Question about electricity

[McMurphy]

I'm not an electrician or a physicist so don't slam me if this is a ridiculously dumb question but I was changing a lightbulb the other day and it occurred to me that the lighbulb would probably last a lot longer if we used direct current rather than alternating current because alternating current puts extra stress on the filament because it causes the filament to vibrate because it alternates back & forth. ( I think ) and by using direct current the filament would not be put under as much duress and would last longer. So why aren't we using direct current?

Also if I were to rewire my entire house so that all my electrical appliances ran on direct current how would it affect my TV or my kettle & other non-lightbulb appliances like that?

[malden]

There are undoubtedly many reasons why we use DC, but the one I recall first is safety. When someone touches a live circuit, there is a tendency for said person to "stick" to the wire (or vice-versa.) This effect remains as long as there is a nonzero voltage in the wire. With DC, there is a constant voltage. With AC, the voltage follows a sine wave pattern between two equal and opposite values, e.g, +120V and -120V. It oscillates between the two values at 60 cycles per second, so 120 times per second the voltage is exactly zero. This would make it easier for someone to break contact with a live wire.

[Arjuna34]

You can buy a light bulb rectifier here (http://www.len.uniserve.com/faq.html), or build your own (http://members.fortunecity.com/geoffegel/light.htm) (note- you shouldn't build your own unless you're qualified - i.e. an electrician).

These are half-wave rectifiers, where the voltage isn't converted to DC- the negative half of the voltage sinewave is removed, leaving a sort of pulsed DC. This increases bulb life mainly by reducing the overall RMS voltage on the bulb, which reduces the filament heat as well as brightness.

My WAG is that using actual DC voltage of the same RMS value would have much effect filament. The only source of filament vibration from the electricity is from the heating and cooling causing the filament to expand and contract slightly. With a 60Hz line frequency, the filament is being heated and cooled 120 times per second. This is fast for the filament to significantly change temperature, so not much actual vibration is happening.

Arjuna34

[Ikujinashi]

Echoing Arjuna34, I don't think that DC vs. AC (at the same power) will effect the life of your bulb. There definitely shouldn't be any noticeable change in filament vibration (maybe if your light bulbs are in a strong magnetic field).


As to why AC is used, AC is what is delivered to your home (it's really easy to stepup AC voltage via a transformer, which makes it more efficient to distribute). A rectifier as Arjuna34 mentioned can produce a DC voltage from this... but there will always be some inefficiency in doing so.

This brings us to what your DC house would be like. Without changing your appliances... your stove should still work, your fans, and other things with motors won't work, I would guess that with careful selection of your DC voltage more complex electronic devices would still work. Most will have a rectifier in their power supply which can give an appropriate output fed an appropriate DC voltage and from there on the appliance won't notice the difference.

[Padeye]

Some motors will work. "Universal" motors, the kind with brushes can run on AC or DC. Some power tools are explicitly made to run on AC or DC. Don't assume that an appliance with a universal motor will safely run on DC. Univeral motors are the loud, fast kind used in blenders, saws, drills, etc. Induction motors, brushless, quiet and slower, must run on AC.

[Una Persson]

Low-voltage DC lights are starting to become vogue for many custom home wiring applications now.

AC is what is delivered to your house, because until recently it had the lowest transmission line losses and it is very easy to transform its voltage. And that makes it easiest to just use AC for the lights. People claim far and wide that converting the outlets to DC extends bulb life, but I am not sure if that is proven.

[TheLoadedDog]

Seems like the jury is still out on this one, though there seem to be guys making money out of DC conversions.

Here (http://boards.straightdope.com/sdmb/showthread.php?threadid=59692) is a slightly related thread in which the AC versus DC subject is mentioned.

[Crafter_Man]

My two cents…

TheVoiceofReason: 60 Hz AC does put more stress on an incandescent bulb's filament than DC of equivalent power, but only slightly more. This is because the period of the sinusoidal power curve is 8.3 ms, which is much shorter than the filament's thermal time constant, so the coefficient of expansion due to power fluctuations would be negligible. Heck, even if the coefficient of expansion was significant, it probably still wouldn't matter, because a red-hot filament is quite flexible.
You also asked if you could "rewire my entire house so that all my electrical appliances ran on direct current." This would work for some appliances (such as heaters), but would not work for synchronous motors and anything containing an input transformer (such as a linear power supply). However, you can run most switching power supplies off DC, so there's a good chance your computer would still work. Not sure about your monitor, though.

malden: It is true there is sometimes a "sticking effect" when a person is electrocuted, but it normally occurs at higher voltages. (I've never heard of it occurring at 120 VAC). In my opinion, 120 VDC is safer than 120 VAC. Why? Because you can be killed by an instantaneous high voltage. 120 VAC is an RMS value, and thus the actual voltage swing is between approx. +170 V and -170 V relative to ground. In other words, with 120 VDC, the most you'll ever feel is a 120 V potential, while with 120 VAC, you'll be jolted with +170 V, then -170 V, then +170 V, then -170 V, etc. every 8.3 ms. Ouch.

Anthracite: You said, "AC is what is delivered to your house, because until recently it had the lowest transmission line losses and it is very easy to transform its voltage." I agree with the latter, but not the former. Both AC and DC would have the same I^2 * R losses (all else being equal). But AC (unlike DC) has reactive losses due to line impedance, which means DC is actually less lossy than AC.

Of course, the real reason AC is used over DC for the power grid is because insulation is cheaper than aluminum or copper. Think about it.

[Catenary]

Tesla and Edison had this argument years ago (Tesla won). AC can be transmitted long distances because it can be put through a transformer, unlike DC. Take 120 volts at 100 amps and you get 12000 watts (P=IE). Run it through a transformer to lower the current, 120,000 volts at .1 amps = 12000 watts. No loss of power (perfect conditions), low current = low loss from heat. DC sees the windings of a transformer as a direct short. Edison argued for DC and actually electrocuted an elephant to prove that Tesla's AC was dangerous.

[Una Persson]

Originally posted by Crafter_Man
Anthracite: You said, "AC is what is delivered to your house, because until recently it had the lowest transmission line losses and it is very easy to transform its voltage." I agree with the latter, but not the former. Both AC and DC would have the same I^2 * R losses (all else being equal). But AC (unlike DC) has reactive losses due to line impedance, which means DC is actually less lossy than AC.

I understand that, but AC until recently was more efficient to transfer long distances because it could be much more easily stepped up to very high voltages, and stepped back down on the other end. So all else was not equal, since the DC was at a lower voltage, and had a larger "I" component. The reason I said "until recently" in my prior post was because I was referring to the new high-voltage DC transmission lines that are starting to go up in the West, which are showing themselves to be more efficient. But it took some technology to get to this point.

[Crafter_Man]

Originally posted by Anthracite
Originally posted by Crafter_Man
Anthracite: You said, "AC is what is delivered to your house, because until recently it had the lowest transmission line losses and it is very easy to transform its voltage." I agree with the latter, but not the former. Both AC and DC would have the same I^2 * R losses (all else being equal). But AC (unlike DC) has reactive losses due to line impedance, which means DC is actually less lossy than AC.

I understand that, but AC until recently was more efficient to transfer long distances because it could be much more easily stepped up to very high voltages, and stepped back down on the other end. So all else was not equal, since the DC was at a lower voltage, and had a larger "I" component. The reason I said "until recently" in my prior post was because I was referring to the new high-voltage DC transmission lines that are starting to go up in the West, which are showing themselves to be more efficient. But it took some technology to get to this point.


I fully understand that an AC system overall is (usually) much more efficient than a DC system because transformers can be used with AC. But I interpreted your comment to mean "the loss along transmission lines is lower with AC than DC", when in fact it's just the opposite. Sorry about the confusion.

[Diver]

Regarding safety of AC versus DC.
If you plot AC frequency versus death you can see that the worst possible frequency to use is - yup - 120 hz, the one chosen for household supply. At that frequency only about .006 amps can kill. The natural rhythm of the heart is disrupted and heart failure occurs.

[Mikahw]

120Hz? Standard wall power, at least in the US, is 60Hz.

[Crafter_Man]

Originally posted by Mikahw
120Hz? Standard wall power, at least in the US, is 60Hz.

It is 60 Hz. But if something doesn't "care" about current direction, then the resulting "pulsations" will occur at 120 Hz.

For example, take an incandescent light bulb. It doesn't care which way the current is flowing; the filament will heat up and expand when the current is flowing in either direction. Therefore, its filament physically "pulsates" ever-so-slightly at 120 Hz, not 60 Hz. Same with an incandescent bulb's light intensity: it also modulates at 120 Hz.

I guess what Diver is saying is that, when it comes to current direction, "your body doesn't care." Therefore, 60 Hz will induce 120 Hz pulsations in your heart. (I'm not endorsing this statement; I'm simply trying to explain what I believe Diver is trying to say.)

[Mikahw]

Yeah, but maybe whatever chart he was reading referred to 120hz current, which would pulsate at 240hz.

[gazpacho]

People have been talking about a DC power distributing system. Is it also high voltage to cut down the loss due to resistance?

[casdave]

One reason that bulbs blow more frequently is that when one does go pop it's often in a place where it is needed most.

Householders often don't have another handy so they will grab one from some other location which is less used.

Trouble is the act of removing, carrying and refitting weakens the filament which is more brittle than a brand new one since it has already had some use.

Bulb life on AC system can be extended by using zero crossing switches, these turn the power on as the voltage hits zero in its journey between +ve and -ve. It isn't cost effective in industry where most lights are flourescants and anyway incandescants are so cheap.

What really does do the damage is not the normal AC variations but switching on which causes a mechanical force to be exerted when the filament is not up to full working temperature, as has been said already, the instantaneous switching voltage can end up significantly higher than nominal.

In the UK we use 220v so our peak voltages are around 300V.This also has a side effect of meaning that insulation has to be thicker for ac than dc to cope with peak voltages, in fact heavy cables often have two figures moulded onto them, one for ac and one for dc.

Using low voltage d.c over anything but short distances of less than say a few metres is not an economic option because the conductors have to be thicker to reduce the resistance losses.

[DrDoom]

Geez ... this only goes to prove that only the people change ... not the basic fundamentals of science ... Westinghouse (A/C) and Edision (D/C) argued this same principle about 100 years ago ... ??? Go Figure !!!

[Diver]

Oops... I meant 60 hz. I was thinking 60 hz and 120 vac and typed the wrong thing.
Sorry about that...
The point is that with the low current required at 60 hz, even fairly low voltages can kill. As I recall, skin resistance varies from around 10,000 ohms to 1000 ohms or so depending on how moist the skin is. At 10,000 ohms only about 60 vac would push .006 amps through the body. That's enough to disrupt the heart rhythm.

[Crafter_Man]

Originally posted by casdave

Using low voltage d.c over anything but short distances of less than say a few metres is not an economic option because the conductors have to be thicker to reduce the resistance losses.


Hmm. Correct me if I'm wrong, but both AC and DC suffer from I^2 * R losses, right?

For a given amount of power, the voltage of an AC distribution grid is typically very high; this is done to keep the current as low as possible, thus requiring smaller, cheaper, and lighter conductors. (Or, you can use large conductors, and enjoy a smaller voltage drop.) But you can also do the same with a DC system, i.e. create a high voltage and keep the current as low as possible, thus requiring smaller conductors. In fact, when you just focus on the power that is transferred strictly along the conductors, a DC system is actually more efficient, because there are no reactive losses to deal with. (Both AC and DC systems suffer from the same I^2 * R losses.)

Anyway, here's my point: moving power from point A to point B along conductors is more efficiently done with DC than AC because you have reactive losses with AC. The problem only occurs when you have to step the voltage up or down. That's where AC has the big advantage, and that's why AC is predominately used for the power grid.

Now I may be completely in left field on this one, so please correct me if I'm wrong.

[Arjuna34]

In addition to the reactive losses Crafter_Man mentioned, AC systems suffer from the skin effect, which causes the current to concentrate more towards the edge of the conducter, rather than being uniformly distributed. DC doesn't suffer from this loss. Although you don't normally associate skin effect with frequencies as low as 60 Hz (the losses are neglible unless you've got some thick wire or buss bar), it is present. I think certain types of stranded wires are used to help minimize the effect, also.

Arjuna34

[Crafter_Man]

Originally posted by Arjuna34
In addition to the reactive losses Crafter_Man mentioned, AC systems suffer from the skin effect, which causes the current to concentrate more towards the edge of the conducter, rather than being uniformly distributed. DC doesn't suffer from this loss. Although you don't normally associate skin effect with frequencies as low as 60 Hz (the losses are neglible unless you've got some thick wire or buss bar), it is present. I think certain types of stranded wires are used to help minimize the effect, also.

Arjuna34

Very true. Though, like you said, the effect would be almost negligible due to the low frequency. But it is there.

[Arjuna34]

For large conductors the skin loss actually is significant at 60Hz - the "skin depth" at 60Hz is about 1/3 inch. For 500 MCM wire (rated at about 380 amps in conduit), the 60Hz resistance is 1.8% higher than the DC resistance. 2000 MCM wire has a 24% higher resistance at 60Hz than DC, which might explain why you don't see it too often :)

ACSR (aluminum conductor, steel reinforced) wire is common- this is aluminum wire with a steel center. Since not much current is carried in the center of the wire (due to the skin effect), steel is used there to increase strength.

Hollow conductors are sometimes used in high-voltage transmission lines. This reduces skin effect losses, and the larger diameter reduces the corona voltage. The wire dissipates heat faster with a larger surface area, so you can pump more current into, too.

Arjuna34

[justwannano]

Well it's been a long time since Electronics class but I do remember a little something about increasing efficency of AC with something called three phase.

In the real world three phase is the most commom way AC is recieved into your house. Thats why there are three wires. Each phase is 120 degrees out of phase with the other two.
That is one reason light bulbs last as long as they do. It smoothes the "AC hum" Actually you are looking at something like 180cycles (oops) hertz.The eye cannot detect that in an incandecent light.

Actually the skin effect does come into effect(can I say that?)with AC high tention lines. They are copper coated steel,hense high tention.

A house wired for DC would lack many of the little luxuries that we have today. Rechargable anything comes to mind first. Although it is possible to set up a lower voltage with DC you would have to set up a voltage divider and would lose most of the 120v to ground.

[CurtC]

justwannano wrote:
In the real world three phase is the most commom way AC is recieved into your house. Thats why there are three wires. Each phase is 120 degrees out of phase with the other two.
That is one reason light bulbs last as long as they do...

The electric company transmits the power as three-phase, but all the appliances in your house run on the voltage across two of them, which is single-phase. This includes light bulbs. Some heavy-duty industrial machinery uses three-phase, but you don't have any of these in your house. Common voltage for three-phase is 208 volts.

then he wrote:
...AC high tention lines. They are copper coated steel,hense high tention.

The "tension" in "high tension lines" is just another word for "voltage." It's not that the wires are stretched really tight, though they probably are. I think "tension" is how the French say "voltage," but I don't know why you often see high-voltage lines called that over here.

[Mikahw]

Over here at least, we don't have 3 phase going to the houses.

The 3 wires are on either side of the secondary coil of the transformer, and the centertap of the secondary coil.

Simple, really. hook up something to both sides, and you have 240V. Hook something up to one side and the centertap, and you have 120V.

I do remember about some place that uses 3 phase power for it's homes though. New York, maybe?

[justwannano]

:O I told you it was a long time ago:O

I forgot about Delta Wye circuits.

Not so sure about the tension though.

[Catenary]

If DC is transmitted to your house (or sub-station) at high voltage/low-current, you're stuck with low current. High voltage can be reduced to a usable level but, unlike AC, this doesn't increase the amount of current available.

[Arjuna34]

Originally posted by Catenary
If DC is transmitted to your house (or sub-station) at high voltage/low-current, you're stuck with low current. High voltage can be reduced to a usable level but, unlike AC, this doesn't increase the amount of current available.

With the right type of DC step-up/step-down circuitry, the amount of current can increase. It's possible to make a "DC transformer" with power electronics. The challenge is making it as low-loss and cheap as a transformer.

For the record, here's Cecil on single-phase/three-phase power: How come the U.S. uses 120 volt electricity, not 240 like the rest of the world? (http://www.straightdope.com/classics/a3_292.html)

Arjuna34