3.1.3 Influence of deviations in supply voltage. Osram HQI-E 150 W/NDL CL
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3.1.3 Influence of deviations in supply voltage
When operating metal halide lamps on a choke, the lamp parameters change depending on the supply voltage. To limit the associated variation in lamp photometrics, a maximum deviation in supply voltage of
5% from the nominal values for the supply voltage is permitted in the short term, or maximum 3% in the long term. For deviations over a longer period of time, suitable ballast tap must be selected. As choke impedance also influences the lamp parameters via the correspondingly adjusted lamp current, this is allowed to deviate from the nominal values by maximum 2%.
Remote mounting can also cause noticeable decreases in voltage (see also chapter 7.4 “Leads to luminaires”).
Long term reductions in lamp wattage cause the luminous flux to decrease, with a shorter service life and a deviation in colour from the nominal values, as also explained in chapter 5: “Wattage reduction in high
intensity discharge lamps”.
Percentage of the nominal supply voltage
UL in %
IL in %
PL in %
FL in %
UL ... Lamp voltage
IL ... Lamp current
PL ... Lamp power
FL ... Luminous fl ux
Fig. 12: Lamp parameters of a typical OSRAM HCI ® lamp over supply voltage
3.1.4 Capacitor for power factor correction
If the supply voltage is too high, the arc tube is operated at too hot a temperature, causing increased blackening and a shorter service life.
UL in %
IL in %
PL in %
FL in %
Percentage of the nominal supply voltage
UL ... Lamp voltage
IL ... Lamp current
PL ... Lamp power
FL ... Luminous fl ux
Fig. 11: Lamp parameters of a typical OSRAM HQI ® lamp over supply voltage
The capacitor for power factor correction is necessary to correct the power factor of the system when operating discharge lamps at electromagnetic ballasts. Inductively stabilized discharge lamps achieve power factors of only about 0.5 because of the dephased current fl ow. The power factor of a load is defi ned as the ratio of effective power to the apparent power actually withdrawn from the grid (kW to kvar) and is referred to as cos
ϕ
. The apparent power comprises the effective power used by the consumers to create e.g. heat or mechanical energy and the idle power that is used to develop magnetic or electric fi elds of inductivity and capacity. However the latter fl ows back into the grid after a half cycle length, i.e. it is not actually “used”. The closer cos
ϕ
is to one, the smaller is the share of wattless power withdrawn from the grid. A higher share of wattless power results in a higher fl ow in current for which the supply lines have to be rated. Similarly, power dissipation in the supply lines increases in a square progression with the current. In order to achieve the values demanded by the utility companies of more than 0.85, a grid parallel capacitor must be selected according to the lamp or choke current to approximately correct the shift in phase. By including an exactly calculated capacitor, the inductive wattless load required by an electric consumer can be offset with a capacitive wattless load. It is thus possible to reduce the wattless power withdrawn from the grid; this is called the power factor correction or wattless power compensation.
The capacitors are differentiated as follows, depending on the arrangement and form of use:
INDIVIDUAL OR FIXED COMPENSATION, where the inductive wattless power is corrected directly where it occurs, relieving the strain on the leads (typical for individual consumers usually working in continuous mode with constant or relatively large wattage – discharge lamps, asynchronous motors, transformers, welding equipment, etc.)
11
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