1 Introduction
Have you ever encountered this situation? You have completed the design of a large-scale lighting renovation project in which you replace the old magnetic ballasts in thousands of fluorescent lamps with advanced electronic ballasts. Customers are looking forward to saving energy, reducing maintenance costs and getting better lighting results through this transformation. Unfortunately, electrical contractors have discovered significant problems in your design because the components of the lighting control system have begun to break during installation.
It was soon discovered that the mechanical relay contacts were fused together, but why did this happen? The circuit is designed in accordance with the requirements of the National Electrical Code (NEC). The contractor is constructed according to the engineer's drawings. The electronic ballast is included in the UL-listed product, and the rated load of the lighting control relay is the most. Value design is reasonable. So why are the contacts of the relay melted together?
The most plausible answer is: this is caused by the inrush current of the electronic ballast.
2. What is the inrush current?
Surge current is not a new problem for lighting designers. In the IES lighting manual, it can be quickly found that the tungsten filament of the incandescent lamp has a relatively low resistance in the cooling state. When the electric energy is supplied for the first time, the electricity is passed through the tungsten wire. The flow rate is 20 to 25 times larger than the amount of current that is passed when the tungsten wire reaches the normal operating temperature. Fortunately, this usually happens within a few milliseconds. Mechanical relays designed for incandescent lamp loads therefore typically have oversized contacts to handle the initial impact of this current.
Surge current has little effect on the core and coil ballasts used in fluorescent lamps. The current flowing into the lamp is controlled by an inductor that has a higher impedance when power is first supplied. The amount of inrush current can usually be limited to 10 times or limited to the operating current value in less than 10 milliseconds. Therefore, circuits designed for incandescent lamp loads are also suitable for handling surge currents caused by ordinary ballast loads of fluorescent lamps.
However, the times have changed. As a design choice, the National Appliance Energy Protection Regulations have virtually eliminated conventional magnetic ballasts, and the latest design is electronic ballasts. The electronic ballast has the advantages of small size, light weight, low energy consumption, elimination of stroboscopic, and can provide dimming characteristics for advanced lamps. The reliability of electronic ballasts has been addressed in earlier designs, and the only remaining legacy is the inrush current problem.
3. Simple electronic ballast design concept
The design concept of the electronic ballast is relatively simple. There are two main problems in this simple design. First, current can flow into the capacitor only when the input voltage from the rectifier is greater than the voltage of the capacitor. The capacitor is fully charged at the peak of each AC cycle, with the result that the input current is not a sine wave, causing a large amount of harmonic distortion in the converter. This can cause the power line to overheat, causing great trouble to the utility company.
Manufacturers of ballasts have two design choices to deal with this problem. First, they can install a passive filter consisting of an inductor, a capacitor, and a resistor, placed in front of the AC-DC converter in the circuit. This filter allows current to flow smoothly into the bridge full-wave rectifier to produce a sine wave with a total harmonic distortion (THD) that is controlled between 20% and 30% (this also helps to increase the power factor of the ballast). ).
The second design choice is to use an active filter that is mounted after the bridge full-wave rectifier. This is actually an electronic switch that both filters the current flowing into the capacitor and provides a high power factor for the electronic ballast.
Manufacturers of ballasts are often willing to choose active filters for several reasons: active filters can control harmonic distortion to be lower than passive filters, typically less than 10%. More importantly, active filters using electronic components have the advantages of being cheaper than passive filters, small in size, light in weight, and energy saving.