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Why AC instead of DC and why 50/60 Hz?

By on April 25, 2016
Why AC instead of DC and why 50/60 Hz?

 

 

 

 

 

 

 

 

If you ever asked these questions to yourself, welcome to the club. These two questions are amongst the most frequently asked questions by Electrical/Electronic engineering students. And they are very pertinent questions indeed.

To be honest some might have been asked out of frustration, after a few weeks of dealing with DC where everything was a straight line. With DC they were in an environment based on a solid and stable ground where voltage and current lived a happy marriage under the friendly Ohms law. Suddenly they faced AC, this never ending rotating thing. They had to deal with frequency, period, phase, radians, vectors, cos φ and ω related formulas etc… So many things that appeared to have been invented to complicate the student’s life… And this AC world was sitting on a shaky ground where voltage and current seemed to live separated lives, often ignoring Ohm’s law! So why don’t we use DC only?

The War of Currents!

In fact, the question was still open when the world’s electrification began in late 1800’s, and remained open for quite a long time, opposing two companies in what was called the “War of Currents”. Edison Electric Light Company founded by Thomas Edison (1847-1931) promoted DC, while Westinghouse Electric Company founded by George Westinghouse (1846-1914) and backed by Nikola Tesla (1856-1943) promoted AC.

war of currents

AC supporters argued that they could easily step up the voltage using transformers, then transport electricity over much longer distances. The idea is simple, to supply 10 kW under 100 V would need 100 A (I = P/V). The same 10 kW under 10 kV would need 1 A only! Less current means smaller wires (less copper) and fewer losses in the line. On the other side, DC could not be transformed easily (at that time) and would have to be generated and transported at the user’s voltage. This would need huge cables to transport a large amount of power; hence DC was not suitable for long distances. The figure below illustrates this advantage (ignoring the losses in the transformers for simplification!):

simplification

DC supporters argued that the AC high voltages were dangerous and would kill people. The battle became nasty with Edison electrocuting animals in public to make his point…

The “War of Currents” ended in 1892 when Edison Company merged with one of their AC competitors, Thomson-Houston, creating a company named General Electric and embracing the AC technology. AC had won the battle thanks to the invention of the transformer! In the meantime both sides had built equipment, plants and grids so the electrified world was using partly DC and partly AC. DC was still used well into the 20th century as DC grids existed until the 1960th. Consolidated Edison in New York cut off its last DC customer in 2007 after 125 years of services!

I can remember fixing those AC/DC radio receivers still common in the 60th and I almost died on one of those… They were transformer less, hence a real danger as one side of the mains would be connected to the chassis, depending on which way you inserted the plug. One of the designer’s challenges had been the vacuum tube filaments which, on other types of radio receivers, were all connected in parallel to a 6.3 VAC winding of the transformer’s secondary. As AC/DC receivers had no transformer, special vacuum tubes were designed for this application. All the tubes filaments were designed for a common current of 100 mA with different voltage drops depending on the tube’s power requirement, so they could be connected in series. The total voltage for 5 tubes filaments in series would be approximately 115 V. In the US those receivers were named “All American Five” as they had five vacuum tubes and could be used anywhere in America whether the supply was AC or DC! The European version used a large resistor to drop the filament voltage from 220 V down to 115 V! The figure below shows one of the versions of those AC/DC radio power supplies.  There were several variants but all based on the same principle. Interesting to note on the figure below that if you reverse L and N the chassis will be Live through the chain of filaments when the switch is OFF.  This is why I nearly died…a real danger!

ac dc radio power supply

Interestingly, the first vacuum tube TV sets took advantage of this design idea, but that time for economical and practical reasons. Without transformer, TV sets were lighter and cheaper. However, they were also dangerous for the same reasons mentioned above. Fortunately at that time they didn’t need to be connected to any peripherals (except the antenna!) and were totally isolated. Most TV sets remained that way until they became transistorized!

Despite having lost the “War of Currents” DC is not dead yet and we are currently witnessing an amazing recovery. With modern technology converting AC to DC and DC back to AC is economically possible even for high power/high voltage applications. Then it was found that transporting high voltage DC (HVDC) over very long distances (over 600 km) is cheaper than using AC… many links already exist around the world and this field in in full expansion. Amongst other advantages, HVDC links can interconnect unsynchronized AC grids having different frequencies. Many people (including my students!) start to think that Thomas Edison might have been right after all!

Can we replace AC with DC in your house?

 

 

Most of us in the repair world take the AC supply for granted. We always had it this way! But try to imagine what would happen if your electricity supplier suddenly decided to power your entire home with DC instead of AC, assuming maintaining the same voltage (110 or 220V). Would everything work? This is a heated debate and can generate interesting thinking, here is a summary:

–        All the appliances presenting a resistive load such as heaters, toasters, grills, irons etc… should work; providing they do not include electronic gadgets such as timers or other fancy stuff.

–        Incandescent lamps will work.

–        Appliances using “Universal Motors” such as vacuum cleaners, electric tools and many kitchen appliances should work.

–        Appliances using induction motors such as cooling fans, air-conditioning etc… would not work unless the motors were replaced with DC or Universal Motors.

–        What about most of the SMPS used as chargers or power supply for our electronic apparatus and gadgets. Would they work if we supplied the same voltage in DC instead of AC? My feeling was a yes, because they convert the AC input into DC anyway. But I was a little worried about the voltage. When you convert AC into DC using a rectifier and filter capacitor the resulting voltage will be higher than the AC rms value, actually around 1.41 times the rms value…  So an input of 240 VAC (rms) will result on around 340VDC. The figure below illustrates this:

smps input voltage

To make my point and learn a bit more I could not resist putting an SMPS under test. First I connected the SMPS into its normal conditions to confirm the voltage across the filter capacitor:

smps under test

While we are at it, let’s do some extra measurements and calculations (mainly for the fun of it!)

wattmeter cheap

Back to our subject, if we power the SMPS with DC instead of AC the voltage across the capacitor will be the same as the DC input voltage because DC doesn’t have different rms and peak values. The rectifier and capacitor will have no effect except for a very small voltage drop in the rectifier’s diodes. So 240 VDC at the input will give 240 V across the capacitor (instead of the 340V we had with AC). Will it work? You bet it will:

switching power supply

This SMPS is rated 100-240 VAC, which will correspond to around 141 to 340VDC across the capacitor (remember we multiplied by 1.414). With 240 VDC we are well into specs. But will it work under 110 VDC? Theoretically not because it would be out of our 141-340V range but let’s try:

wattmeter to check on wattage

I reduced the AC input to around 90V until the voltage across the capacitor reaches 110V (almost!) to simulate a 110 VDC input. And the SMPS is still working fine. So the designer took some margins.

However, even if something will work on DC, that doesn’t mean it will work reliably. Relays, switches, thermostats etc… have a different current rating for AC and DC. Because DC forms arcs and doesn’t pass through zero to reduce them promptly, switches DC current rating is usually much lower than AC current rating. This could be a difficult problem to solve.

But yes, it is possible and you will find a lot of animated debates on this subject on the Internet… with the event of alternative energy sources like solar panels generating DC the subject became hot again as many people dream about converting their house power supply to DC! The question is the voltage and how to avoid too many conversions. They cannot supply a 1200W iron with 12VDC (providing it would be designed for that voltage!) as it would take 100A.  Some proposed two supplies, one with higher voltage (110 or 240 VDC) and one with lower voltage (12 VDC or even 5VDC) …

Now what about the frequency?

Was there a “War of Frequencies”? Not as far as I know… Why couldn’t the frequency be lower, like 10 Hz for example or higher, like 100 Hz or something totally different? This is another interesting question and the answers are found in history and compromises.

Let’s first look at the lower and upper limits:

At the lower limit side we find one of the big electricity consumers, lighting. If the frequency were too low, lamps would be flickering as the voltage passes through zero twice every period. Any frequency below 25 Hz would not be acceptable for lighting.

At the higher limit we find the electricity producer! Electricity is generated using rotating machines, called alternators. A higher frequency would request the alternators to run faster or having more poles with all the mechanical problems, cost and reliability issues that may include. And then, some history: the main users for electricity at that time were traction motors, mainly for the railways. DC-powered series-wound traction motors had already become common because of their excellent starting torque and relatively easy speed control. Theoretically, they can run on AC as well; however those early motors did not work well with higher frequencies because their large windings and heavy pole pieces had too much inductive reactance and eddy current losses.  This is why some countries had one frequency for lighting and another for traction!

Then some technical limits:

The losses in the transmission lines increase with the frequency because of the inductance and capacitances of the lines. The lower the frequency, the better! DC is even better! The following figure shows how a transmission line appears to DC, then to AC. In AC the losses are function of the frequency as we can see in the formulas.

transmission line equivalent circuit

Then, to make the decision even more difficult, transformers and induction motors are more efficient at higher frequencies… so they can be smaller!

So what? The frequency could not be too low and also could not be too high… many different frequencies such as 40 Hz, 100 Hz or others have been tried at various places before the current standards, when 60Hz was adopted in America and 50Hz was adopted in Europe. For some other countries, the frequency adopted would depend on where they purchased their equipment or on some politico/commercial decisions! Japan for example still has two frequencies in use, 50 Hertz in eastern Japan and 60 Hertz in western Japan. In some countries they still use a lower frequency for the railway systems only, such as 16.7 Hz. I still remember the flickering lamps at the railway stations in Switzerland during my childhood. The engineers who designed the whole railway system based on 16.7 Hz must have thought that flickering lamps should not be a problem while you are waiting for your train. Trains were always on time so the waiting should be short… Now the Swiss railway is still running at that 16.7 Hz frequency but the stations lighting is connected to a 50 Hz supply… I hope that trains are still on time!

Aviation use 400 Hz to take advantage of smaller and lighter transformers and motors. As the power doesn’t have to be transported over long distances (within and aircraft) the losses in conductors are negligible.

There are a lot of debates about which one 50Hz or 60Hz is the best. You can say that 60Hz allows for more efficient transformers while 50Hz allows for more efficient energy transmission but people will still dispute … Actually it doesn’t really matter as far as we are concerned. Most of our equipment will work well with either frequency. But beware that synchronous motors will run faster with 60Hz than with 50Hz.

Conclusions

DC is catching up quickly because of lower losses in the transmission lines. DC also does not have those annoying phase shifts between voltage and current producing reactive power and power factor issues… and you don’t need to worry about the frequency! DC also can be stored into batteries!

AC is simple to produce and to transform. A simple rotating magnet near to an inductor produces a nice AC sine wave voltage as does the dynamo of your bicycle… AC has been serving us well for many years. Industries appreciate the simplicity, reliability and efficiency of 3 phase motors. Small transformers can be used to build simple, well isolated and interference free power supplies.

The “War of Currents” is not over yet but instead of opposing each other both systems will end up working together for the benefit of all of us.

gerald musy

Gerald Musy

Penang, Malaysia

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You may check out his other article below:

https://www.jestineyong.com/how-to-measure-current-with-your-scope/

 

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24 Comments

  1. Paris Azis

    April 25, 2016 at 3:24 pm

    Hi Gerald

    Another perfect article in every aspect of it. Although I knew about the story, I enjoyed your presentation of it. Thanks a lot for sharing it.

    Greetings

    Likes(0)Dislikes(0)
    • Gerald

      April 26, 2016 at 4:24 pm

      Hi Albert,
      Thanks for your support and your enthusiastic comments. But I need to clarify a few points:

      When I wrote that “Many people start to think that Thomas Edison might have been right after all!” This was not the conclusion of my article and I did not mention that I was amongst those “many people”. Indeed I never support any side regarding history, first because history about any subject is different depending on who wrote it and second because I was not there to judge, actually I was not even born 🙂

      Anyway this comment, made by others, was purely technical and meant that many reasons to disqualify DC at that time are no longer valid today. HVDC is a perfect example. If they mentioned Edison is because he was representing DC, I assume this part is correct…

      Regarding motors efficacy etc… the Internet is full of contradictory stories but also interesting information. Some are (almost religiously) supporting of the DC brushless motors, which in fact are some kind of AC motors… I also read a comment that Telsla would be using tree phase AC motors in their electric car!

      The suitability of a particular type of motor cannot be generalized as it depends on the application. Well this is another subject in which I prefer not to be involved. I hope this removes at least a part of the confusion.

      Cheers,
      Gerald Musy

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    • Gerald

      April 26, 2016 at 4:25 pm

      Hi Paris, thanks for your comment and support.

      Likes(0)Dislikes(0)
  2. Lee

    April 25, 2016 at 3:28 pm

    Nice informitive artical Gerald

    Likes(0)Dislikes(0)
    • Gerald

      April 26, 2016 at 4:29 pm

      Thanks Lee

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  3. albert van bemmelen

    April 25, 2016 at 4:26 pm

    Talking about Energy Gerald, I admire the Frequency and the great Power of information in the articles you write. However you left me confused a bit after the conclusion that Edison maybe was right all along about using his DC Generators. I think it was the great inventor Tesla that invented and patented things that Edison never could have and that made the world as we know it today. Just as Tesla invented Wireless Radio Transmission and not Marconi (who stole his ideas). And Edison in a way bankrupted Tesla in a horrendous malicious and scandalous way. Mainly because Edison (who never really invented new things but had in fact a trial and error testing Lab at home) couldn't bare his defeat.

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    • Albert van Bemmelen

      April 25, 2016 at 8:05 pm

      PS: Is the fact that DC Generators need a more demanding design with a Commutator, which needs more maintenance in time, not the main reason why the AC Generators are in use everywhere today? As they will last longer too.
      That is what I remembered. I read that people in the early years of the 1900's before the AC Generator was invented by Tesla himself (!) thought only DC motors could exist? Until Tesla proved otherwise.

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      • albert van bemmelen

        April 26, 2016 at 5:31 am

        PS2: In all new (LG etc.) Washingmachines, DC motors with those fast wearing DC motor brushes are replaced by terrific working Paykel Fisher AC Motors. Which are perfectly rPM controllable too. The nice part is that they even can be used as Power Generators. On Windgenerators etc.

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        • Albert van Bemmelen

          April 27, 2016 at 5:36 am

          Thanks for your much appreciated reply on my added little comments Gerald. I was merely pointing out why the brushless AC Turbines are chosen over the early DC generators then and now and why Tesla's AC system had won. Tesla won also because the AC Generators were mechanical much better and could generate more power more reliably than Edison's Generators could. Tesla was in the beginning when still working for Edison, send out many times to fix broken DC Generator Systems for Edison's company. Tesla said that he could redesign Edison's inefficient motor and generators, making an improvement in both service and economy. But Edison didn't come through on his promise of payment after months of work which Tesla fulfilled. So Tesla resigned. This is a historical fact but was somehow not mentioned. Which is also why the better AC generators are now over about 120 years used in Power Plants. And added to the longer distances that can be reached with AC lines.

          AC is even better than DC today in automotive style Alternators. For the same amperage output, an Alternator is usually lighter in weight, more compact in size and doesn't need the old DC type anymore. An AC Alternator also gives its necessary Stationary Battery charging Output at much lower rPM's compared to the previous also much heavier DC Alternators. Which is also why Edison lost the DC 'War'. So I hope your students do not think that Edison made our future as we know it. I think he mainly tried to stop good progress.
          Cheers!

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  4. Robert Calk

    April 25, 2016 at 5:29 pm

    Thanks Gerald. I say keep A/C.

    Likes(3)Dislikes(0)
    • Gerald

      April 26, 2016 at 4:28 pm

      Thanks Robert, you are a wise man.

      Cheers,
      Gerald

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  5. Parasuraman S

    April 25, 2016 at 8:59 pm

    Very informative, refreshing article. I went back to the 1980s, when I did my private Diploma in Radio Engineering (Something I had forgotten to mention in my profile!) It is well written and presented!

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  6. Matsuoka

    April 25, 2016 at 9:17 pm

    Well Gerald, you woke up my memories of the days I many times got elec-shocked when playing with my self-assy 5-tube receiver with 35W4, 50C5, 12BE6, 12BA6, 12AV6 lamps. When grew up I dealt with b/w Japanese TVs with 25E5, 30AE3 etc... and got shocked not by hot chassis but earphone output socket. Thanks a lot because it was 50yrs ago and those mostly faded away in my mind. Regards. Matsuoka.

    Likes(3)Dislikes(0)
    • Gerald

      April 26, 2016 at 4:31 pm

      Thanks Matsuoka,yes it was a long time ago but I still remember the feeling 🙂

      Cheers,
      Gerald

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  7. Humberto

    April 25, 2016 at 11:41 pm

    Hi Gerald, thanks for this article. Very informative. Keep up.

    Likes(0)Dislikes(0)
    • Gerald

      April 26, 2016 at 4:32 pm

      Thanks for your support Humberto

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  8. beh

    April 26, 2016 at 12:04 am

    Hi Gerald
    thank you so much of your very interesting comparison between Ac and DC.
    beh

    Likes(0)Dislikes(0)
    • Gerald

      April 26, 2016 at 4:32 pm

      Thanks for your support Beh

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  9. Anwar Shiekh

    April 26, 2016 at 12:22 am

    I recall a story about large US 60Hz transformers being used abroad on 50Hz; they were saturating and so burning out. The solution was to run them at lower voltage to avoid the saturation at the lower frequency.

    I once built a small inverter and ran it at 400Hz so the transformer could take a higher load, and very few devices seemed to mind the higher frequency. 400Hz is about the limit one can push an iron cored transformer.

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    • Gerald

      April 26, 2016 at 4:38 pm

      Thanks Anwar for your support and for sharing your experience. Yes 400 Hz is used in aviation for good reasons and I assume that they have different core transformers.

      Cheers,
      Gerald

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  10. Yogesh Panchal

    April 26, 2016 at 3:24 pm

    Sir,
    Thank you for refreshing our fundamentals.

    Likes(0)Dislikes(0)
  11. Luis

    April 28, 2016 at 12:21 am

    Mr. Musi,
    In one phrase, "well put". Those of us who were associated with General Electric for a long time know that most of the industrial and up to the largest utility machines they've built to date are AC.

    Likes(1)Dislikes(0)
  12. Gene

    June 21, 2016 at 2:04 pm

    Here in America in large plants that handle materials that change depending what mixture, concrete, asphalt, etc for what application, we use 3 phase motors 240, 480 vac. to control the speed we use variable frequency controls (VFD's) to change the 60 hertz frequency to what ever frequency to get the speed needed for the job. 35 years in electrical a/c and d/c I still find things that just amaze me. I can't imagine how Tesla and Edison did it back then. thank you for your article

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  13. Sunil

    February 10, 2017 at 5:44 pm

    Thanks for explaining diffrance between ac and dc

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