# Alternating Current vs Direct Current

1. Jun 14, 2006

### george1

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..

2. Jun 14, 2006

### G01

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!

3. Jun 15, 2006

### mathman

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.

4. Jun 15, 2006

### george1

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

5. Jun 15, 2006

### chroot

Staff Emeritus
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.

- Warren

6. Jun 15, 2006

### george1

ok thanks .... but i also hear that many things that run DC are more expensive... then AC...why is that?

7. Jun 15, 2006

### chroot

Staff Emeritus
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

8. Jun 15, 2006

### george1

oh i see cool thanks for the info...hopefully this will help me for my exam tomorrow :O LOL..

9. Jun 16, 2006

### mathman

One other need for dc is recharging storage batteries, such as needed to run cars.

10. Jun 16, 2006

### george1

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!

11. Jun 17, 2006

### 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.

12. Jun 17, 2006

### rcgldr

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.

http://www.northeast.railfan.net/diesel_faq.html

13. Jun 17, 2006

### G01

OK Good Point

14. Jun 18, 2006

### rbj

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.

Last edited: Jun 18, 2006
15. Jun 18, 2006

### rcgldr

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?

16. Apr 14, 2008

### halfANDhalf

how is ac and dc generated? please message me.
thanks

17. Apr 14, 2008

### halfANDhalf

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

18. Apr 14, 2008

### chroot

Staff Emeritus