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Alternating Current vs Direct Current

by george1
Tags: alternating, current
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george1
#1
Jun14-06, 05:24 PM
P: 5
Can someone explain breifly or in detail the differnece between ac and dc. Maybe advantages and disadvantages. From what i have researched i know that ac the current flows in every direction while in dc it only flows in one. In ac the voltage can be put down and up using transformers as they use ac in them. But can someone tell me a little more regarding these two..
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G01
#2
Jun14-06, 09:41 PM
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Well lets start with the basics...

DC stands for direct current. It is current that starts at one place and flows in one direction to the end destination, hence the name Direct.

AC stands for Alternating Current. The current flows in one direction for a period of time and then switches direction, going the opposite way. It switches diretion over and over again continuously. In the united states the AC current in power lines goes switches direction, forward to backward, then backward to forward, 60 times each second. This is a frequency of 60 Hertz and is called 60Hz AC electricity.

Many electrical appliance, computers for example use DC current. Yet our power outlets supply AC current. The computer has a power supply that contains a step down AC to DC converter. The AC to DC part is self explanatory and the step down part means that the voltage is lowered from the 120 V coming in to whatever the computer requires. You are right when you say that tranformers are used to change voltages in AC current, but your reasoning is incorrect. The transformer does not use AC electricity up in anyway. There is, ideally of course, no energy lost in a transformer. How a transformer actually changes the voltage of a current requires some knowledge of magnetic fields and a good understanding of what a voltage actually is. In my experience many people do not really understand voltage. That is a separate topic and I dont have room to get into it here. For more information I suggest the book Electricity Demystified, or if your really interested in voltages and magnetic fields(as you should be there are very interesting concepts!!!), then I suggest picking up an intro to physics text.

Anyway, you may be wondering why, when many devices use DC current, we produce AC current. This is because AC current is much easier to produce. AC generators can come in many different forms and be powered by many different things, wind, Nuclear, oil, hydro-power, etc. These generators use magnets in such a way that they produce AC current. This has to deal with magnetic fields and how magnetic fields can induce electric current. Again its kinda complicated to fit into the post, so I suggest a book on electricity or an intro to physics text.

Hope this helped!
mathman
#3
Jun15-06, 04:15 PM
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One major (maybe the major) reason for using ac rather than dc is the need for long range transmission. Transmission line power loss is proportional to the square of the current. To keep the current to a minimum while keeping the energy transmission at a high enough level it is necessary to use transformers. These raise the voltage to a very high level and lower the current accordingly. Direct current cannot be transformed so transmission would have to be at the low voltage safe for customer usage.

When electric generation first started (before 1900), there was competition between ac (Westinghouse) and dc (Edison). However the need for long distance transmission killed off the dc generation industry.

george1
#4
Jun15-06, 04:51 PM
P: 5
Alternating Current vs Direct Current

ok thanks guys but then why use dc at all...like why do many thigns use dc? instead of ac like many computers and other electronics
chroot
#5
Jun15-06, 04:59 PM
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Microelectronics use DC because they are composed, essentially, of transistors. Transistors are small "switches" which use voltages as control signals. If the control signals were changing polarity many times a second, the switches would open and close many times a second, leading to, well, a mess. It'd be very hard to make a digital computer which could use AC.

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george1
#6
Jun15-06, 07:08 PM
P: 5
ok thanks .... but i also hear that many things that run DC are more expensive... then AC...why is that?
chroot
#7
Jun15-06, 07:34 PM
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You mean like hair dryers and refrigerators and so on? Generally, devices with motors can be made to use either AC or DC. Neither is inherently more expensive to make, really, but supply and demand economics encourages whichever is less widely used to be more expensive.

- Warren
george1
#8
Jun15-06, 08:50 PM
P: 5
oh i see cool thanks for the info...hopefully this will help me for my exam tomorrow :O LOL..
mathman
#9
Jun16-06, 04:33 PM
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One other need for dc is recharging storage batteries, such as needed to run cars.
george1
#10
Jun16-06, 06:58 PM
P: 5
okk cool thanks again my exam went pretty good there was only one question about ac vs dc and it was why is ac used to transmit electricity over long distances rather than dc? And i think i put a good enough answer for that question at least:) so thanks again..

multiple choice was a little tricky somewhat but it was all good

thanks again to everyone!
rbj
#11
Jun17-06, 09:54 AM
P: 2,251
Quote Quote by G01
...
Anyway, you may be wondering why, when many devices use DC current, we produce AC current. This is because AC current is much easier to produce. ...
i think it's more accurate to say that AC is much easier to transform than DC. in many, many, contexts DC is easier than AC to produce.
rcgldr
#12
Jun17-06, 02:00 PM
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Regarding ac versus dc motors, it's easier / cheaper to make dc motors with a wide range of rpms. AC motors require a frequency change in order to change rpm which is generally done by going from ac to dc and back to ac again, but there are cases where ac motors are used with a wide range of rpm, such as locomotive diesel electric ac traction motors. The link below metions one advantage of the AC motor is that there is reduced maintainance because there are no brushes to service. I don't know if or why brushless dc motors couldn't be used as locomotive traction motors.

Some links:

http://www.epanorama.net/links/motorcontrol.html

http://www.northeast.railfan.net/diesel_faq.html
G01
#13
Jun17-06, 10:40 PM
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Quote Quote by rbj
i think it's more accurate to say that AC is much easier to transform than DC. in many, many, contexts DC is easier than AC to produce.
OK Good Point
rbj
#14
Jun18-06, 01:23 AM
P: 2,251
Quote Quote by Jeff Reid
Regarding ac versus dc motors, it's easier / cheaper to make dc motors with a wide range of rpms. AC motors require a frequency change in order to change rpm which is generally done by going from ac to dc and back to ac again, but there are cases where ac motors are used with a wide range of rpm, such as locomotive diesel electric ac traction motors. The link below metions one advantage of the AC motor is that there is reduced maintainance because there are no brushes to service. I don't know if or why brushless dc motors couldn't be used as locomotive traction motors.
not all AC motors are brushless. some AC motors have external current applied to the rotor windings and you need "brushes" to do that.

some AC motors are brushless which means that the rotor is either some kind of permanent magnet (not likely except for illustrative models, perhaps stepper motors have PM rotors) or the rotor has self-looping windings and the current in those windings are induced. The speed of rotation of these induction motors is not constant, but decreases as a mechanical load increases. if the angular speed of the rotor was synchronous to the frequency of the AC applied to the stator windings, then the current in the rotor windings would have to be DC (making those windings into fixed magnets like PMs). so the speed of rotation of the rotor is slightly less than the synchronous speed when the motor is "running light" (no external mechanical load) and decreases as the load (torque) increases. also as the speed of the rotor decreases, the frequency of the AC induced into the rotor winding increases. this frequency is proportional to the quantity we call "slip" (which is 1 - (rotor speed)/(synchronous speed)). at "running light" the slip is almost zero and the rotor current frequency is nearly 0, but not quite. at "blocked rotor" (where the rotor is fastened so it cannot turn and juice is still applied to the stator windings), the slip is 1, the frequency of the rotor current is the same as the AC applied to the stator, and this motor is equivalent to a transformer with a short circuited secondary winding (which is the rotor winding). "blocked rotor" or "short circuited power transformer" with power applied is scary (smoke fills room).

oh, and a brushless DC motor must have power electronics that do the current reversal or commutation in a winding that the brushes would do if it had them.
rcgldr
#15
Jun18-06, 03:34 AM
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a brushless DC motor must have power electronics that do the current reversal or commutation in a winding that the brushes would do if it had them.
I'm familiar with these, as brushless dc motors are very popular for radio aircraft these days. What I was wondering is what is the advantage of a brushless AC motor over a brushless DC motor for use as a traction motor on a locomotive. The AC motor can be used as a generator during braking (saves power), is this not possible with a DC brushless motor?
halfANDhalf
#16
Apr14-08, 04:46 PM
P: 13
how is ac and dc generated? please message me.
thanks
halfANDhalf
#17
Apr14-08, 05:38 PM
P: 13
well i was wondering if you guys can help me on this same subject because i have a pretty crappy science teacher and he doesnt do much so please help me out on this.
thanks
chroot
#18
Apr14-08, 05:41 PM
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Stop resurrecting ancient threads to ask for help with your homework. Please post your homework questions in the appropriate forum.

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