Alternating Current vs Direct Current

[uwochris]

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.

 

[FN FAL]

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?

 

[uwochris]

Affirmative! Don't tell anyone my secret identity

 

[tarp]

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?

 

[Jafar]

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...

 

[Ralgha]

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.

 

[A Squared]

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.

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

Also not true, see previous.

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.

 

[Meathead]

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).

 

[FN FAL]

Your question can have a thousand answers.

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.

 

[mzaharis]

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