Control algorithm for an electronic dimming ballast of a UV lamp

Abstract

A control algorithm for operating a fluid disinfecting system by UV radiation, wherein the UV radiation is generated by at least one UV lamp including a pair of heating cathodes having a discharge voltage (U.sub.D), the UV lamp is operated by an electronic ballast unit equipped with the control algorithm for adjusting the UV power of the UV lamp by pulse-width-modulation to reduce UV power. The control algorithm decreases the current to a level (I.sub.kmin), increases the voltage amplitude (U) above the discharge voltage (U.sub.D) until a desired UV power level is reached. The pulse width (PW) is decreased with increasing voltage amplitude (U) until PW.sub.min is reached. The decrease in current and the increase in voltage generate an ineffective current-voltage-ratio in which excess current heats the cathode. An electronic ballast equipped with the algorithm and systems equipped with such ballasts are also disclosed.

Claims

1. A method for controlling operation of a fluid disinfecting system by means of UV radiation, wherein the UV radiation is generated by at least one UV lamp comprising a pair of heating cathodes having a discharge voltage (U.sub.D), said UV lamp operated by an electronic ballast unit equipped with a control algorithm for adjusting UV power of the UV lamp by pulse-width-modulation to reduce UV power, the method comprising using the control algorithm of the electronic ballast unit to perform the steps of: decreasing a current generated by the electronic ballast unit to a level (I.sub.kmin); increasing a voltage amplitude (U) generated by the electronic ballast unit above the discharge voltage (U.sub.D) until a desired UV power level is reached; and with increasing the voltage amplitude (U) decreasing a pulse width (PW) generated by the electronic ballast unit, until PW.sub.min is reached, wherein decreasing the current and increasing the voltage amplitude (U) generates an ineffective current-voltage-ratio in which excess current heats the cathode.

2. The method of claim 1, wherein the operating voltage of the UV lamps has a frequency between 40 kHz and 80 kHz.

3. The method of claim 1, wherein the operating voltage of the UV lamps has a frequency of about 65 kHz.

4. The method of claim 1, wherein the voltage amplitude (U) is 110% to 180% of the discharge voltage (U.sub.D) during a major part of the pulse width.

5. The method of claim 1, wherein the voltage amplitude (U) is 135% to 150% of the discharge voltage (U.sub.D) during a major part of the pulse width.

6. The method of claim 1, wherein the at least one UV lamp is a low-pressure UV lamp.

7. The method of claim 1, wherein the fluid is drinking water or treated wastewater.

8. The method of claim 1, wherein the operating voltage of the UV lamps has a frequency between 40 kHz and 80 kHz, the voltage amplitude (U) is 110% to 180% of the discharge voltage (UD) during a major part of the pulse width.

9. The method of claim 1, wherein the operating voltage of the UV lamps has a frequency of about 65 kHz and the voltage amplitude (U) is 135% to 150% of the discharge voltage (UD) during a major part of the pulse width.

10. The method of claim 8, wherein the at least one UV lamp is a low-pressure UV lamp and the fluid is drinking water or treated wastewater.

11. An electronic ballast unit for controlling a UV lamp comprising a pair of heating cathodes having a discharge voltage (U.sub.D), said electronic ballast unit equipped with a control algorithm for adjusting the UV power of the UV lamp via pulse-width-modulation by performing the steps of: decreasing a current generated by the electronic ballast unit to a level (I.sub.kmin); increasing a voltage amplitude (U) generated by the electronic ballast unit above the discharge voltage (U.sub.D) until a desired UV power level is reached; and with increasing the voltage amplitude (U) decreasing a pulse width (PW), generated by the electronic ballast unit until PW.sub.min is reached, wherein decreasing the current and increasing the voltage amplitude (U) generates an ineffective current-voltage-ratio in which excess current heats the cathode.

12. The electronic ballast unit of claim 11, wherein the operating voltage of the UV lamp has a frequency between 40 kHz and 80 kHz.

13. The electronic ballast unit of claim 12, wherein the operating voltage of the UV lamp has a frequency of about 65 kHz.

14. The electronic ballast unit of claim 11, wherein the voltage amplitude (U) is 110% to 180% of the discharge voltage (U.sub.D) during a major part of the pulse width.

15. The electronic ballast unit of claim 14, wherein the voltage amplitude (U) is 135% to 150% of the discharge voltage (U.sub.D) during a major part of the pulse width.

16. The electronic ballast unit of claim 11, wherein the UV lamp is a low-pressure UV lamp.

17. A system for disinfecting fluid by means of UV radiation, the system comprising: at least one UV lamp for generating UV radiation, the at least one UV lamp comprising a pair of heating cathodes having a discharge voltage (U.sub.D); and the electronic ballast unit of claim 11 configured for operating said at least one UV lamp.

18. The system of claim 17, wherein the at least one UV lamp is a low-pressure UV lamp.

19. The system of claim 17, wherein the fluid is drinking water or treated wastewater.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows a schematic illustration of a prior art voltage and current curve generated by a ballast unit for a UV module with a plurality of UV lamps.

(2) FIG. 2 shows a schematic illustration of a voltage and current curve according to the present invention.

(3) FIG. 3 shows a block diagram of a UV lamp controlled by an electronic ballast according to the present invention.

DETAILED DESCRIPTION

(4) An electronic ballast unit 302 (see FIG. 3) for a UV radiator like a low voltage gas discharge UV lamp 304 (see FIG. 3) preheats the coils of the lamp prior to starting the gas discharge, and generates an ignition voltage to start the discharge. The power of the connected UV radiator is automatically controlled by a pulse-width modulation. It is driven by a pulse-shaped voltage obtained from rectified AC (see FIG. 1). The example of FIG. 1 shows a dimmed operation with a UV power output and a corresponding electric energy input of 30% of the nominal power rating of the lamp. However, the cathodes 306/308 (see FIG. 3) are constructed for 100% nominal power at which a predetermined cathode temperature is generated. At 30% of the nominal power the cathodes are too cold, which negatively affects the service life time of the UV lamps.

(5) FIG. 2 shows the change in voltage and current over time according to the present invention. The output current I and voltage U have an essentially rectangular shape with a frequency of around 65 kHz. The current signal I and voltage signal U have almost the same shape, because a commonly used choke is not present. The power or rather the effective current I is controlled by pulse width modulation (PWM).

(6) During rated operation the voltage amplitude should be equal to the lamps' discharge voltage U.sub.D. If the burn voltage U is higher than the discharge voltage U.sub.D, hardly more UV power is produced; rather energy is lost by heat generation.

(7) As shown in FIG. 2, at the beginning of a pulse the voltage increases for a short time until it decreases to a predefined level U.sub.kmin for the rest of the pulse length, creating a sharp peak followed by a plateau. The given current I.sub.kmin leads to a drop of the operating voltage U to U.sub.kmin. This mode generates an ineffective current-voltage-ratio, wherein the too high current is used for cathode heating.

(8) The electronic ballast unit is preferably equipped with two control algorithms. The control variable is UV power. To reduce UV power, the current is decreased to I.sub.kmin and held at this level. After that the voltage amplitude is increased until the desired UV power is reached. With increasing voltage amplitude the pulse width decreases, until PW.sub.min is reached.

(9) The intermediate voltage circuit is preferably designed in such a way that the desired voltage range is given without hardware modification.

(10) In order to reach 30% UV power with acceptable electrode heating, in one embodiment the pulse width is 35% of rated operation and the voltage amplitude is 40% higher.