Thanks as always to ecg for taking the time in all OPs like this (which are often mine, BTW) to focus on the needs of OP directly (not a knock on anyone else here) with patient and persuasive knowledge in anything on this electromagnetics and day-to-day experience.
Wish ecg was my next door neighbor.
Thanks to everyone who answered.
I was aware about ocean waves; that they move energy without moving the physical water. If I’m understanding correctly, electricity is similar.
And…
In my field, this is exactly what happens in both cases. In addition to weight savings, there’s a cost reduction, because copper and iron are expensive. When you use an inverter, you can use three phase power, which is cheaper and can avoid or reduce surcharges for power factor.
An example of this in manufacturing is medium-frequency direct-current resistance welding. Three phase 480 VAC in, 780 VDC out, via the rectifier. This is immediately passed to a variable frequency inverter, still 780 volts, and DC, but it works like AC (only the ground reference is relative). Pass it through a light-weight, high-frequency transformer, and rectify it again, and you’re welding at 15,000 amps.
For that specific question, I think the best analogies would be a Newton’s cradle, or a good opening break in a game of pool. You move one ball, it hits another ball, first ball stops moving and the second one starts moving so it carries the energy away.
For AC, you can take the same analogy and stretch it till it breaks. Ball comes in from the left and hits the center ball, you suck energy off the center ball into your device. Then a ball comes in from the right and hits the center ball, suck more. etc…
To answer this question, let’s go back to the bicycle-chain analogy, but instead, I like to use the belt-and-pulley analogy. (These were used in 19th-century factories to power machinery from one large steam-powered engine in the factory.)
So picture a simple belt-and-pulley system with a hand crank attached to a pulley on one end and another pulley on the other end. A continuous belt wraps around the two pulleys.
If you turn the crank in one direction, and another person simultaneously places their hand on the belt at the far end to slow the pulley, friction will cause their hand to warm up. You are now transferring energy from one end of the system to the other. Work at the crank end is getting transferred to heat at the far end. This is analogous to DC power, and a simple light bulb works like this. Friction in the resistive element in the light bulb causes it to light up.
Now envision rapidly turning the crank back in forth, several times a second. If a person places their hand on the belt at the far end, friction will still cause their hand to warm up. This is analogous to AC. The same thing happens to a light bulb connected to an AC circuit. Even though the direction of current is rapidly switching back and forth, the resistive element in the light bulb still causes the light bulb to light up.
The other nice thing about the analogy is that it enables you to see why you need a complete circuit. If the belt does’t wrap around both pulleys in a continuous loop, it doesn’t work.
Also note that you don’t have to have a section of belt make it all the way to the far end for energy to be transferred to the far end. On the contrary, as soon as the belt starts moving, energy is being transferred to the far end. This is similar to how a DC circuit works (and in an actual DC circuit, energy is transferred much faster than the drift speed of electrons in the wire).
In the case of going back and forth (like in an AC circuit), any given section of the belt basically stays in place, but energy is still transferred to the far end.
In fact, pretty much everything is much faster than the drift speed of electrons in the wire.
Some data about a specific long distance DC system read the Wikipedia article on the Pacific DC Intertie. Capable of sending 3,100 megawatts from the Columbia river to LA.
Note that the article mentions the skin effect as one reason DC is more efficient than AC. Hmm. 8.5mm. Maybe. I guess.
yes, since these cables can be several cm in diameter. it’s also why many are aluminum cable wrapped around strands of reinforcing steel cable; if almost all of your current is carried within the skin depth then the lower conductivity of the steel reinforcement is not so important.
Here is some useful information, including a cross section picture. Apparently, they pack data cables in with the power these days.
The increase in wire resistance due to the Skin Effect is a serious issue for AC power systems.
There is a misconception that, as long as the radius of the wire is larger than the skin depth, then the effects due to the Skin Effect can be ignored. But compared to DC, the resistance of a wire is always greater at AC, regardless of the wire diameter and regardless of the frequency. (Of course, the effect can sometimes be rendered negligible depending on the frequency and wire diameter.)
So when delivering power using AC, having too low of a frequency is not good, since the transformers would have to be very big and heavy. OTOH, having too high of a frequency is not good, since - when trying to size the wire for the amount of current - you will reach the point of diminishing returns by simply increasing the diameter of the wire due to the Skin Effect.
One way to get around the problem of Skin Effect is to use multiple, insulated wires in parallel instead of one big wire. That’s what they do on aircraft for the wires coming off the 400 Hz generators. For higher frequency systems, Litz wire is used.
I’m not up on development progress for fuel cells, but battery technology, capacity and cost has been steadily improving for decades. We are now at the point where a 1 MW/h battery is planned to even out the wind generation cycles in a commercial (30MW) facility in Scotland. The wind generation is currently operable, the battery, not yet.
We have a Tesla 100MW/100MWh battery here in Oz that is happily working in concert with quite a lot of wind and solar. It has been operational for nearly 6 months now. There are a few more batteries slated to go in now that this one has proven a success. What there isn’t is capacity to power the grid for an extended time.
There are current moves to add thousand of batteries and solar generation panels to individual houses as both generation and stabilisation - to the order of some additional hundreds of MW and MWh of capacity.
Good. Thank-you! Now, understanding that, why does reversing the current allow this to continue to work as designed? Presumably, using your prior answer, reversing the current means the marbles shift to going the other direction. The OP has trouble seeing how that wouldn’t result in the motor/appliance/thingy wanting to go the “other” way, too. Yet, obviously that’s not what happens…
A couple things:
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When you plug a device into an electrical receptacle, the power company does not reverse the current 60 times a second through the device. It reverses the voltage 60 times a second across your device. The current is simply the result of the voltage across the device and the characteristics of the device’s impedance. Unlike the voltage waveform - which is a pretty sine wave of fixed amplitude - The current can look like anything. The current might be reversing 60 times a second, or it might not be reversing 60 times a second. The current might look like a nice sine wave, or it might not look like a bunch of random noise. The key thing to remember is that the power company supplies you with a voltage sine wave, and the ability to provide whatever current waveform the load wants.
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AC devices are designed to operate properly when the voltage reverses 60 times a second (i.e. a sine wave). Heaters and incandescent light bulbs don’t care. For electronics, a rectifier circuit is used. And motors don’t care.
This isn’t the correct way to solve this problem. The correct way to solve this problem is to generate electricity in the Sahara desert, and then make it right into* synthetic fuel* right there. Probably liquid methane. Send it via pipeline or train to the places it needs to go.
The advantage is that each ton of liquid methane is 15472 kilowatt hours. A ton per second is moving 55 gigawatts.
And it’s not even that. Presumably a sufficiently beefy power line using superconductors could be made. The advantage of using methane is you can store very large quantities of it at the destination. So if the supply line is cut, power isn’t lost instantly, you could have several weeks or even months of fuel stored.
Oh, but I did answer your question. At a certain scale and manufacturing ability level, you’d make a superconducting power line. It would resemble a pipeline and would probably be run underground or up on pillars. (it would be much too heavy to suspend) The bigger the cable is, the smaller the relative ratio of the walls of the pipe, which are where heat is lost and you have to spend more energy on refrigeration. You’d use extremely high voltage DC and the actual line losses would be “zero”, though in reality longer the line is, the more you spend on refrigeration.
robby’s explanation upthread covers part of this pretty well. Simple incandescent light bulbs and heating elements don’t care which way the current flows. Using robby’s example, if you have a belt and pulley and you put your hand on the belt, it doesn’t matter if the belt is constantly moving in one direction or if it is alternating back and forth. Your hand gets hot either way.
Motors are a different story. There are many different types of motors. If they are designed for DC and you give them AC instead, with some designs they will still spin, but with others they will just sit there and shake. Some of the motors will even overheat and could destroy themselves.
Similarly, if you have an AC motor and you give it DC, some motors will spin and others won’t. Some will overheat and destroy themselves, and some won’t.
So, depending on how the motor is designed, it might actually want to go the “other” way when the sine wave reaches the negative part of its cycle.
Motor design is a whole topic to itself, and it would take pages and pages to describe all of the different types of motors out there and how they work. Details vary quite a bit, but the underlying principle is usually the same. Run electricity through a coil and it generates a magnetic field, which can push off of another magnetic field (which could be a permanent magnet or it could be another coil generating a magnetic field) to make motion.
Without going into too much detail, here are a couple of simple examples.
One thing you need to know is that motors typically have a part that rotates, which (displaying the incredible creativity we engineers have… ahem…) is called the rotor, and a part that remains stationary, which (again, displaying our immense creativity) we call the stator.
A “universal” motor was mentioned upthread. In a universal motor, the rotor and stator are electrically connected in series. When DC is applied to it, the rotor and stator push off of each other, and the motor rotates. If AC is applied to it, the changing alternating magnetic field are in phase with each other during every part of the AC cycle, and the rotor and stator push off of each other and the motor rotates. So it works on both AC or DC.
On the other hand, if the rotor and stator are connected in parallel, the motor will rotate with DC applied, but the rotor and stator will not be in phase with each other when AC is applied, and the motor will shake back and forth at 60 Hz (or whatever the local line frequency is) as it tries to follow the current in each direction. This type of motor is DC only.
It should also be noted that while some DC motors will run with AC applied, with some of them, the varying AC will will make some of the internal contacts spark and arc, which will wear them out pretty quickly. So for some of them it’s like yeah, it will run from AC, but you don’t want to run it that way for very long.
Motors in some analog clocks do very much care. It has to be AC and the frequency has to be very stable or this happens.
I did not choose my words very well. What I was trying to get at was this: if I have a single-phase motor that is powered from 120 VAC / 60 Hz, it means the motor was purposely designed to operate on a voltage that switches polarity 60 times a second.
As for synchronous clock motors, you’re exactly correct. There was a time when the long-term accuracy of 60 Hz frequency (i.e. over the course of many days) was very tightly controlled. This was done to maintain the accuracy of clocks powered from 60 Hz, obviously. But I recall reading somewhere that they “loosened” the specs about eight years ago, and now the long-term accuracy is not as good.
Well, it was a quickie explanation. But an induction motor still tries to be a frequency times some factor (related to number of poles) and then minus the slip. That’s why most are 1725 rpm, not 1800. Increase the voltage and nothing happens (until it burns out). Change the frequency and the motor turns at a different, but predictable rpm.
As for universal AC/DC motors - um, they are a mystery to me.
Dennis