Welcome to Flightinfo.com

  • Register now and join the discussion
  • Friendliest aviation Ccmmunity on the web
  • Modern site for PC's, Phones, Tablets - no 3rd party apps required
  • Ask questions, help others, promote aviation
  • Share the passion for aviation
  • Invite everyone to Flightinfo.com and let's have fun

Alternating Current vs Direct Current

Welcome to Flightinfo.com

  • Register now and join the discussion
  • Modern secure site, no 3rd party apps required
  • Invite your friends
  • Share the passion of aviation
  • Friendliest aviation community on the web

uwochris

Flightinfo's sexiest user
Joined
Dec 21, 2001
Posts
381
Hey guys,
Can someone explain the difference between alternating current and direct current output? I just don't really understand why some systems require AC, while others require DC. What makes one system require one type of power input, but not the other?

Thanks in advance.
 
uwochris said:
Hey guys,
Can someone explain the difference between alternating current and direct current output? I just don't really understand why some systems require AC, while others require DC. What makes one system require one type of power input, but not the other?

Thanks in advance.
MacGyver...is that you?
 
Affirmative! Don't tell anyone my secret identity ;)
 
Your question can have a thousand answers.

Let me try to read your mind and give this answer:

AC power by it's nature can send vast amounts of voltage (power) at very long distances.

DC power conversely is very difficult (and heavy) to send high power long distances.

Batteries by their nature are DC. However, you can change DC to AC by using an "Inverter". You can change AC to DC using a "Rectifier".

Read a book.

You have a simple airplane with simple needs.....lights, a flap motor, radios, maybe an electric fuel pump. 14 or 28 Volts DC is fine to run these relatively low powered things and the battery is already DC.

Now step up to a modern jet. Start with WX-Radar - you need a bunch of volts and amps to get a really strong RADAR. Do you beef up a DC system or do you Invert to an AC system? Add in TCAS, TAWS or GPWS. Let's throw some CRT's in the panel. How about the toilet motors? Electric heat for windows, windshields and the tips of the elevator horns and rudder horn. You are sending a lot of electricity a long way. Do you want to do this in AC or DC?

That just about sums it up. Airplanes usually add up to a gross weight. If to run all those high energy products in DC wiring and systems is going to cost you 1,000lbs of airplane parts, but will cost you 600 lbs in AC parts, which do you want to purchase?
 
Direct current flows one way. Alternating current alternates directions, forward and back(Once every 50th of a second). Almost all household appliances use DC at some stage in the circuitry by converting the AC to DC. AC creates electromagnetic fields that can be useful in circuits that don't require or use a constant, unchanging flow of electrons. With AC it is also easier to vary voltages through the use of transformers.

Hope that helps, it's been a couple years since I took electromagnetism so it isn't real fresh...
 
Jafar said:
Direct current flows one way. Alternating current alternates directions, forward and back(Once every 50th of a second). Almost all household appliances use DC at some stage in the circuitry by converting the AC to DC. AC creates electromagnetic fields that can be useful in circuits that don't require or use a constant, unchanging flow of electrons. With AC it is also easier to vary voltages through the use of transformers.

Hope that helps, it's been a couple years since I took electromagnetism so it isn't real fresh...

U.S. power supply is at 60 Hertz, or alternates every 1/60th of a second. AC in airplanes varies from 50 Hertz to 300 or more Hertz depending on designer desires.
 
tarp said:
AC power by it's nature can send vast amounts of voltage (power) at very long distances.



No, not true Voltage is not power, and AC is not inherently more suited to long distance transmission.

tarp said:
DC power conversely is very difficult (and heavy) to send high power long distances.

Also not true, see previous.

tarp said:
Batteries by their nature are DC. However, you can change DC to AC by using an "Inverter". You can change AC to DC using a "Rectifier".

This, at least, is true. the remainder of your post, being based on incorrect understanding, is misleading at best. Sorry. I don't mean to be rude, but there's very little useful information in your post.

Like Jafar says. DC is electron flow (or positron, or hole-charge flow, or whatever theory you like to explain the phenomena of electricity) in one direction. AC is when the polarity of the voltage, and thus the flow of electricity reverses periodically. Like the name says, Alternating current flow is both directions, alternately. Household electricity reverses polarity 60 times every seconds, or 60 Hertz (hz). A hertz is one cycle per second, 60 hertz is 60 cycles per second. Most AC aircraft systems are 400 hz systems.

The primary virtue of AC is that it's very easy to increase or decrease the voltage with relatively small energy losses. You can step up the voltage or step down the voltage of AC with a transformer, which is a very simple device with no moving parts. Not so with DC. You can decrease the voltage fairly easily, but unless you have a fairly sophisticated voltage converter your losses are about equal to the change in voltage. Ie; changing 24 volt to 12 volt in a simple "brute force" voltage converter, half the energy is wasted. That is why DC voltage converters always have cooling fins, half the energy input gets turned to heat. It is relatively difficult to *increase* the voltage of DC. Not impossible, but the simplest way is to use an inverter to change it to AC, run it through a step-up transformer, then turn it back to DC with a rectifier. Compare that to AC, if you have 115 V AC in your aircraft and you need say 2,000 Volts for a Cathode Ray Tube (radar, efis, etc) you just have a transformer built into the unit to step 115v up to 2Kv (or whatever) .

I gotta go, but I'll add some more information later.
 
A Squared: You are scaring your compadres - Not to hack on you, but Tarp is exactly correct.

AC power has 2 principal advantages:

1) Less efficiency loss in transmission (very significant over long distance) and

2) Smaller conductors required to transmit the same power (equals smaller wire or cable size, and smaller and lighter motors as compared to the DC equivalent)

Here's why: a good approximation of power is given by

Power = Voltage X Amperage

Easy to see that to make the same power in a 115 volt system requires far less amperage than say a 28 volt system. Why is that a big deal?

Anytime you move current anywhere, resistive heating (same effect that makes the heating element in your oven hot) takes place. The more current, the more heating. This heating (or loss unless your cooking) is approximated by

Power loss = Amperage squared X resistance.

So as the current goes up, the loss increases exponentially.

This is why AC is used on larger aircraft, as well as for bulk power transmission (ie across the country). Precisely for the purpose of limiting heating losses (and transmission line sag), voltage for most transmission systems is 345000 volts AC (or even 510kV way out west).
 
tarp said:
Your question can have a thousand answers.

Col. W.E. Kurtz said:
I watched a snail crawl along the edge of a straight razor. That's my dream. That's my nightmare. Crawling, slithering, along the edge of a straight... razor... and surviving.













:eek:
 
EDIT - Meathead, sorry that I covered some of the same territory as your post. I'm mostly trying to cover the fact that it's easier to step AC up or down than DC.

The reason AC is more efficient for long distance transmission is that you can step it up to extremely high voltages via a transformer. Remember that power (watts) is potential (volts) X current (Amps). Resistive loss is proportional to the current squared. So if you want to send a certain amount of power (watts, Kilowatts or Megawatts) down a power line, you can convert it to an extremely high potential(volts), low current (amps) via a transformer. It then is transmitted across the power lines efficiently, with little loss. When you get to the house, you can then step it back down with another transformer.

The reason why this is not done with DC is that there's no direct equivalent of a transformer for DC to easily step up or down the voltage efficiently by a factor of 100-1000 - the principle of a transformer is based on AC current. There are mechanisms to step direct current up to high voltage/low current, and back down again, and it is sometimes used for long distance transmission, but it's easier to use transformers and AC.

There are some limited uses where DC High Voltage transmission is preferred, but in most cases, AC is used, due to the ease in stepping voltages up and down.

A couple of interesting Wikipedia articles:
http://en.wikipedia.org/wiki/Electric_power_transmission
http://en.wikipedia.org/wiki/High_voltage_direct_current
 
Last edited:
Death, Money, and the History of the Electric Chair



The history of the electric chair and death by execution.

During the 1880's two developments set the stage for the invention of the electric chair. Beginning in 1886, the New York State Government established a legislative commission to study alternate forms of capitol punishment. Hanging was then the number one method of carrying out the death penalty, even while considered too slow and painful a method of execution. Another development was the growing rivalry between the two giants of electrical service. The Edison General Electric Company founded by Thomas Edison established themselves with DC service. George Westinghouse developed AC service and started the Westinghouse Corporation.

What is AC? What is DC?

DC (direct current) is electric current that flows in one direction only. AC (alternating current) is electric current that reverses direction in a circuit at regular intervals.

The Birth of Electrocution

DC service depended on thick copper electrical cables, copper prices were rising at that time, DC service was limited by not being able to supply customers who lived beyond a few miles of a DC generator. Thomas Edison reacted to the competition and the prospect of losing to AC service by starting a smear campaign against Westinghouse, claiming that AC technology was unsafe to use. In 1887, Edison held a public demonstration in West Orange, New Jersey, supporting his accusations by setting up a 1,000 volt Westinghouse AC generator attaching it to a metal plate and executing a dozen animals by placing the poor creatures on the electrified metal plate. The press had a field day describing the horrific event and the new term " electrocution " was used to describe death by electricity.
On June 4, 1888, the New York Legislature passed a law establishing electrocution as the state's new official method of execution, however, since two potential designs (AC and DC) of the electric chair existed, it was left to a committee to decide which form to choose. Edison actively campaigned for the selection of the Westinghouse chair hoping that consumers would not want the same type of electrical service in their homes that was used for execution.
Later in 1888, the Edison research facility hired inventor Harold Brown. Brown had recently written a letter to the New York Post describing a fatal accident where a young boy died after touching an exposed telegraph wire running on AC current. Brown and his assistant Doctor Fred Peterson began designing an electric chair for Edison, publicly experimenting with DC voltage to show that it left the poor lab animals tortured but not dead, then testing AC voltage to demonstrate how AC killed swiftly.
Doctor Peterson was the head of the government committee selecting the best design for an electric chair, while still on the payroll of the Edison Company. It was not surprising when the committee announced that the electric chair with AC voltage was chosen for the statewide prison system.

Westinghoused

On January 1, 1889, the world's first electrical execution law went into full effect. Westinghouse protested the decision and refused to sell any AC generators directly to prison authorities. Thomas Edison and Harold Brown provided the AC generators needed for the first working electric chairs. George Westinghouse funded the appeals for the first prisoners sentenced to death by electrocution, made on the grounds that "electrocution was cruel and unusual punishment." Edison and Brown both testified for the state that execution was a quick and painless form of death and the State of New York won the appeals. Ironically, for many years people referred to the process of being electrocuted in the chair as being "Westinghoused". Edison's plan to bring on the demise of Westinghouse failed, and it soon became clear that AC technology was vastly superior to DC technology. Edison finally admitted years later that he had thought so himself all along.


LINK




eP.
 
The simple explanation you were probably looking for is...

Direct current flows one way in a cirlce. So positive lead on the airplane battery goes to electrical stuff and then returns to negative lead. The power travels around in a cricle.

Alternating current (into your house) has a single wire from the power station and the power goes back and forth over a single wire. There is no negative wire back to the power station.

http://science.howstuffworks.com/electricity1.htm
 
cynic said:
The simple explanation you were probably looking for is...

Direct current flows one way in a cirlce. So positive lead on the airplane battery goes to electrical stuff and then returns to negative lead. The power travels around in a cricle.

Alternating current (into your house) has a single wire from the power station and the power goes back and forth over a single wire. There is no negative wire back to the power station.

http://science.howstuffworks.com/electricity1.htm

Check out the cord on your lamp. If you were to cut it open, you'd find out that there are two strands (don't do this, at least until you unplug it). That's why you have two prongs. To put it in simple terms, the electricity still flows in a loop. It's just that, in alternating current, the direction of the flow reverses, on the order of 120 times a second (so it changes back to the same polarity 60 times a second). In simplified turns, the two wires take turns being the "out" and "in" wire.

Actually, it's not a hard reversal (square wave), but a smooth transition (sinusoidal wave).

EPilot22, I love that story. We always have this saintly image of Edison, but he could be an SOB when it came to competition.
 
mzaharis said:
EPilot22, I love that story. We always have this saintly image of Edison, but he could be an SOB when it came to competition.

Sure sounds like it. How about the animal cruelty? Not a very nice image either.


I like the compounded words electric and execution. If it wasn't for the electric chair we wouldn't have the word electrocution. I wonder what it'd be called when we got shocked?




eP.
 
ePilot22 said:
I wonder what it'd be called when we got shocked?
Go make crazy faces and wave your arms wildly in front of a TAZER toting traffic cop and let us know how you do.:nuts:
 
I prefer matter/anti-matter. Much cleaner. 500 lightyears per crytsal.
 
FN FAL said:
Go make crazy faces and wave your arms wildly in front of a TAZER toting traffic cop and let us know how you do.

I would imagine it would be something like this... KRUMPING.




eP.
 
Meathead said:
A Squared: You are scaring your compadres - Not to hack on you, but Tarp is exactly correct.



No, he is absolutely *not* correct.

Meathead said:
AC power has 2 principal advantages:

1) Less efficiency loss in transmission (very significant over long distance) and

2) Smaller conductors required to transmit the same power (equals smaller wire or cable size, and smaller and lighter motors as compared to the DC equivalent)


No, sorry, you are every bit as mistaken as Tarp. Efficiency of transmissin and the use of smaller wires is absolutely not a characteristic of AC.

Here's what *IS* correct:

It is more efficient to transmit power (and you are able to use smaller wires) using higher *VOLTAGE* not "because it is AC". This effect has absolutely *nothing* to do with being AC and has everything to do with the voltage. The exact same is true with higher voltage DC. THat's the reason that many small airplane manufactureers have switched from 12 volt DC systems to 24 volt systems; Because it is more efficient to transmit the same power at higher voltages. If you need 120 watts at the wingtip (just for example) , you need 10 amp in 12 volt system, but only 5 amp in a 24 volt system. You can use smaller (therefore lighter) wire to transmit the same power with the 24 volt system.

The effect is completely independent of whether it's AC or DC. In fact, if you had an airplane which had 115 Volt DC power and 26 Volt AC power (unlikely arrangement, but it's a hypothetical situation) it would be much more efficient in terms of power loss and wire weight to transmit power to the high power items using the DC rather than AC.

Once more, in case you missed it the first five times: It has nothing to do with being AC.


Meathead said:
This is why AC is used on larger aircraft, as well as for bulk power transmission (ie across the country). Precisely for the purpose of limiting heating losses (and transmission line sag), voltage for most transmission systems is 345000 volts AC (or even 510kV way out west).

Nope. There are a lot of reasons why AC power is used for transmission, Efficiency isn't one of them. A 510KV DC transmission line would be exactly as efficient as a 510KV AC transmission line.

So why is AC used? As mentioned, it's primarily because it it's easier to change it's voltage. Some other reasons are because it's nicer to work with in instrumentation. It's easier to make an AC gyro which spins at a very stable speed than it is a DC gyro. DC motors tend to vary speed with voltage fluctuations more than AC motors. It's easier to make more accurate sensors and indicators with AC than DC. A capacitance fuel guage, for example requires AC. There are Capacitance guages on some DC planes, but the instrument itself will have to have a small ac power supply, often built into the unit.

So why aren't *all* planes AC powered? Because AC power is more difficult to make. Actually it's not, AC power is very easy to make, much easier than DC power. All rotating generating devices inherently produce AC. That's the only thing they *can* produce. In DC alternators and generators, that AC power is rectified, either using diodes or a commutator (which is essentially a mechanical rectifier) What *is* difficult to generate is AC power with a stable frequency. It requires a Constand Speed Drive for the generator, which is heavy, expensive and complex .... Or it can be made from DC which requires an inverter, which adds weight and expense, and loses energy.

SO for small airplanes, where the electrical power requirements are small, the benefits of AC don't outweigh the extra weight, cost and expense.

On larger airplanes, with higher power demands and complex electrical systems, the benefits of AC make it worth the added complexity to produce it.
 
And remember, when it comes to gauge needles: AC lies, DC dies.
 
Why 400 Hz for aircraft power?

Some of you might be wondering why the aircraft that use AC power use 400 Hz instead of the 60/50 Hz that is generated by land based power systems. The reason is to save weight. In order to change the voltage of an AC power system a transformer is used. A transformer has two coils of wire wrapped around an iron core. The voltage change is proportional to the ratio of the number of turns of wire in the two coils. A 2 to 1 turns ratio means a 2 to 1 voltage ratio.

The efficiency of a transformer depends on the amount of magnetic field that saturates each of the coils of wire. The magnetic field depends on how much iron is used in the transformer. More iron means a more efficient transformer. It turns out however, that if you use a higher frequency, you can get the same efficiency with less iron (and weight) than you could using a lower frequency current and a bigger iron core. So aircraft systems use 400 Hz power instead of 60 Hz power so that they can use smaller lighter transformers.
 

Latest resources

Back
Top