Friday, September 30, 2011

Hurricane Irene and inverters

During the weekend of August 27-28, 2011, hurricane Irene wreaked havoc along the east coast of the United States. I live in northern New Jersey where we got more than 10 inches of rain in a short time! Flooding, downed trees, and power outages were rampant! My mother called me during the storm to tell me her basement was flooded. She still lives in the house where I grew up, and I know that basement had not flooded in decades. But she lost power disabling her sump pump, so the heavy rain resulted in several inches of water in the basement saturating the carpet and ruining furniture and other personal items. What a mess! And my brother, who lives in another NJ town, has a restaurant that ended up with 4 feet of water in it!! Fresh fish, anyone?

So when my mother called me for help, I gathered up various tools, buckets, hoses, extension cords, flashlights, my wet/dry vac, and stopped at a friend’s house to borrow an inverter he used when camping (thanks, Andy!). An inverter takes DC in and puts out AC. My hope was to power the inverter from my car battery and plug in my mom’s sump pump to empty out the water in her basement. Luckily, as I was driving to her house with my friend who was coming to help (thanks, Nyla!), my mom called my cell phone to let me know the power was back on, so the sump pump kicked in and pumped out the bulk of the water. Of course, a soggy mess was left behind (7 hours of wet vacuuming made only a small dent in the cleanup, but it was a start). So, it turns out I did not use the inverter at her house (it would not have provided enough power anyway), but when I went to work the next week, I figured I’d play around with it in our lab area. Here are some of the things I found…

This inverter is a Coleman Powermate (model PMP400) 400 W inverter. It takes 12 V DC in and has a 40 A fuse on the input side, and two outlets with an on/off switch on the output side.


The output is a modified sine wave (looks more like a modified square wave to me, but OK, I’ll call it by its rightful name), at nominally 120 Vrms and 60 Hz, which are the standard AC mains voltage and frequency in the US. The waveform below was captured with a scope (an Agilent MSO7054A) and shows the actual output of the inverter with 12V DC in (from an Agilent N6754A installed in an N6705A) and a light load (~32 W) on the output.



Below is what the standard AC line looks like in the US, so you can see that the inverter's output (shown above) is only an approximation of the waveshape, although the inverter does maintain the correct rms voltage and frequency:


As a load on the inverter, I powered up another one of our DC power supplies (an Agilent 66332A) by plugging it into the inverter output. I could then program the output of the 66332A power supply to a voltage (20 V), connect it to one of our DC electronic loads (an Agilent 6063B) and vary the load current (up to nearly 5 A), thereby changing the loading on the 66332A, which in turn, changed the load on the inverter.


The inverter output frequency remained very close to 60 Hz for all loading conditions, and the output voltage dropped slightly (just a few volts) as I increased the loading on the inverter. The maximum power I drew from the inverter was limited by my input power source, the N6754A, which is a 300 W, 60 V, 20 A power supply. Since I was using it at 12 V, I set the current limit on it to the maximum of 20 A providing a maximum of about 240 W to the inverter input. So I was able to exercise the inverter up to only a little over one half of its 400 W capability.

The 66332A power supply I used as my load for the inverter has a standard AC input and seemed to operate just fine when powered by the modified sine wave coming from the inverter output. Regarding other loads you might plug into the output of an inverter, I think most AC motors would operate when supplied by a modified sine wave, however other devices such as audio equipment, fluorescent lighting, and some laser printers might not work properly or at all. Inverters are available with pure sine wave outputs to more closely mimic the power supplied by your utility company, however, these tend to be much more costly – sometimes several times the cost of an equally powered modified sine wave inverter.

I looked up a few numbers about waveforms and found that a pure square wave has a THD of about 45% while a modified sine wave has a THD of about 24%. Here is an interesting article on this topic:
http://powerelectronics.com/mag/608PET21.pdf

So if you ever lose AC mains power and need to run one or more AC powered devices, you could temporarily use an inverter powered from your car battery. Just be sure to get an inverter with enough power to handle the load you will put on it, and make sure the type of inverter you choose (modified or pure sine wave output) is appropriate for the load you want to power. Although it turned out I did not need it for my mom’s sump pump, the 400 W inverter I borrowed would not have been powerful enough for the pump. The current rating on the pump was about 6 A, so at 120 V, that is 720 VA (120 V x 6 A) which is more than the 400 W inverter could provide. But how do you compare VA (volt-amperes) to W (watts), you ask? The power that a device consumes expressed in W will always be less than or equal to the power in VA, but I’ll leave that discussion for another post! For now, if you think you’ll need an inverter, get one with a W rating higher than the total VA you require. This approach may be a bit overkill, but you will definitely have enough power.

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