Is there any one who could explain to me the basic calculation scheme for the RLC components in Line Filtering? Is there any site explianing more details about the Power Line Filtering, I mean, for example,the output filtering for inverter designs? My problem is especially with inductors, how can I design and construct an inductor for line filtering? I found many links but most of them explain about signal filtering, antialiasing, and some DSPs but not the topic I am having problems with.
"Myauk" wrote in message news: snipped-for-privacy@g47g2000cwa.googlegroups.com...
A line filter is the same as any other signal LC filter, but with the requirements that it handle much more current and much higher voltage than typical signal filters. To design a filter, you first need to decide on the response shape. If you are doing this professionally, you should avail yourself of a filter design book - - the most useful of them being "Handbook of Filter Synthesis" by Anatol I. Zverev. Then you need to decide what frequencies to attenuate. Obviously the frequency to pass from your inverter is 60Hz, but a filter to eliminate all frequencies above the fundamental would certainly require rather large inductors and capacitors. I don't know much about how you create the 60Hz sine wave in your inverter, but if you can allow frequencies of 50KHz and below to pass through the filter unattenuated, then the component values will be much more reasonable in size. Lets say you need 60dB attenuation above 500 KHz, so the hash in your source can't interfere with AM broadcast band reception, and a 3-pole filter is all you have room for. Select a Butterworth response shape with cutoff at
50KHz since that will reach 60dB at ten times the cutoff frequency. Now we have a bit of a problem - - filters normally have to be terminated with a specific resistive source and load to behave according to the typical response curve. It is also possible to terminate the filter at one end only, with the other end either a "zero impedance" voltage source or an open circuit. These are called extreme termination designs. The impedance of the source in a line filter may be assumed to be close to zero, let's say, and since the load voltage and current is known, the filter can be designed around that single resistive termination. The normalized component values for a 3-pole Butterworth singly terminated for a zero impedance source are (from Zverev) L1 = 1.500 henries for the source end, C2 = 1.3333 farad, and L3 = 0.5000 henry at the load end, where the load R = 1.000 ohm. To de-normalize the design to your desired cutoff frequency and load resistance, calculate a value for w = 2 x pi x Fc, where Fc is 50KHz in this example, so that w = 314,159.27. Each inductor is de-normalized by the formula L = L' x (Rt /w) where L' is the normalized value, and Rt is the actual termination resistance. Each capacitor is de-normalized by the formula C = C' x (1 / (w x Rt)) where C' is the normalized value. In this example, lets say your load pulls 2 amps at 120 volts AC, so the Rt is 60 ohms. De-normalized values for 50KHz and 60 ohms are L1 = 286 microhenries, C2 = 0.0707 microfarads, and L3 = 95.5 microhenries. These may still be too large inductance values to be reached with open core ferrite inductors, so you may need to consider ferrite EI core or pot-core or toroid inductor design, or re-think the cutoff frequency or the number of poles in the filter. If you use more poles, you can reach 60dB attenuation with a higher specified cutoff frequency, and the components, though more numerous will be smaller. Or you might decide 40dB attenuation is sufficient, which similarly will allow you to raise the cutoff frequency and use smaller components.
In general, line filters are just passive filters designed with components that handle line voltage and load current. So any tutorial on passive low pass filters would be applicable, to some extent.
A good place to see some typical filter configurations is the data sheets for commercial filters:
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A feature of line filters that you will not often see in signal filter designs is the common mode choke. This is a transformer with two similar windings, connected so that each winding carries the load current each way. But they are phased so that the line currents create canceling magnetic fields, so that a high permeability core can be used without saturating. This connection and construction allows the component to act as a high inductance between the line and load for signals that are in common to both lines, because those signals create magnetic fields that add in the core, instead of cancel. The symbol for a common mode choke is often a pair of coils with an arc with two arrow heads between the coils, representing the shared core flux.
Thank you very much for your precious hints! Now I could be able to learn and design filters for my UPS. My design include 3524 and the output wave form is modified sine wave We studied the Thai Design and improve it to fit in the local conditions of our country, Myanmar. I have studied the voltage regulation concepts, the inverter concepts, the control procedures, form the design and now I could be able to calculate and change the component values except the line filters. As far as I have studied, the output filter design should include the consideration of the type of load, and the estimate of RLC values of the load in addition to the consideration of output wave form produced without the filters. However I would like to calculate and estimate the required values for line filters practically. The design is intended to use for under 500VA with four FETs, push pull. We are also trying to design with PICs Right now my design has spikes when it has no load. When the load is connected the spike level is reduced to some extent. Do I need to make additional circuit for over 500VA designs?
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