Power Management

LEDs are highly energy efficient, but watch out for that first millisecond

Today’s light emitting diode (LED) technology is a marvel of model physics. LEDs for many years were relegated to use mostly as indicator lights and alpha-numeric displays, but now LED technology is replacing other traditional forms of lighting in many different types of applications.

Why the switch?

LEDs use far less energy, they are much more rugged and last much longer than other light sources, plus the fact that control can be easily embedded.

What’s the catch?

Well, until recently the purchase price of LED lighting was relatively high, but increasing demand and improvements in manufacturing have reduced LED cost to the point where it can be no longer a significant concern.

But there is still a bit of a catch — and to appreciate how easy it is to deal with, you first have to understand how LEDs work and how they differ from other lights.

The diode part of an LED is a semiconductor, specifically a two-part piece of silicon with a little coating of antimony added on one half and a little boron on the other. The result is a one-way conductor that emits photons of light as electric current passes through it. These diodes typically are built into a rounded plastic bulb that provides control of the optics, helping to focus and concentrate the light being emitted.

Nick Holonyk Jr. invented the first visible light LED back in 1962. Today, LEDs are available in numerous colors and are far brighter than the early versions. In addition, LED technology is very energy efficient, consuming 75% less energy than traditional incandescent lighting, and the technology continues to improve.

In the middle, in terms of efficiency, are fluorescent and other similar light sources that typically are configured either in long or compact spiral tubes. These lights rely on a dab of mercury, an inert gas, and a fluorescent coating on the inside of the tube to produce visible light.

Strength in numbers

Whereas getting more light out of incandescent and fluorescent lights usually requires longer filaments or longer tubes, one popular way of boosting LED lighting is to cluster individual LEDs together. You’ve probably seen the variety of bulb-shaped LEDs (which are actually a number of individual LEDs assembled into one unit) offered to consumers as replacements for individual incandescent bulbs, much the same as when compact fluorescent bulbs were developed. These are good for home use, but that’s not where the catch is.

Manufacturers have been expanding their product lines to serve office, retail, industrial and other markets by producing complete LED-based luminaires. Because LEDs operate on low-voltage direct current, these luminaires needs an electronic converter (called driver). And that is where the “catch” begins to show up.

Unlike fluorescent light sources that take a moment to warm up, LEDs go on instantaneously when power is applied. However, that is accompanied by a relatively large inrush current that is many times larger than the LED’s steady state current usage. You may sense what’s coming next.

In a typical commercial or industrial installation, perhaps two dozen luminaires may be on the same circuit. That means they all get energized at once. For traditional lighting, that may not be a problem. However, the cumulative inrush current from these LEDs being energized all at once can be significant.

How significant is that inrush current?

When energizing LED luminaires in laboratory tests, we measured transient currents as much as 253 times the rated current. In a real-world test, we replaced 25 existing luminaires on a 20-amp circuit with 25 LED luminaires, each rated at 56 watts, for a total lighting load of 1400W – not a huge load. When power was supplied, however, the circuit breaker tripped, which was no surprise; the measured transient load at startup was 237 amps. Even though the in-rush current happens very quickly – it’s over on the order of a millisecond – it’s enough to trip the breaker.

A simple solution

One way to deal with this is to derate the circuit breaker when using it with LED luminaires, which may require reconfiguring the circuit(s). However, another solution is to use a zero voltage crossing contactor. Installed inline with the circuit breaker, the zero voltage contactor delays energizing the circuit until the voltage wave is crossing zero, which minimizes the in-rush current to the point that no derating of the circuit breaker is required. Problem solved.

Power surges are another concern when using LED luminaires. This topic will be addressed in a separate post.

For more information on how to successfully upgrade to LED lighting, download our white paper “Impact of LED Lighting on Electrical Networks.”

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