based on my power monitoring over the years, that transfer time is abrupt and often has high voltage transients that are going to get past the UPS (not yet switched to inverter) and the TVSS (designed for high voltage, high frequency transients, not relatively low voltage sub cycle transients)

In addition to the unusual outage profile, emergency power can bring to the dance frequency fluctuations, voltage distortion, and out of phase transfers that some UPS have problems with. Again, not what the utility normally does, so perhaps the highly engineered to take cost out UPS’s are not designed to deal with.

If you have access to a power monitor, I might suggest monitoring (a) a normal emergency power test (no UPS) and (b) the output of your UPS. I suspect you will walk away kind of amazed that you are not having more issues.

My advice: for critical system on emergency power, use an online UPS (not stand-by or line interactive) – these will provide no-break power. Yeah, they will cost more. There are a lot of vendors out there who do that sort of thing, and I don’t have a particular favorite – the key is to look for online, double-conversion technology. APC has some of these although maybe not in a small (750 VA) size that we are discussing.

You also might consider that the UPS you are using in undersized. Your load might power up fine while on utility, but the inverter might be collapsing due to load inrush. So the load will not power up when the UPS is on battery. If you mean “generator power” being restored, its possible that the UPS does not think the generator power is good enough (frequency, voltage level, voltage stability, waveform) and so is still running from battery / inverter. I would take a known good UPS and try to power up the load with the line cord unplugged – I suspect the issue is that the UPS is undersized to supply the inrush current while on inverter.

I’m not APC or standby / line-interactive bashing here – that’s what I use in my office for my server / desktop. The price is right, they make a ton of them so reliability is good, they work well for normal computer loads and normal power, and the software interface to my network is good. But in my experience, emergency power systems cause problems for them often enough that I would not consider myself “protected” unless I had been through a few months of emergency power testing without an issue, and had a good idea what the emergency power system testing looked like (power monitor) on the line side as well as the load side of the UPS.