Operating instructions

4.6 Cable capacitance 4.7 Start-up behavior of metal halide lamps

The capacitance of the supply cables between lamp and ignition unit depends on various general conditions. These include the size and structure of the cable (diameter, distances and insulation together with number of individual cables, dielectric coefficients of the materials). The capacitance also depends on the grounding and shielding of the cable and where it is fastened, e.g. close to grounded surfaces. Commonly used power cables are not suitable for this purpose, because of their relatively thin PVC-isolation, where the wires lie comparably close together. The capacitance here is about 100 pF/m. Because of the high ignition voltage for discharge lamps the lead wires have thicker insulations and they are normally not placed close to each other. The capacitance of the lead wires will therefore be lower than for the power cable.Capacitances only form limited resistance to high-frequency voltage components of the ignition pulse. The capacitance attenuates the ignition pulse, with resulting ignition pulses possibly not reaching the amplitudes required to ignite the lamp. Certain load capacitances must therefore not be exceeded, depending on the specifications of the ignition unit.

After igniting the lamp and heating the discharge, the discharge runs initially only in the start gas. The mercury and the metal halides are still in liquid or solid form on the arc tube wall. The voltage across the discharge is initially still very low. The start gas argon radiates a little in the visible range (weak violet light), which is why the luminous flux in the initial phase is still very low.

Through power consumption in the lamp, first the mercury and then also the metal halides begin to evaporate. The individual filling particles evaporate at different rates, resulting in differing ratios of the particles during runup. The dominance of individual particles in the start-up phase results in the colour phenomena during this period shown in Fig. 22. Only after a few minutes, having reached the steady state, is the required composition achieved, producing the full luminous flux and the required light colour.

after 20s after 40s after 60s after 80s

CRI=66 after 100s

CRI=29 after 120s

CRI=36

CRI=92

CRI=70 CRI=85

Fig. 22: Course of light parameters of a HCI ® -T 150 W/NDL during start-up

21

22

The new round ceramic arc tube (POWERBALL ® ) has a uniform wall thickness without thick ceramic plugs as in the cylindrical ceramic type. The mass is therefore only about half that of the cylindrical version. This means less energy and therefore less time is needed to bring the POWERBALL ceramic arc tube up to operating temperature. The times required to achieve the lit-up status are therefore clearly shorter than in the cylindrical version, as shown in Fig. 23.

The time it takes to reach a steady state depends on the start-up current and the associated wattage input.

If the current is too high, the electrodes will be damaged, causing the walls to blacken. The standard for metal halide lamps (IEC 61667) therefore limits the start-up current to twice the nominal lamp current.

With OSRAM POWERTRONIC ® , the start-up is faster than with a conventional ballast, as shown in the following Fig. 24.

cylindrical HCI-T 150 W

HQI-T 150 W at OSRAM PTU

HQI-T 150 W at CCG

Time in s

Fig. 23: Start-up behavior of luminous flux in various metal halide lamps operating with an OSRAM electronic ballast

Time in s

Fig. 24: Start-up behavior of luminous flux of a HQI ® -T at various ballasts