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| DC bias in transformer |
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| Stefan:
Hi! I'm currently trying to better understand transformers, and there is something I don't get. We use AC for transformers because if you put DC though a winding, the magnetic flux will build up until the core saturates. But what if there is a small DC bias on, say, the mains? Shouldn't that, after a while, run the core into saturation as well? Ruining the efficiency? As far as I can tell transformers run on mains directly without any coupling caps or anything. If, for example, I drive a transformer from an H-bridge, and my FETs are not all equally fast, a small DC Bias can easily occur. Do I have to do anything about this? Thanks, Stefan |
| Simon:
no it won't be a problem, you don't need a perfect balance. you can infact send DC pulses to a transformer and you will get AC out the other side although I guess it is not the best method |
| Andy Watson:
The current "builds up" to the point at which it is limited by the DC resistance of the coils. Hopefully, the DC offset, and therefore the DC current will be insignificant compared to operating current of the inductor/transformer - so saturation is not a problem. |
| Stefan:
Ahh, I think I see it now. The voltage bias gives a time-increasing offset to the magnetic flux, which creates a DC current propotional to it. That in turn produces a DC voltage over the wire resistance which counters the DC bias. A negative feedback loop. The loss will be dc_offset2 / wire_resistance + additional core losses due to the higher magnetic flux density. Thanks for the answers! |
| c4757p:
--- Quote from: Stefan on September 06, 2013, 12:47:13 pm ---But what if there is a small DC bias on, say, the mains? Shouldn't that, after a while, run the core into saturation as well? Ruining the efficiency? As far as I can tell transformers run on mains directly without any coupling caps or anything. --- End quote --- The mains is unlikely to have a significant DC bias, because your transformer is run through a coupling transformer - i.e., the one outside your house (or down the street, depending on where you live). Also, you seem to have confused a bit of the function of a transformer with the memristor. The transformer doesn't store more and more flux as charge moves, and keep it when the current is gone. The flux through a transformer winding is phi = Li - directly proportional to the instantaneous current. The flux is higher during the half of the wave that is biased up, but still drops to zero* at the zero points, it doesn't just build and build. *Real transformer cores have hysteresis due to the ferromagnetic core, but the basic idea still stands. The hysteresis will cause a bit of this type of behavior, but unless the DC bias is large, it will be insignificant - the core will not become heavily magnetized. |
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