The AC/DC Affair

Most often in our homes, we are surrounded by equipment that uses the direct current (DC) as their source of energy. The examples of such device include the cell phones, electric toothbrush, laptops, LED (light emitting diodes), etc. are all dc powered. Even though they may be plugged into an alternating mains supply, there is a rectifier inbuilt that transforms the AC to DC to make it work.

Though sometimes in the conversion of Ac to DC we may experience some losses. The other day, I showed my friend the loss in this conversion in his portable inverter setup that comprises two 12V 200AH battery and a 1.5KVA inverter system.

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First, we checked the DC part of the inverter by measuring the battery voltage and the DC current supply to the inverter.

Here are the values:

Battery voltage= 25.4V

The direct current supply to the inverter = 4.82A

The Power= Current x Voltage = 4.82 x 25.4 = 122.428Watts

Moving on to the AC part, i.e the output supply of the inverter, which is now converted to alternating current, we have the following values:

Output load current= 0.51A

Output voltage= 226V AC @ 50Hz

Power factor= 0.63

Power factor is a ratio which shows the power that does useful work. So in this system, a pf of 0.63 shows that (1-0.63) or 37% of power is used up by either the capacitive or inductive element of the system leaving only 63% for actual work.

The real power or true power in an electric ac circuit is

P= VI cosφ

cosφ= power factor

Therefore the real power in the above setup is

P= 226 x 0.51x 0.63 = 72.6138 watts

If you compare the DC power of 122.428Watts with the AC power of 72.6138 watts, you would see that we have lost power in the conversion process.

The big question is why do we use alternating current instead of direct current? Why bother going through the troubles of converting DC to AC and converting it again to DC, in some instances, for devices such as our laptops, phones, LED bulbs, etc? Note, the load on the inverter was the same and the measurement on both the DC and AC values was carried out simultaneously.

Let us go back some years in the past. It was the 1880s and there exist two geniuses in their own right who together electrified the world. One was Thomas Edison, the famous inventor of the light bulb, and the other was Nikola Tesla. Edison was working on his direct current project after his invention of the direct current incandescent light bulb in 1879. Edison saw some of the limitations of DC and according to Tesla, offered a $50,000 payment to him if he could come up with ways to improve the limitation of DC, chief amongst which is its inability to transmit energy over long distances. Why was that a problem? At that time, the DC voltages of the Edison generators are low, in other to transmit it over long distances it would require an incredible massive size of wires to convey the electric power.

Tesla, a man who is said to think in three dimensions was able to come up with the solution to Edison's dilemma- the alternating current. When Tesla came back for his $50,000 reward, which should be six figure sum today did not get the promised amount, instead, Edison told Tesla it was a joke. The idea of an alternating current was one not welcomed by Edison. Tesla had to leave him in 1885 and then the "War of Currents" officially sets off. Tesla set up Tesla Electric Light and Manufacturing, a rival company to his former boss, Edison. He soon became a bigger threat to Edison's DC electric company when the big financier/industrialist George Westinghouse of the Westinghouse Electric and Manufacturing Company offered him financial help to Tesla.

Now we have seen a little of the history of these two kinds of electricity, we may now take a look at the features of the two.

The alternating current just as the name describes "alternates" or changes. The flow of charges (electrons) produces current. If the flow of current changes (reveres) direction as it flows, we have an alternating current.

The frequency of an alternating current is the number of complete cycles (from 0 to 360) that the alternating waveform makes in a second. The frequency of an alternating current is the number of complete cycles (from 0 to 360) that the alternating waveform makes in a second. So if the alternating waveform makes ten complete cycles (we have one complete cycle in the diagram on the right) the alternating current frequency is 10 Hertz. In Nigeria, the frequency of operation is the same as the European countries; at 50 Hertz or 50 cycles per second at 220-230 voltage.

The waveform of an alternating current is not usually sinusoidal as depicted in the diagram above. It may be in other waveforms such as square, triangle, or sawtooth in shape.

If you observe the various waveforms there is a certain similarity in all. First, the current moves from zero until it reach the maximum positive value, then it retraces (falls) to zero again and enters the negative part of the journey. That also gets to the maximum value and returns to zero completing a full cycle.

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Direct current, on the other hand, is unidirectional, i.e it does not alternate but rather it flows in one direction, either in forward or in the backward direction. It can only flow in a particular direction at a particular time.

The direct current is not the electricity of choice in transmission as it is not economical when employed in short distances. It only becomes economical when the minimum transmission distance is at least 400KM, for overhead transmission.

In 2016, a 12GW 3000KM went to ABB to build in China an ultra high direct current voltage (1100KV DC) which sets the record for the longest distance transmission. You can watch the video of the 1100KV UHVDC transformer and see the ABB engineers talk about the successful testing here.

But the major reason, alternating current apparently have a huge popularity is because of the ease of change of both voltage and current via the use of a transformer at a very cheap rate.

For instance, some generating stations in Nigeria has 10.5KV of the voltage at the generator's terminal, this voltage is stepped up to 330KV for transmission to neighbouring states. On getting there it is stepped down to 132/33KV. This voltage is further stepped down again to 11KV/ 0.415KV (phase to phase) or 0.230KV line-to-neutral voltage.

In the house, this 230V undergoes further changes, via various transformers in our electronics, home appliances, etc.

The AC, because of the above is more preferred to DC in use around the home and in shorter distance transmission.

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^References

• ^Powerfactor
• ^{Nikola Tesla vs. Thomas Edison: Who Was the Better Inventor?}
• ^{Edison vs. Westinghouse: A Shocking Rivalry}
• ^{High-voltage direct current}
• ^{ABB achieves breakthrough with world’s most powerful HVDC transformer}

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