• FAQ
  • Why do we need ballasts?

    For many users, ballasts are a mystery. Electrical distribution systems deliver fixed AC voltage (50 or 60 Hz) and expect connected electrical loads to limit the current drawn from the source. Low pressure and high pressure arc discharge lamps exhibit "negative impedance." Without a ballast, the arc will extinguish or draw increasing current until some circuit element burns up. Ballasts provide system stability by limiting the current that can be drawn. Ballasts use inductive and capacitive components because they impede alternating current with little power consumption. Resistive components generate high loss and are usually avoided. This is true of conventional electromagnetic ballasts as well as electronic ballasts.

    Warm-up
    Unlike low pressure lamps, HID lamps have a low initial arc voltage following GAT and warm up over several minutes to final operating voltage. In HPMV lamps this involves the evaporation of a fixed amount of mercury. In traditional metal halide and Uni-Form pulse start lamps, a fixed amount of mercury evaporates and the metal halide salts partially evaporate. For most HPS lamps, this involves the partial evaporation of mercury and sodium as the lamp reaches thermal equilibrium. Traditional and pulse start metal halide lamps have sustaining voltage requirements after GAT to assure the lamp will continue to operate. HPS lamps have a lamp power vs. lamp voltage space (see trapezoid) that has been defined to assure stable warm-up and operation.

    Ballast history

    Most of the world uses "lag" type ballasts for the operation of high intensity discharge (HID) lamps. Another common name for the simplest type of lag ballast is "reactor". These ballasts are constructed from steel laminations and wire coils. The term "lag" derives from the inductive nature of the ballast; the input current lags the input voltage by up to 90 electrical degrees. Several input taps may be provided to accommodate small local variations in nominal voltage. Reactor ballasts provide outstanding lamp performance, with excellent efficiency, at the lowest possible cost.

    Lag ballasts that can accommodate a wide range of input voltages are made using an autotransformer stage in front of an inductive element. These use two coils and are referred to as HX or high leakage reactance autotransformers. The losses and material content are higher resulting in higher operating and initial costs. The lamp performance benefits are retained.

    The CWA, or constant wattage autotransformer ballast, became popular in North America for mercury vapor lamps after World War II. The primary application was roadway lighting. The circuit delivers relatively constant lamp current, which, in turn, translates to relatively constant lamp power as long as lamp voltage does not vary with power input during life. This is a good assumption for mercury vapor lamps. It allowed utilities to start a roadway circuit with as much as +13% input voltage at the beginning of a string of lights and allow for sag to 13% at the end of the string. The resulting lamp power variation was an acceptable ±15%. A small "peaking" capacitor across the lamp terminals provided enough voltage to start lamps outdoors with modest OCV. The strategy had little to do with temporal variations in line voltage, but rather addressed the economics of lighting circuits along long stretches of road.

    When HPS lamps were introduced, they were incompatible with CWA ballasts because they required a high starting voltage. The constant current characteristic led to unstable operation. Lag and HX ballasts with electronic ignitors became the preferred circuit types. Later, CWA circuits were developed for HPS lamps that depart from a constant current characteristic and incorporate ignitors.

    Metal halide lamps were introduced in the 1960s. They required a higher peak starting voltage than mercury vapor lamps, but were incompatible with "peaking capacitors." The lamps would start and promptly "drop out." By adding saturable elements to the magnetic circuit of the ballast, the OCV could be "peaked" to start the lamps. Probe start metal halide lamps and "peaked lead" ballasts launched metal halide lighting in North America. Internationally, the same lamps operated on lag ballasts by adding simple low cost ignitors. Multiple input voltage taps for CWA ballasts were readily accommodated. More ballasts could be operated on a circuit than lag or HX ballasts of the same wattage. However, the current wave shape left little margin for input voltage fluctuations during starting, had poor energy efficiency and provided poor regulation of lamp power with respect to lamp voltage. Evidence suggests that maintained lumens of most metal halide lamps operated on CWA ballasts are worse than those operated on lag circuits

    HID ballasts perform the following functions:

    Provide voltage to breakdown the gas between the electrodes of arc lamps and initiate starting.

    Provide voltage and current to heat the electrodes to allow a low voltage, high current arc mode to develop (referred to as glow-to-arc transition, GAT).

    Provide enough current to heat and evaporate the light emitting components after an arc has been established. Provide enough sustaining voltage (see Vss) to maintain the arc during warm-up and operation.

    Operation
    The ballast determines the lamp current in normal operation. by providing the impedance. The combination of lamp current and voltage determines the power consumed by the lamp. The lamp power, in turn, determines light output and color. For example if a 320 watt lamp is accidentally operated on a 350 watt ballast, the lamp will run over wattage at 350 watts because the nominal lamp voltage is the same for both lamps and the ballast delivers the current required for a 350 watt lamp. Color will be warmer, light output will be higher and lamp life will be shorter.

    In stable operation, lamp power varies with supply voltage and lamp voltage. Electronic ballasts can be designed to minimize both sources of power variation. On lag and HX ballasts, lamp power varies about 2% for each 1% of line variation. On CWA and CWI ballasts, lamp power varies about 1% for 1% of line variation. These ballasts amplify lamp voltage variations into power variations while lag and HX ballasts minimize the same.

    Definition of Ballast

    According to the particular features of HID lamps described previously, a ballast, as it is shown in Fig. 2, having an input which is connected to a given (usually 50/60 Hz sinusoidal) voltage source, can be considered as an HID ballast if the output connected to a HID lamp acts:

    1.as a symmetrical AC current source providing:

    a)nearly constant effective current between zero and the minimum lamp voltage at nominal lamp power;

    b)nearly constant effective power equal to the nominal lamp power between the minimum and maximum lamp voltage;

    and

    2.it includes an appropriate ignitor for starting purpose.

    According to the definition of a ballast for HID lamps, the lamp current (I) vs. lamp voltage (V) and the lamp power (P) vs. lamp voltage V(ballast curve) diagrams are illustrated in Fig.2. All values should be interpreted as effective values.The lamp voltage(arc discharge voltage!) at cold start is approximately 20V(30V). In the definition, for simplicity, zero(short circuit) value was used as minimum output voltage. The current in the range of 0 < Vout< 20V can be lowered but it should be sufficiently high forcing the transition from glow discharge to arc discharge at a certain glow discharge voltage determined by the lamp.

     

    3. Efficiency. The efficiency and the closely related energy savings, ambient temperature handling capability and reliability can be considered as a crucial factor according to the practical application of ballasts. Therefore the following sub-classification of ballasts with respect to the efficiency may be justified:

    1. Conventional (core & coil)

    low efficient (< 80% )

    high efficient (> 85%)

    2. Electronic

    very low efficient ( < 85% )

    low efficient (85% - 90% )

    high efficient( 90% - 93% )

    very high efficient( > 93% )

    The average temperature inside an electronic ballast (this is a very global approach, separate temperature measurments are recommended for crucial components) depends on the external ambient temperature (which can be high as 50°C for industrial HID applications) and the temperature rise which is directly related to the power loss of the ballast. Therefore the efficiency of an electronic ballast for HID lamps (especially at high lamp power range) can be a crucial limitation factor according to the applications.

    4. Power Factor. High power factor ballast are recommended especially in the high power range(> 150W).

    High power factor: PF > 90%

    Low power factor: PF < 90%

    Low power factor equipments can result an increased harmonic distortion and effective value of the current in the power line. On the other side an extra unit (power factor preregulator) is required decreasing the efficiency and reliability. The cost of ballast can be approximately increased by 30%

     

     

    电容Oil-filled capacitors
    Oil-filled capacitors come in metal cases and are filled with a dielectric fluid. They are rated up to 100°C, although 90°C is the most common rating. They usually have two 1/4" spade terminal lugs located on the top for connection with the ballast. Most ballasts come with the mating terminals already attached to the appropriate leads. Oil-filled capacitors are very reliable and available in ratings up to 525V. For some higher wattage HID ballasts, they are the only choice.

    Dry-Film capacitors
    Dry-Film capacitors do not use a dielectric fluid. Originally, these capacitors were limited to applications where voltages did not exceed 330V. Recent advances have pushed this to 400V. They are available in temperature ratings of 100°C and have become an attractive alternative to oil-filled capacitors. They are packaged in plastic housings which do not need to be grounded and do not need any special clearances above the terminals.
     

    点火器capacitor

    There are HID lamps available internationally that incorporate internal ignitors. The pulse voltage appears on the ballast output terminals. These may not work with all ballast circuits, and could damage insulation. Request technical support for help with these.

    Venture® ignitors and ballasts are capable of continuous pulsing at maximum rated case temperature.

    Prolonged continuous operation (weeks to months) degrades ballast insulation and reduces ballast life. Best practice is the timely replacement of failed lamps to prolong ballast life. Ignitor case temperature limits must be observed. There is little safety margin, so expect short ignitor life if the limits are exceeded.

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