The Rock 'n Roll Connection
If you are a rock music lover, then you must know AC/DC. They are a hard rock and blues band that was formed in Sydney by two brothers in 1973. Before beginning this post, I had to watch one of their music videos on youtube. It was called Highway to Hell and it was truly electrical. Of course, this rock band has nothing to do with AC and DC in electricity. However, if you love Rock 'n Roll, you may click the picture below to see the video.
[Original picture taken with my Redmi Note 3 Pro Prime: 2.8KVA Blue Gate Sine Wave Inverter]
It was this Rock 'n Roll band that popularised AC-DC because most people never heard of the War of Currents. War of Currents is the name given to the battle for supremacy between the American greatest inventor, Thomas Alva Edison (February 11, 1847 – October 18, 1931) and his Serbian-born arch-rival Nikola Tesla (1856 - 1943). The feud between the two had many facets which include the fact that Tesla once worked for Edison and was cheated by Edison, they had disagreement in scientific methods, lifestyle and so on. In the end, it was disagreements on how best to get electricity into homes that were the biggest disagreement.
Nikola Tesla believed that the best way to deliver electricity to homes was through the transmission of Alternating Current (AC) while Thomas Edison found Tesla's proposition "utterly impractical". Edison's proposal was the transmission of Direct Current (DC). While Edison tried different means, including some pretty devious ones such as electrocuting an elephant and supporting the use of AC for execution of the death penalty, Tesla ended up winning. He won because, in spite of the danger of electrocution, AC transmission offers various advantages in the transmission of electric power over DC transmission. A comparison between AC and DC transmission would reveal these advantages.
| S/No | Factors | AC | DC |
|---|---|---|---|
| 1. | Conductors Required | Requires three conductors | Requires two conductors |
| 2. | Inductance, capacitance etc. | Existence of inductance, capacitance, phase displacement and surge problem | No such problems |
| 3. | Skin effect | Skin effect exists | Absence of skin effect, so the whole cross-section of the conductor is utilised |
| 4. | Corona effect | More corona effect | Less corona effect and hence more efficiency |
| 5. | Potential stress on insulators | High | Low |
| 6. | Voltage regulation | Low | High due to absence of voltage drop |
| 7. | Transmission Distance | Maximum distance of transmission is high | Limited to a one-mile radius |
| 8. | Compatibility with Transformers | Compatible with transformers and therefore capable of being changed in power transmission | Not compatible with transformers |
| 9. | Switches and Circuit Breakers | Efficient switches and circuit breakers | Switches and circuit breakers with limited capabilities |
| 10. | Maximum Transmission | Can be generated at up to 11 kV and 21 kV in some countries | Can be generated at high voltage but cannot be stepped down |
Table 1.1: Comparison between AC and DC
A close look at Table 1.1 would have you wondering why people did not see it that AC transmission was "impracticable" as Edison put it since DC offers so many advantages and such few risks! Well, it turns out that AC has all the advantages that count when it comes to transmission of electricity over long distances, since we must generate electricity far from users we must transport it to our homes. However, we cannot just allow high voltage alternating currents to flow through our home appliances, frying our circuits. Therefore, we must find a convenient way to convert AC to DC in our appliances. This is the job of rectifiers which we discussed here.
Things, however, are never that simple. If we somehow manage to generate DC voltage in our home by using Solar panels, we may not be able to use the DC in our appliances because these appliances have been designed to take in AC and convert to DC for use. Therefore we cannot change the input anytime we want. This is why we use inverters - machines which take DC voltage and convert it to AC voltage so that our appliances can receive the type of voltage with which it was designed to function. Let us see what the inverter is all about.
Inverting with Magic Hands
We know that DC is current moving in one direction much like soldier ants on a mission. AC, on the other hand, is current that is flowing in cycles, which means that it flows in one direction then changes and flows in the opposite direction. In Nigeria and many other countries except the US, the AC changes direction 50 times per second.
This alternation is the reason we say that the frequency of the alternating current is 50 Hertz (in Nigeria and neighbouring countries) and 60 Hertz (in the US). So, if you are a magician with swift fingers, all you have to do is stand at the output of the battery in the DC circuit of Fig 1.1 and switch the A terminal to the other terminal, fifty times per second and you would have succeeded in converting the DC circuit to an AC circuit. Excellent.
Even though you have invested your magic in converting the DC to AC, the AC you have obtained is not quite the way it is meant to be because the waveform of such an AC is not sinusoidal. In other words, the alternation of the current would be quite abrupt as the current switches direction suddenly, and this can be quite brutal to electronic equipment, often causing damage to home appliances. This is the kind of AC obtained by using a mechanical inverter built with an electric motor or a variety of automated switching device.
The waveform of such an inverter is either a saw-tooth shape or a square shape as shown in Fig. 1.2. Sometimes it is a triangular waveform. However, what is desired is a sine wave in which the current changes direction in a slow and smooth manner as shown in the sine wave. While an inverter that produces the square, triangular, and saw-tooth waveforms are still being manufactured and used today, our focus is on the type that produces a sinusoidal waveform.
Types of Inverters
Inverters can be classified based on the shape of the waveform that the output AC takes or based on how the inverter is connected to the home. Based on these two criteria, the following are some of the classifications of inverters:
- True/ pure sine wave inverters
- Quasi/ modified sine wave inverters
- Standalone inverters
- Grid-connected inverters
Grid-connected inverters are such excess power generated from say a solar panel installed in the home, is sold to the utility through the grid. However, this connection may cause problems because if there is a power outage from the network, then the home that uses a grid-connected inverter would be in a blackout, just as if there was no solar power generation system installed in the house. To solve this problem, there are bi-modal or bi-directional inverters which can work as either standalone or grid-tied but never as both.
Pure Sine Wave Inverters
We have talked about almost everything except how to arrive at an inverter that gives AC with a pure sine wave as an output. The process is slightly complicated but let us try a simple explanation. Pure sine wave inverters employ the concept of Pulse Width Modulation(PWM). Pulse width modulation is a technique employed in communication engineering to encode messages in a pulse signal. Even though this is the main application, it is also useful in pure sine wave inverters. The process used in the inverters to convert DC to AC using PWM is two folds:
- Generate the DC as pulses
- Averaging the pulses.
This is how PWM works. The DC voltage fed to the inverter input is produced as a series of DC pulses with varying widths. The width of-of the pulses is made wide for the areas where the sine wave should have a higher amplitude. The second step is to obtain the average of these pulses. The result is a sine wave such the one below:
In practical terms, the process described above is implemented with a Full Bridge Inverter Circuit The circuit is made up of four switches, a DC source and load connected as Fig 2.1 here.
From Fig 2.1 b, when the transistor switches D1 and D2 are closed, current flows through the load in one direction. When D1 and D2 and closed so that D3 and D4 are closed, the current would flow in the other direction, thereby producing a square wave the likes of which we discussed earlier. To convert this to a smooth sinusoidal waveform, we employ two additional devices called comparators. The output of one is connected across g1 and g4 with an additional NOT gate connected on one of the legs of the comparator output. The output of a second comparator is connected between g2 and g3 with an additional NOT gate on one of those comparator outputs. The function of the NOT gate is to ensure that when the comparator is turned on, only one of D1 and D4 is closed. The same thing applies to D2 and D3.
The first comparator takes a sine wave and a triangular wave as inputs and compares the signal whereas the second comparator takes an inverted sine wave and a triangular wave as inputs. The logic of the comparison is as follows:
- If the sine voltage amplitude is greater than the triangular voltage, output 1; otherwise, output zero. That is
- Vsine > Vtrian = 1
- Vsine < Vtrian = 0
When the output of the comparator is 1, D1 is turned ON while D4 is OFF. This logic produces a waveform such as the one below:
- The above comparison is performed by the second comparator using an inverted sine wave, generating its own set of square waves while turning D3 ON at the same time as when comparator 1 turned on D4 and turning D2 ON at the same time with D1.
Finally, since the output voltage, Vac, is the voltage between A and B, we find the net voltage by between point A and B using the waveform as follows:
As shown in the waveforms above, we have succeeded in generating a sine wave from the pulses. However, our work is not done. To smooth off the sine wave, inductors are connected to the output to smooth off the current and capacitors are also connected to smooth off the voltage. The finer the triangular wave, the more accurate the pulse train we be in producing the sine wave.
We must not forget to mention that a transformer is used in an inverter to step up or step down (usually step up) the input voltage to a higher voltage (up to 220 or 240 V) used by home appliances. We know from the law of conservation of energy that we cannot produce extra power apart from the one at the input. Therefore, even though the voltage of the output AC is increased significantly, the current must be reduced by corresponding amounts so that the output power remains the same or less than the input power due to inevitable losses.
It is always lovely sharing these ideas with you. If you read until this point, congratulations: you are a geek! Thank you for your time. Below are pictures of a 2.8 kV Pure Sine Wave Blue Gate Inverter I used before replacing it.
[Original pictures taken with Xiaomi Redmi Note 3 Phone by 
[Original pictures taken with Xiaomi Redmi Note 3 Phone by
[Original pictures taken with Xiaomi Redmi Note 3 Phone by
References
- Wikipedia | Square Wave
- Spark Fun | Pulse Width Modulation
- Explain That Stuff | How Inverters Work
- Solar Facts | Inverters
- Electronics Tutorial | Single Phase Full Bridge Inverters
- Learn Engineering | How Inverters Work
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