Sparks Electrical News August 2016

LIGHTING

25

DIMENSIONING A UPS/ INVERTER FOR LED CREST FACTOR UPS and inverter backup units can be classi- fied into two main categories: true sine wave and ‘quasi’ or ‘simulated’ sine output, and are usually specified by their VA and Watt rating. Oscillogram 1 shows the voltage (red) and load current (yellow) of typical 600 VA qua- si-sine UPS with 10 x Verbatim 8.5W GU10 LEDs (#52310) without a dimmer – notice the 360 V peak voltage produced by the UPS! The discontinuous voltage output of the UPS causes severe lamp instability when operated with a typical leading edge dimmer, shown in Oscillogram 2 and such UPS systems should thus be avoided when dimming LEDs. By Dr Marthinus Smit (Shuttle Lighting) and Alex Cremer (Verbatim SA) Oscillogram 1: 10 x LEDs on a ‘quasi sine’ UPS – no dimmer (LED shimmer encountered). Time scale: 2.5 ms/div. Voltage (red): 100 V/div. Current (yellow): 1 Amp /div. Oscillogram 2: 10 x LEDs on a ‘quasi sine’ UPS and 600 W leading edge dimmer (extremely poor performance and the risk of LED and dimmer failure). Time scale: 2.5 ms/div. Voltage (red): 100 V/div, Current (yellow): 2 Amp /div.

Oscillogram 3: 10 x LEDs on a true sine UPS with a Shuttle trailing edge dimmer. Stable dimming performance. Time scale: 2.5 ms/div. Voltage (red): 100 V/div, Current (yellow): 1 Amp/div.

Problems can, however, be experienced when dimming LEDs operating from a pure sine UPS due to an obscure parameter known as the load crest factor (CF). The CF, which is typically rated at 3:1 for most UPS and inverter units, is the ratio of load peak current to the RMS current. Illustration 1, for instance, shows two loads that consume exactly the same power and have the same RMS current, but the blue waveform has a CF of 1.414 (pure resistive load) and the red waveform’s CF is 4.24. The large peak current of the high crest factor in the example clearly shows that more current is required from the UPS than for the blue waveform, which requires an associated de- rating of the UPS. LED lamps can have large crest factors when not being dimmed, but when dimming with phase cut dimmers, the crest factors can be extremely large. Illustration 2 shows the meas- ured maximum crest factors of 241 different dimmable LED models with a Shuttle trailing edge LED dimmer; some have a crest factor as high as 25. Leading edge dimming, however, usually results in a much higher crest factor, up to 46 in cases as is shown in the measured results of Illustration 3. Such extremely large crest factors imply a very large current rise or fall time, which can further reduce the ‘usable’ VAs of UPS or inverter. To demonstrate this phenomena, a pro- fessional double conversion true sine wave 2 000 VA/1 800 W UPS was employed to power the 10 Verbatim LEDs. The UPS has a load CF rating of 3:1 and is capable of deliver- ing 8.7 A RMS with a unity power factor load (thus CF of 1.414). The peak current available is thus 12.3 A, and, taking the crest factor into ac- count, the UPS can deliver a maximum current of 26 A (obviously with reduced load wattage). Oscillogram 3 shows the measured load volt- age and current when the LEDs are dimmed with a trailing edge dimmer. The lamp peak current is approximately 0.5 A, resulting in a maximum CF of 2.22 and the lamps have sta- ble dimming performance. When employing a classical triac based lead- ing edge dimmer with the same UPS and lamps, the results change drastically: severe lamp flick- er occurs with 85 W of LED lamps on a 2 000 VA UPS! Oscillogram 4 reveals the origin of the flicker: the dimmer turns on, but then the voltage from the UPS falls away and starts up again. The lamp peak current is about 13.2 A re- sulting in a maximum CF of 11.8 (notice the dif- ference in current scales between Oscillograms 3 and 4). The UPS is thus unable to sustain the large peak current although it is well within the specification of the particular model. The rea- son for this is due to the very sharp load current rise time of about 130 000 A per second, which exceeds the maximum load current rise time of about 120 kA/s for the particular model. Above this limit, the UPS is unable to sustain stable output voltage. The choice of UPS, dimmer and LED model thus directly impacts the maximum rating of the UPS, while the dimensioning of a UPS

Oscillogram 4: 10 x Verbatim LEDs on a true sine UPS with a triac leading edge dimmer (approximately 50% intensity). Notice the dip in the voltage waveform, which causes lamp flicker. Time scale: 2.5 ms/div. Voltage (red): 100 V/div, Current (yellow): 5 Amp /div.

Illustration 1: These two waveforms have a RMS current of 5 A, but their crest factors are very different. (Source: Ametek)

Illustration 3: Measured Crest Factor of 46 LEDs with leading edge dimming

Illustration 2: Measured Crest Factor of 241 LEDs with trailing edge dimming.

backup for LEDs, especially with leading edge dimmers, must not only take the maxi- mum crest factor into account, but also the current rise time.

Enquiries: M C Smit (Shuttle Lighting) +27 82 465 2299 / Alex Cremer (Verbatim SA) +27 11 253 4940

Dimmers for LED + Halogen + Incandescent

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Deep dimming. No flickering. NRCS RCC certificate for CoC. Silent. Recommended by all leading LED manufacturers. Maximum load varies by lamp. Minimum load 1 lamp. Details available on website under Resources tab – www.shuttlelighting.com

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Enquiries: sales@shuttlelighting.com • Website: www.shuttlelighting.com Head Office: 021 448 8229 • Laboratory: 012 991 2521

SPARKS ELECTRICAL NEWS

AUGUST 2016