No Need to Press Restart

Ohm's law on a chalkboard

While working on the renovation of a second-floor master bedroom and bathroom, my business partner and I found ourselves with limited electrical-supply options. Most of the old circuits were disconnected, and the new wiring was being roughed in. At the same time, several other contractors were working on projects in the space, requring a variety of power tools.

While we were framing out the shower enclosure and installing the shower pan, there was an opportunity for the hardwood flooring contractors to do their part, weaving in new boards and installing missing ones.

Among the tools needed were a table saw, power miter saw, air compressor for pneumatic nailing and a router. The one existing energized receptacle in the bedroom was used for the table saw, air compressor and the fluorescent drop light we were using in the shower area. Another circuit from another bathroom provided power for the miter saw and router. Both were 20A circuits fed from the breaker panel in the basement.

Every time one of the installers turned on the table saw, the fluorescent light would go out and then come on about 3 seconds later. We are all used to “blinks” on the job when significant current-drawing tools turn on, such as various saws or when the air compressor groans. None of us recalled a time when temporary light sources went completely dark, especially not for several seconds.

Kirchoff's law on a chalkboard

Though officially retired—for the second time—as a power quality engineer, the inner geek in me took over. My first instinct was to go home and get a PQ analyzer to record the voltage and current waveforms to confirm my suspicions. However, only one other person on the job seemed the slightest bit interested because it interrupted her work in the shower.

I tried to curb my enthusiasm about discovering the source of mysterious performance of the fluorescent light, but I couldn’t contain myself.

“It’s just the classic motor inrush current causing a voltage to drop low enough for the cutoff voltage or minimum necessary to sustain the arc stream within the fluorescent light,” I said.

Seeing the “Really?” stare back from my explanation, I decided not to go into detail about rapid-start versus instant-start versus programmed-rapid-start ballasts. Since the light consistently restarted on its own once the table saw got up to speed, work proceeded without further discussion.

This was another textbook example of Ohm’s and Kirchhoff’s laws, which I also didn’t explain then. They are worth repeating here, as they go at the heart of the overwhelming majority of power quality problems, and no properly conducted case study has proven these laws wrong.

From harmonics to interharmonics to transients to swells to unbalance, the manner in which the voltage, current and impedances of the supply and loads interact is also what makes the voltage sag, typically the most common PQ phenomena. Voltage is the product of current, and impedance, or current, flowing through an impedance causes voltage to appear across the impedance. When looking back toward the source impedance, we often lump them all into the miles, or feet, of wire and transformers and anything else in the path into a single value, or the source impedance.

Load impedances vary greatly among different load types. Using the example of the table saw, or any electric motor, the load’s impedance varies during the different stages of operation. When the saw motor is turned on, the rotor is stationary, which is similar to a locked rotor, so the impedance is very low, and it draws lots of current. It is typically 6–10 times the current compared to when the motor is running at steady state. While running, the current goes down because the impedance goes up as it spins at rated speed. This also can vary as wood passes through the blade, though usually not nearly as much as the start-up.

During the start-up, the large inrush current value causes a proportional voltage-drop to be developed across the combined source impedance. Assuming that the voltage supplied by the source itself remains constant, the available voltage is reduced by the time it gets to the load.

Though not actually measured, my guess is that the voltage was reduced to 60–70% of nominal when the saw was started. It recovered in a second or so to about 95% of nominal. The voltage sag level was enough to reach the light’s cutoff voltage. Although it recovered in a second or so, the fluorescent light (with the rapid-start ballast) has a soft start where it spurts on for another second before going to full brightness. This happens whether the switch is turned on or the arc was extinguished by the voltage sag.

No need to press restart in this case.

About the Author

Richard P. Bingham

Power Quality Columnist

Richard P. Bingham, a contributing editor for power quality, can be reached at 732.248.4393.

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