| Electronics > Beginners |
| Isolation transformer and electrons |
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| rstofer:
This question of transformer return current is easy: The electrons have to complete a circuit back to the transformer from which they came. Utility transformers have one side grounded and called the Neutral. Every ground fault has to get back to this neutral point to complete the circuit and it does it by traveling through some path (perhaps dirt) back through the point where the neutral was grounded. This is actually easier to think about when you consider a 3 phase transformer in a Y connection with the center point grounded. A phase to ground fault MUST get back to that center point by any means necessary. The resistance of the round trip path partially determines how much current will flow. On some higher voltage installations, that midpoint is not grounded with a piece of wire but rather with a resistor. This is done to limit the available fault current and is quite common in motor control centers for large motors. On other, older, installations, the mid-point isn't grounded at all. This means the first ground fault won't trip the circuit, it just guarantees that the other phases are at some high voltage relative to ground. We used to have neon lights on the phases that, when one went out, indicated the presence of a ground fault somewhere in the system. This type of installation is no longer in use. The reason is that the second ground fault (assuming the first wasn't cleared) is now a phase-to-phase fault and the fault current is much higher. |
| rstofer:
I think you might find some 1-wire medium voltage installations in rural areas of the US. Apparently, New Zealand and Australia are also using 1-wire distribution schemes for rural electrification: http://www.nzdl.org/gsdlmod?e=d-00000-00---off-0hdl--00-0----0-10-0---0---0direct-10---4-------0-1l--11-en-50---20-about---00-0-1-00-0--4----0-0-11-10-0utfZz-8-00&a=d&cl=CL1.18&d=HASH01a419b0834cf9dbfb8643d0.9.fc |
| rstofer:
Four simple equations would suffice to describe all of these situations: https://en.wikipedia.org/wiki/Maxwell%27s_equations |
| Brumby:
I think a lot of that might be a bit too advanced for the OP just yet. We are just getting sorted out on the fundamentals of current paths. --- Quote from: nForce on December 14, 2018, 06:17:31 pm ---I have one question: Our electrical outlets at home are voltage sources. Because the voltage is constant and the current varies. But what are limitations that outlets would be current sources? --- End quote --- If we are talking about household general purpose mains outlets, the big issue here is practicality. For a constant voltage supply, you present a given voltage at the outlet and the devices that get plugged in will only draw the current they need. Aside from efficiency factors, there is no inherent waste of power. For a constant current supply, the source will attempt to push the same amount of current through a device, irrespective of its needs. As a result, the voltage presented will rise or fall as the case may be in order to maintain that constant current. For a known load having a specific current requirement that does not change during operation, this could work - but for a constant current supply that wants to supply more current than needed, there will need to be a dummy load to "bleed off" the extra current that the load does not want. Very wasteful. If there is any fluctuation in the current requirements, then the source is going to "throttle back" when current demand rises and "open up the throttle" when it falls in order to keep the current constant. This becomes a practical problem in many cases - such as starting and running an electric motor. Have the supply provide the current for running and it will not provide the necessary current to get it going from a standing start. Have the supply provide the current for starting and it will want to continue to supply that same current during the running period. The voltage response will skyrocket. Another problem would be with a power board. Have a general purpose outlet provide a set amount of current and you plug in one device. Let's assume that this setup works because the current supplied matches the current required. Now plug in a second device with the same requirements (for simplicity). That constant amount of current will be shared between two devices, so each will only get half. Most devices will not work properly, if at all, in such a situation. Fill a 4 way power board and it gets even worse. But more than that the question has to be asked: Where do we place the constant current regulator? Do we place one at each power point in a house? Do we place it on each power circuit in a house? Do we place it on the mains supply to a house? Do we place it on the pole transformer that services several houses? With the variations in loads, there is no real answer you can give here. It's the power board problem all over again - at a bigger scale. The only way around this is to have the constant current supply programmed to adapt to each load independently. This would require the electronics to provide a constant current at every distribution point - and that electronics would need to be cheap, reliable and able to handle variations across a wide range of devices from a night light to a vacuum cleaner. There would also need to be a communications protocol between the device and the supply so that the supply knows what current to deliver. This is all getting way too messy. What is needed is a system where the devices get to choose what current they require, without having to go through some convoluted process. A system where adding extra devices through a power board, for example, just works. A system that is simple, reliable and inexpensive. Such a system does exist: Constant voltage supplies. Present a given voltage and the device will only take the current it needs. Paralleling loads is not a problem - so long as the capacity of the circuit is not exceeded. There is no need for electronics in basic systems. Reliability is high and costs are low. The only time constant current supplies are generally beneficial is where you have a single functional unit - such as an LED - that operates more effectively and safely with such a supply. This is one of the key functions of an LED driver. |
| IanB:
There is one case where constant current circuits have been used, and that is lighting circuits. In particular, street lighting (in the past), and airfield lighting (maybe still today). In the case of street lighting there would be one long loop containing many lamps in series, and a constant current power supply designed for a specified current with a large compliance voltage (maybe 1000's of volts). Each lamp used in such a circuit is specified with two numbers: the rated current, and the output lumens. Because the lamps are in series, there are certain challenges with this system. For instance, if a bulb blows it will break the circuit and all lights will go out. Unfortunately, if a bulb blows the full compliance voltage will also appear across the break in the circuit, leading to a bright, hot arc with lots of smoke, flames and molten metal. Therefore each bulb has a special bypass link that fuses and shorts out the bulb if it blows, preventing the arc and keeping the other bulbs lit. Why might such an awkward arrangement be used? Presumably because it lowers the wiring cost and also because keeps all bulbs at the specified brightness. There is no issue of a bulb at the end of a long wiring run being dim due to voltage drop in the cable. Every lamp will give the specified brightness no matter how far away from the power source. This might be an advantage on an airfield where every lamp must perform exactly to specification. The wiring cost would be lower because 50 bulbs use the same current as 1 bulb, and therefore you don't need fatter cables to carry 50x the current. |
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