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