A brief characterization of HID lamps, and the definition/classification of related electronic ballasts are presented.

 

A. Characterization of HID Lamps

A brief characterization of HID lamps (HPS and MH lamps) and the related ballast requirements are summarized in the following points.

1. Ignition. HID lamps need an appropriate voltage across the electrodes to initiate and mantain glow discharge. Furthermore the ballast should provide sufficient current at glow discharge voltage(appr. 90V for HPS and 180V for MH) forcing the glow-to-arc transition. Therefore, the ballast should provide increased open circuit voltage (>600V) for MH(Type I, 2+1 electrodes) lamps and high voltage pulses (2000 - 3000V, 1µs) for MH (Type II, 2 electrodes) and HPS(2 electrodes) lamps.

2. Warm up time. The warm up time for HID lamps is several minutes (shorter for MH and longer for HPS lamps). In this period the resistance of the lamp (measured by applying square wave current) continuously increases from a low value [6W (400W, MH)] to an essentially higher nominal value [40W (400W, MH)]. Therefore, the ballast should act as a nearly constant current source providing sufficient increasing (nearly linear) power for the lamp.

3. Lamp Voltage Rise. HPS lamps in particular, have an excessive rise in lamp voltage during their life time. This voltage rise can achieve approximately one hundred seventy percent (170%) of the one hundred hour operation value. Therefore, a ballast should keep the lamp power within an acceptable power range derived from the ballast curve.

4. I-V Characteristics. If the lamp current is forced to change with a certain value (DI) the lamps can respond in two different ways as it is shown in Fig 1.

If the current is changed slowly, (i.e. within a minute), and with a certain value (DI) the lamp voltage changes only with a small value . In this case the lamp acts like a non-ideal bidirectional Zener diode. Furthermore, if the change is fast (< 1s) a decreased lamp voltage is produced by the increased lamp current and vice versa.Therefore, if a lamp is connected directly to a voltage source, a highly unstable state can be resulted. Any small current fluctuation can cause extinction or a very fast current increase, which can damage the lamp resulting a practically short circuited voltage source. Evidently, a ballast should act as a current source allowing the lamp to determine its voltage.

5. Acoustic Resonance. At high frequency (f > 4 kHz) operation of HID lamps, standing pressure waves (acoustic resonances) can occur in the discharge tube. This phenomenon may lead to visible arc distortions, resulting in decreased lamp life time and, in some cases, cracking of the discharge tubes. Acoustic resonances are driven by periodic instantaneous lamp power. In conclusion it may be stated that the occurrence of acoustic resonances at high frequency can be considered as a limitation factor for a wide and reliable application of high frequency (< 60kHz) electronic ballasts supplying HID lamps.

6. Cataphoretic phenomenon. Cataphoretic effects may result when a lamp is operated with DC current. Such operation results in demixing of the gas-filling as the sodium is transported toward the cathode side of the tube, making the lamp inadequate for lighting purposes. Therefore, the polarity of the lamp current should be periodically changed by the ballast (i.e. every 10 ms) providing an axially homogeneous discharge. An approximately zero DC component is recommended. Obviously the situation is different for special HID lamps designed for DC operation.

 

B. 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.

C. Ballast Classification

With the temperature modulation depth in the central discharge channel (flickering, reignition peak), maximum current density in the electrodes, and acoustic resonances, the frequency and the crest factor of the lamp current (or power) can be considered the logical starting points for a simple classification method of ballasts. From the ballast perspective, the efficiency (power loss) can be considered as a basic parameter, directly affecting the temperature rise. The ambient temperature surrounding the electronic ballast will affect the reliability and, necessarily, the expected product lifetime. Furthermore, the energy saving is also directly determined by the efficiency.

1. Frequency. From practical viewpoint the following frequency ranges can be taken into consideration.

Low frequency range: 50 Hz < f < 500 Hz
High frequency range : f > 20 kHz,

2. Crest Factors. The lamp current and lamp power are fluctuated periodically where frequency of the instantaneous power is twice of the lamp current frequency with the exception of the square wave operation where the instantaneous power is constant. The fluctuation can be characterized by crest factors as it will be shown in the following part.

Current crest factor: Ci = Im/Ie (Ci > 1) where Im is the amplitude (or max. value) and Ie is the effective value of the lamp current . Ci depends strongly on the current wave form: Ci = 1 (square wave), Ci = 1.4 (sinusoidal), 1.1 < Ci < 1.7 (piecewise exponential). For current pulse operations Ci can be essentially higher than one.

Power crest factor: Cp =Pm/Pe (Cp > 1), where Pm is the maximum instantaneous power and Pe is the effective power . If the lamp resistance is nearly constant in a period time, then Cp is approximately equal to Ci2. In the case of a square wave lamp current, Cp = Ci = 1. Furthermore if Cp > 1 acoustic resonances can occur at high frequency operation.

Using the frequency and current crest factors a simple classification of HID ballasts is shown in Fig.3. The current pulse operation ( Ci >> 1 ) has some specific features such as decreased light output, with a slightly increased color temperature at low frequency operation, stronger acoustic resonance problems and practical circuit difficulties at high frequency operation. At square wave operation there are no flickering, reignition peaks and acoustic resonance related problems, but the ballast circuit is more complex and more expensive.

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%.

 

Bibliography

Further readings:

1. The high pressure sodium lamp, J.J de Groot, J.A.J.M. van Vilet, 1986 MacMillan.

2., The need for high-pressure sodium ballast classification, M.C. Unglert,, Lighting Design and Application, March 1982.

3. An elementary arc model of the high pressure sodium lamp, J.F. Waymouth, Journal of IES/April 1977.

4. Ballast Curves for HPS Lamps Operating on High Frequency, J. Melis, IAS 1992 Technical Conference, Houston,Texas.

5.A power controlled current source, circuit and analysis, J. Melis, APEC' 94, IEEE Technical Conference, Orlando, Florida.

6. An output unit for low frequency square wave electronic ballasts, J. Melis,
SOUTHEASTCON' 94, IEEE Technical Conference, Miami, Florida.

Some HID lamp related technical papers:

7. A theoretical investigation of the pulsed high-pressure sodium arc, C.L. Chalek and R.E.Kinsinger, J.Appl. Phys. February 1981.

8. Study of HID lamps with reduced acoustic resonances, S. Wada, A. Okada, S. Moori, JOURNAL of the Illuminating Engineering Society, Winter 1987.

9. Characteristic of Radiation-Dominated Electric Arc, J. J. Lowke, J.Appl. Phys. May 1970

10. High-Intensity Sodium Lamp Design Data for Various Sizes, W. C. Louden, W. C. Matz, LIGHT SOURCES II preprint no. 13.



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