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| What determines the minimum/maximum frequency an AC transformer will work at? |
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| T3sl4co1l:
There's no such thing as "DC" to a transformer, because a DC voltage implies a linearly rising flux density with respect to time. The highest flux densities reached on Earth are around 60T (sustained) to 500T (explosively pumped). Unless you're planning a trip to a neutron star, I wouldn't worry about anything even approaching DC. Tim |
| Beamin:
--- Quote from: ejeffrey on December 01, 2019, 07:06:58 am --- --- Quote from: Beamin on November 30, 2019, 06:04:48 pm ---So my question (like most of my questions) is more theoretical then practical. I'm not interested at say 30hz, or building one, but rather what happens at 1Hz or a gigantic transformer at less then 1hz. When does AC cease to become AC and just turn into pulsed DC with no transfer of energy? --- End quote --- There isn't really any limit at the low end as long as you avoid core saturation, but eventually the primary reactance becomes lower than the resistance which limits your power transfer. --- Quote --- i.e. why is glass an insulator of EMF at lower frequencies but a good one at high frequencies if we go past the simple answer of: refraction down fiber optics make it work, or what happens between light and RF)? --- End quote --- It isn't that glass is an insulator at RF and a conductor at DC. It actually behaves exactly the same for RF and light -- a low loss dielectric with eps_r ~ 2. The difference is that with light we are always talking about free space waves whereas with RF we are interested in both signals carried by wires (i.e., PCB traces or coax) as well as free space waves (radio). A large enough diameter glass rod would be a perfectly good "fiber" for RF signals! --- End quote --- So how would the large glass rod work if the radio freq. RF energy (Long photons) don't have a metallic "sea of electrons" skin to travel down? My understanding of why glass is transparent is that it is really complicated but has to do with photons being absorbed and reemitted at the same wave length, that's much different then from an AC signal traveling through a metal antenna. --- Quote --- --- Quote ---Imagine a magic DC to daylight signal gen what would the outputs on the back look like assuming you started at 1hz and cranked it up to 1000THz and noted what happens to your transmission lines as you went up in small 10hz steps. --- End quote --- --- Quote ---At 10 Hz you would need wires/coax for your signal as a waveguide would be prohibitively large. By around 100 GHz you would want to switch from coax to waveguides when at all possible as the center conductor losses on coax would be very high. At 200 THz even a metal waveguide would have too much loss and you would need to use an optical fiber made of only dielectric such as glass. --- End quote --- --- End quote --- So what happens in the gray area at 100 thz or 10thz or where ever things begin to change from really small microwaves/mm waves and long IR, imagine you are cranking the freq. dial up and noting the changes; its easy to say glass is for light metal for RF but whats the transition like? Is it sharp or more intriguingly gradual as is usually the case. |
| ejeffrey:
--- Quote from: Beamin on December 01, 2019, 02:40:08 pm --- --- Quote from: ejeffrey on December 01, 2019, 07:06:58 am --- --- Quote from: Beamin on November 30, 2019, 06:04:48 pm ---So my question (like most of my questions) is more theoretical then practical. I'm not interested at say 30hz, or building one, but rather what happens at 1Hz or a gigantic transformer at less then 1hz. When does AC cease to become AC and just turn into pulsed DC with no transfer of energy? --- End quote --- There isn't really any limit at the low end as long as you avoid core saturation, but eventually the primary reactance becomes lower than the resistance which limits your power transfer. --- Quote --- i.e. why is glass an insulator of EMF at lower frequencies but a good one at high frequencies if we go past the simple answer of: refraction down fiber optics make it work, or what happens between light and RF)? --- End quote --- It isn't that glass is an insulator at RF and a conductor at DC. It actually behaves exactly the same for RF and light -- a low loss dielectric with eps_r ~ 2. The difference is that with light we are always talking about free space waves whereas with RF we are interested in both signals carried by wires (i.e., PCB traces or coax) as well as free space waves (radio). A large enough diameter glass rod would be a perfectly good "fiber" for RF signals! --- End quote --- So how would the large glass rod work if the radio freq. RF energy (Long photons) don't have a metallic "sea of electrons" skin to travel down? --- End quote --- Same way RF travels through air or vacuum. There is no need to have a conductor to propagate em waves of any frequency. Metal conductors mostly help you do it in a smaller space. --- Quote --- --- Quote --- --- Quote ---Imagine a magic DC to daylight signal gen what would the outputs on the back look like assuming you started at 1hz and cranked it up to 1000THz and noted what happens to your transmission lines as you went up in small 10hz steps. --- End quote --- --- Quote ---At 10 Hz you would need wires/coax for your signal as a waveguide would be prohibitively large. By around 100 GHz you would want to switch from coax to waveguides when at all possible as the center conductor losses on coax would be very high. At 200 THz even a metal waveguide would have too much loss and you would need to use an optical fiber made of only dielectric such as glass. --- End quote --- --- End quote --- So what happens in the gray area at 100 thz or 10thz or where ever things begin to change from really small microwaves/mm waves and long IR, imagine you are cranking the freq. dial up and noting the changes; its easy to say glass is for light metal for RF but whats the transition like? Is it sharp or more intriguingly gradual as is usually the case. --- End quote --- No abrubt change. You can use a dielectric waveguide at ordinary rf if it is big enough. The point wasn't "metal for microwaves glass for light". The point is the waves are the same the difference is how we humans interact with them. That is limited because we are about 2 meters tall and only have materials on the periodic table |
| CatalinaWOW:
--- Quote from: T3sl4co1l on December 01, 2019, 09:32:52 am ---There's no such thing as "DC" to a transformer, because a DC voltage implies a linearly rising flux density with respect to time. The highest flux densities reached on Earth are around 60T (sustained) to 500T (explosively pumped). Unless you're planning a trip to a neutron star, I wouldn't worry about anything even approaching DC. Tim --- End quote --- No such thing as DC to an ideal transformer. Resistance limits current to a constant at DC in a real transformer, which means no flux change and no output. But this the essence of the physics making low frequency transformers hard. The secondary current is proportional to the field rate of change, so drops linearly with frequency. The asymptote is zero, which can only be conquered by an infinite number of turns in the secondary. High frequency limits on transformers are set by parasitics as stated previously. You have to get into the interactions of waves with atomic level structure to understand the transitions with frequency in fibers and other materials. I am not aware of any one sentence, or even one paragraph explanations that don't depend on much much background in the area. The following link provides some overview of the behavior. https://www.google.com/imgres?imgurl=https%3A%2F%2Fars.els-cdn.com%2Fcontent%2Fimage%2F3-s2.0-B9781437778175000043-f04-12-9781437778175.jpg&imgrefurl=https%3A%2F%2Fwww.sciencedirect.com%2Ftopics%2Fchemistry%2Fdielectric-constant&tbnid=P-EQ1ho6oMaUyM&vet=12ahUKEwj79MKA75TmAhU1IH0KHeTHCRUQMygFegUIARD3AQ..i&docid=lkphjELth_8z1M&w=470&h=314&q=dielectric%20function%20vs%20frequency&ved=2ahUKEwj79MKA75TmAhU1IH0KHeTHCRUQMygFegUIARD3AQ |
| Beamin:
--- Quote from: CatalinaWOW on December 01, 2019, 04:31:26 pm --- --- Quote from: T3sl4co1l on December 01, 2019, 09:32:52 am ---There's no such thing as "DC" to a transformer, because a DC voltage implies a linearly rising flux density with respect to time. The highest flux densities reached on Earth are around 60T (sustained) to 500T (explosively pumped). Unless you're planning a trip to a neutron star, I wouldn't worry about anything even approaching DC. Tim --- End quote --- No such thing as DC to an ideal transformer. Resistance limits current to a constant at DC in a real transformer, which means no flux change and no output. But this the essence of the physics making low frequency transformers hard. The secondary current is proportional to the field rate of change, so drops linearly with frequency. The asymptote is zero, which can only be conquered by an infinite number of turns in the secondary. High frequency limits on transformers are set by parasitics as stated previously. You have to get into the interactions of waves with atomic level structure to understand the transitions with frequency in fibers and other materials. I am not aware of any one sentence, or even one paragraph explanations that don't depend on much much background in the area. The following link provides some overview of the behavior. https://www.google.com/imgres?imgurl=https%3A%2F%2Fars.els-cdn.com%2Fcontent%2Fimage%2F3-s2.0-B9781437778175000043-f04-12-9781437778175.jpg&imgrefurl=https%3A%2F%2Fwww.sciencedirect.com%2Ftopics%2Fchemistry%2Fdielectric-constant&tbnid=P-EQ1ho6oMaUyM&vet=12ahUKEwj79MKA75TmAhU1IH0KHeTHCRUQMygFegUIARD3AQ..i&docid=lkphjELth_8z1M&w=470&h=314&q=dielectric%20function%20vs%20frequency&ved=2ahUKEwj79MKA75TmAhU1IH0KHeTHCRUQMygFegUIARD3AQ --- End quote --- This kind of explains what I was getting at: https://www.researchgate.net/figure/Frequency-dependence-of-complex-dielectric-constant_fig2_305116040 What does the sigma represent? The scale on the bottom seems to be Hz You are on to something to answer the question |
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