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June-July 2011


11 June 2011

History of US Line Voltage

According to "Men and Volts: The Story of General Electric" (1941), the original distribution voltage set by Thomas Edison - DC, of course - was apparently based upon a nice round number, 100 volts, plus 10% for losses. Edison's first commercial power plan, the Pearl Street Station,  built in 1882, operated at 110V.

Beyond being a nice round number, the choice of 110 volts was based upon the resistance of early carbon filament lamps, 275 ohms being the resistance of Edison's first carbonized cotton thread lamps, although later lamps used carbonized bamboo filaments. (Earlier experiments with metal filament lamps yielded resistance of 5 ohms or so.)

Not long after the Pearl Street station, Edison improved the distribution system, adopting a "three wire" arrangement with - from what I can determine - with bipolar supply, one wire positive, one negative and one neutral or ground, fed from two 110V DC generators in (polarity opposed) series, with the common point being the neutral.

When AC power replaced DC, it was necessary to maintain a similar RMS voltage to support the existing incandescent lamp infrastructure, so AC line voltage started out at 110V RMS.

05 June 2011

After 25 years of service, we recently replaced our old Jenn-Air stove with a Jenn-Air Model JES9800CAB00, as pictured below. (Our stove is black but it's easier to see the detail in the white finished version shown below.)

Instead of an overhead vent, Jenn-Air uses a (I believe unique to Jenn-Air) downdraft arrangement, so it is not cost-effective to replace it with a different manufacturer's product.

My wife has complained since the day it was installed about how poorly designed the smooth top surface is, how easy it is to damage it and how it limits her choice of pots and pans. There seems to be nearly 100% adoption of the smooth top design these days, so there isn't any real choice in style.

In addition, she maintains that the oven temperature varies too much, is too low and takes at least a half hour to stabilize, based upon two analog oven thermometers and baking results.

Today, I captured temperature versus time data, with the results below.

I used a Fluke 189 digital multimeter with a Type K thermocouple and adapter, and a Fluke serial data port adapter to capture temperature readings every 10 seconds. The capture software is something I wrote using EZGPIB, with post-capture plotting using Origin 8.1 software. The particular thermocouple is a bead type, suspended roughly in the oven's center.

The plot below show two test conditions. One (blue) is with a cookie sheet on the bottom rack, and the second (red) is with the cookie sheet removed.

Both show some degree of initial overshoot from the 400°F set point, 60 degrees without the cookie sheet and 45 degrees with it. After the initial overshoot, however, the temperature stabilizes reasonably well, a few degrees above the set point, with perhaps 25°F peak-to-peak variation without the cookie sheet, much less with the cookie sheet in place. I suspect that the cookie sheet reduces direct radiation from the oven heating element onto the thermocouple, and also alters the natural air flow. I suspect that the average temperature error is within the normal error for a home oven, and that the temperature variation is also not unreasonable.

In neither case, however, can I duplicate my wife's observations.



01 June 2011

As usual, the prior month Updates have been moved to an archive page, reachable by clicking here or through the link table at the top of this page.