For power transmission, a lower frequency is better because capacitance and inductance have less effect at lower frequency, dropping to zero at DC. Also, even at 60 Hz, skin effect becomes a factor for wires thicker than about half an inch, which increases the resistance compared to DC. On the other hand, transformers can be made much smaller and lighter (and cheaper) at higher frequencies; this page has a table (power output) that shows the maximum power output for a given frequency; the largest core shown (1.9x2.2x1.6 inches) can output only 22 watts at 60 Hz but over 1.6 kW at 500 kHz (power output isn’t linear because of losses which increase with frequency; 50-100 kHz is more typical in a SMPS, which still provides 700-1000 watts).
Huh. Neat. So what does a DC shock feel like?
I don’t know if you were taught wrong or if you are remembering it wrong, but that’s definitely wrong.
If you have AC, then it doesn’t matter much which wire is which. The current flows one way and then the other, back and forth 60 times a second (50 in some parts of the world). If you touched either wire but not the other, then you would be ok. The only time you get in trouble is if you touch both wires. This is called an isolated system, and it is safer than a grounded system.
But we use grounded systems. Now at first you might think that’s kinda stupid since isolated systems are safer, but in the real world, isolated systems are about impossible to keep isolated. Mother nature likes to randomly throw ground points into your system, by having tree branches grow into your wires and such things. So we intentionally ground one of the conductors. This is called the neutral conductor, since it is neutral with respect to the earth. You can (almost) safely the neutral and touch anything that is earth grounded (like a water pipe, your home’s metal siding, etc) and not get shocked. The other wire is the hot wire, since if you touch it and anything that is earth grounded you’ll get shocked.
The standard colors in the U.S. are black for hot and white for neutral.
For a while that’s all we had. If you had an appliance like an oven with a big metal case, you connected the case to neutral so that it was safe to touch. This could cause you problems, though. If the neutral wire broke, then when you turned on the oven the entire case would become electrically hot an unsafe to touch. So they figured out that it’s much safer to run a completely separate ground wire to use as your protective ground. If the hot wire breaks, nothing happens. If the neutral wire breaks, nothing happens. If the protective ground breaks, nothing happens. You only get bad things if you have multiple failures, such as the protective ground breaking and the hot wire breaking and shorting to the case. This is a much safer system. It also has the advantage that even if current is flowing through the circuit, there is no current flowing through the protective ground at it remains at true earth potential.
In the U.S. the protective ground is green, or bare copper.
You should never tie black and white wires together, and the green and white wires are only connected together at one place, and that’s near your main breaker box. The connection to earth ground should be somewhere near there as well. In the old days you were required to ground to the cold water pipe. Since the common use of plastic pipe, you are now required to have a separate ground, but you are also required to ground your water pipes to it so that they don’t become electrically hot if something were to short to them in a failure.
Sometimes when you are running a switch and you need two hot wires, you can use a black and white wire as long as you cover the end of the white wire with black tape to indicate that it is hot. This is about the only time that you intentionally tie a white wire to a black wire. The only other time that comes to mind is if you have a dead circuit in the walls and you want to leave the wire in place. In order to assure that it’s dead, you tie the black and white wires together. That way if anyone ever connects the wire back up at the other end it will blow the breaker indicating that the wires aren’t connected to anything and shouldn’t be used.
It hurts more because it cancause you to lock down.
When bring a generator on line adjust the govenor so the generator is running slightly faster than the system. As the output comes into sync with the system the breaker is closed as they match. With the slightly higher speed of the oncoming generator it will take a small load and slow down to sysstem cycles. The govenor of the oncoming generator is opened up and the generator will pick up more load. This is now done by computers and the whole system will ballance out at 60 cycles.
I heard of one case where a generator’s breaker was pulled in about 180 degrees out of phase. Blew every breaker and fuse between San Francisco and the where the ship was plugged at the dock.
I saw a 3000 shaft horsepower motor drop out of phase for a few seconds. That motor bounced about 6 to 12 inches.
No, the voltage is alternating. All the time. Regardless of the load (within reason). The current may be switching direction periodically, or it may not be switching direction periodically – it all depends on the load. See my post above.
The power company give you a stiff, constant, known, alternating voltage source. The current is freewheeling.
Sorry to be such a nitpick on this. But I think we do a disservice to people when we state as fact the “current is alternating.” We should always start off our discussion with the fact the voltage is alternating. The signature of the current can be anything, depending on the load. Yes, the current is often alternating, but there are times when it is not. And often the current signature does not look like a nice, friendly sine wave.
Well, even most SMPSs have a transformer. But they’re much smaller and lighter (vs. a linear PS) due to the high switching frequency. And if you don’t care about galvanic isolation, you can buy non-isolated DC-to-DC boost converters that don’t even have a transformer.
Thanks very much for this explanation.
If I have understood you correctly, this works even though the current in each conductor is changing direction? So regardless of whether the positive direction of current in one conductor at any instant is towards the generator or towards the load, the Poynting vector is always in the direction of propagation?
In fact, so-called “DC transformers” are really just AC transformers. You start with a DC source, and then use some fancy electronics to switch the direction of it back and forth really quickly, at a frequency far higher than the typical 60 Hz. Then you put that through a transformer that works exactly the same way as a normal AC transformer, but which can be much smaller and lighter due to the high frequency. Then you take the AC that comes out, and put it through some sort of rectifier to turn it back into DC.
SMPS transformers are driven a bit differently though (I know this because I have designed numerous SMPSs myself); instead of an AC voltage/current into the primary, many converters (flyback converters) drive the primary only one way; that is, a transistor turns on and applies the supply voltage to the primary, then turns off and opens the primary circuit; energy stored in the core is then transferred to the output, which resembles inductor action more than transformer action (which transfers power but doesn’t store it, except for magnetizing current; output voltage also isn’t dependent on the input voltage / turns ratio). Thus, you never have current flowing in both the primary and secondary windings.
Although other topologies (forward and push-pull/bridge converters) do use normal transformer action, but only push-pull converters actually use true bipolar drive, what you would call AC; the output voltage and current are symmetrical so you can generate +/- outputs with one winding by reversing diodes; for the other topologies, you need an additional winding wound in opposing direction (you actually don’t need to do this, but the output won’t be regulated, but can be used at low power levels with a post-regulator, like the -12v output in a computer PSU).
Yep, that’s definitely a valid point. I’ll try to keep it in mind in future explanations. Thanks for the correction.
Oh, as an aside: Once, I was teaching a physics lab on electromagnetic induction, and one of the questions at the end was “Why are there no DC transformers?”. One student answered “Because Marvel got the comic book rights”.
Well, as I said, that isn’t entirely true; net current flow in the windings of a flyback or forward converter is DC; the only requirement for a transformer is that current flow can’t be continuous.
Yes, that’s right. The current and voltage both change sign, so E and H both change sign, so the Poynting vector sign doesn’t change.
That’s when the voltage and current are in phase with each other, both proportional to sin(120 Pi * t), for example. If they are 90 degrees out of phase, say one is proportional to sin(120 Pi * t) and the other proportional to cos(120 Pi * t), then their product changes sign rapidly, and you have what’s called reactive power. Its time average is zero, even though there are both current and voltage.
If the phase difference is somewhere between 0 and 90, you’ll have both real power and reactive power. This is relevant if you’re the electric company, since the total current causes your resistive losses, and limits your line capacity, but you only get paid for the real power. For industrial customers they’ll monitor the power factor, which is something like real power divided by total power, and charge extra if it’s too far from 1. (engineer_comp_geek or Crafter_Man could give more info on this than I.)
[Foghorn Leghorn]I say, I say, that’s a joke, son![/FL]
Thanks again. I was thinking about this overnight and making the “right-hand rule” for each conductor and satisfied myself that the Poynting vector direction doesn’t change.
It’s something I’ve wondered about for years and only now understand for the first time.
As for reactive power, I do understand that. It’s worth saying that from the point of view of the electricity company, not only do they (we) have to carry the reactive power, we also have to burn fuel to generate it.
A big concern with utilities today with the proliferation of electronic equipment (including CFL/LED light bulbs and electronic motor controllers) is not necessarily reactive power per se, as in inductive or capacitive reactance, but nonlinear loads, such as when you use a rectifier and capacitor filter to convert AC to DC; it results in current being drawn in short spikes only at the peak voltage and besides the low power factor, it can also cause distortion of the waveform, flattening out the peaks and interference due to the sharp current spikes which causes (harmonic distortion, a pure sine wave only contains its fundamental frequency). As a result, many power supplies are required to have power factor correction, which requires an electronic circuit (an inductive load can be corrected with a capacitor or vice-versa but nonlinear loads need special circuitry).
I gather this is why aircraft electrics operate on a 400-Hz system. Why not even higher than that?
One would imagine that it comes about for a range of reasons. 400Hz is 24,000 cycles per minute. Clearly a simple alternator isn’t going to be viable, well certainly not reliable running that fast, so a multi-pole alternator will already be necessary, which reduces the physical speed it must rotate at to something sane, and something you would be prepared to trust on an aircraft. Pushing the design either in terms of higher rotational speed or more and more poles was probably the dominant limited factor.
Iron losses increase with increasing frequency, so you don’t get all the efficiency you would hope for. Ameliorating these adds cost, uses more exotic core materials, and may start to increase weight again.
Eventually I suspect the gains simply didn’t get much better, as the fraction of mass of the equipment occupied by the transformers dropped low enough that it no longer mattered enough. At 50Hz a transformer can occupy well over half the mass of many devices, but at 400Hz it may well have been good enough. Once it dropped much below say a quarter of the device’s mass is probably was as close to mission accomplished as was needed.
Not just aircraft. A lot of the the old CDC (Control Data Corporation) and I suspect early Cray computer systems ran off 400 Hz power. Computer installations included a reasonably large genemotor to power the system. This had the advantage of providing superb power isolation.
Higher frequencies allow for smaller transformers, but they also have higher inductive and capacitive losses in longer wires. 400 Hz is a good tradeoff between smaller and lighter components and a reasonable practical length for the wires.