AC vs DC: Choosing the Right Current for Electrical Needs

Electrical energy is produced in two ways: Alternating current (AC) and direct current (DC). Both have their unique characteristics and applications, making it important to understand the differences and choose the right current for your specific needs. In this article, we will explore the key differences between AC and DC, and help you determine which one is best suited for your project or application.

Differences Between Alternating Current (AC) and Direct Current (DC)

The differences between alternating current (AC) and direct current (DC) are:

1. The direction of the current

The current that changes direction and intensity periodically depending on time is called alternating current (AC). The intensity of the alternating current depends on the strength of the source. The current whose direction and intensity do not change over time is called direct current (DC).

2. Waveforms

Alternating current changes direction (both positive and negative). The waveform of pure AC is a sine wave. Other forms of AC waves are square, triangle, and sawtooth waves.

Direct current always flows in the same direction. It’s either positive or negative. Pure DC is a flat line.

3. History and scientists

The first electric distribution system introduced in 1882 was Thomas Edison’s 110 V DC system from the Pearl Street station in Manhattan. This was followed by the Gable Street power plant in Houston, Texas, and several other small generating plants in every city.

With the increase in demand for electric energy, the necessity for improvement in reliability and efficiency grew. Parsons made the efficient generation of electricity possible with the invention of the steam turbine in 1884. In 1881 Frenchman Lucien Gaulard and Englishman John D. Gibbs patented an alternating current transmission system in England. This patent was bought by the American George Westinghouse in 1885. In 1886, William Stanley installed the first practical ac transmission in Great Barrington, Massachusetts. A transformer was used to step up the generated voltage to 3000 V for transmission and stepped down by another transformer to 500 V for use. In 1888, Nicola Tesla introduced the polyphase AC system. It was later found that three-phase systems were better than single or two-phase systems and they became the standard transmission systems. AC is still commonly used in current industries, businesses, and homes throughout the world.

4. Safety

DC power sources have a positive and a negative terminal. Current flows from the negative side of the battery, through the circuit, to the positive side of the battery. Therefore, contacting one side of the circuit will not result in an electric shock because the circuit is not completed. Contacting both sides will complete the circuit and can result in shock.

Since the current in an AC circuit flows in both directions, there are no positive and negative like you find in DC circuits. Instead, they have one or two live, or hot, conductors, a neutral conductor, and possibly a grounding conductor, or ground, depending on the wiring.

Typically, when a circuit is completed current flows back and forth between the hot and neutral conductors. The ground serves as a safety measure, allowing excess energy to discharge in the event of a hazard or fault.

You might think that to receive a shock from an AC circuit, you would have to touch both the hot and neutral, but that isn’t the case, at least in typical AC circuits like those in a building. Because most AC circuits have an earth ground, if you were to touch just the hot side of the circuit, electricity would flow through you and back to the earth to complete a circuit.

5. Transmission and distribution

Transformers were developed by William Stanley that could convert electricity to the desired voltage. In an alternating current system, transformers were used to step up or increase the voltage that left the power plant. This enabled the electricity to travel over long-distance wires. When the electricity reached its destination, another transformer would then step down, or decrease the voltage so that power could be used in homes and factories. The direct current system was unable to use transformers. With the DC system, the voltage dropped as it traveled further and further from the generator. To overcome this disadvantage, power plants would have to be built close to the power users-a costly solution.

Soon, the alternating current system—rather than a more expensive, higher maintenance, and less efficient direct current system—began to get most of the orders.  Another advantage of the alternating system soon became apparent: By allowing central stations to serve wider markets, the AC system also encouraged utilities to build larger stations, which then benefited from economies of scale and lowered their operating costs.

In 1893, the AC system was chosen to move electric power from Niagara Falls to Buffalo. Shortly after that, the AC “universal” system became the new standard for transmitting electricity. Now, one generating station could transmit power relatively cheaply over a wide service area.

6. Conversion to each other

Powerplants produce AC by default, so it would take additional effort to convert it to DC. It is much easier and cheaper to convert AC to DC than to convert DC to AC.

7. Generation

Batteries, fuel cells, and solar cells all produce something called direct current (DC). The power that comes from a generator in a power plant, on the other hand, is called alternating current (AC)

With AC it is possible to build electric generators, motors, and power distribution systems that are far more efficient than DC, and so we find AC used predominately across the world in high-power applications.

8. Efficiency

The power loss in the line (P=I2R) depends on the resistance of the line as well as the current flowing through the line. However, the use of narrower but less expensive conductors must have higher resistance than a thicker radius conductor (R=ρl/A). However, due to the high voltage, the current in the transmission line becomes low and the square of a lower current subsides the increase in resistance of the line giving rise to a reduced overall power loss in the line. Thus, the efficiency of alternating current transmission is higher than direct current transmission.

9. Frequency

The frequency of the alternating current indicates how many times the direction of voltage and current is reversed. If the frequency is 50 Hz, it means that the current changes direction 50 times. The frequency of the direct current is always zero. Because it never changes its direction.

10. Direction

Direct currents are currents that, even though their magnitude may vary, essentially flow only in one direction. In other words, direct currents are unidirectional. Alternating currents, on the other hand, are bidirectional and continuously reverse their direction of flow.

11. The flow of the electrons

In AC, electrons change direction between negative and positive poles consequently. In DC, electrons only move from the negative pole to the positive pole. The battery symbol is used as a generic symbol for any DC voltage source, and the circle with the wavy line inside is the generic symbol for any AC voltage source.

12. Power factor

In a DC circuit, power is equal to the product of voltage and current. This formula also is true for purely resistive AC circuits. However, when a reactance — either inductive or capacitive — is present in an ac circuit, the DC power formula does not apply. The product of voltage and current is, instead, expressed in volt-amperes (VA) or kilovolt-amperes (kVA). This product is known as apparent power. When meters are used to measure power in an AC circuit, the apparent power is the voltage reading multiplied by the current reading. The actual power that is converted to another form of energy by the circuit is measured with a wattmeter and is referred to as the true power. In AC power-system design and operation, it is desirable to know the ratio of true power converted in a given circuit to the apparent power of the circuit. This ratio is referred to as the power factor. The power factor of alternating current varies between 0 and 1. The power factor of direct current is always 1.

13. Calculation

Analysis of AC systems always involved complex numbers, while DC is only a real number, thus simplifying the analysis. Direct current circuit analysis deals with constant currents and voltages while alternating current circuit analysis deals with time-varying voltages and currents.

14. Storing

DC is easier to store, especially on a small scale. When electricity is stored, we can use it when we need it. The best way to store electrical energy for relatively small-scale applications is to use rechargeable storage batteries. AC cannot be stored.

15. Traction motors

Initially, DC motors were the mainstay of electric traction motors. The torque speed and the simple control system for traction demands were its main advantages, however, the DC motors use switches/brushes and collectors, making them less reliable and adequate for working at high speeds. The use of AC motors instead of DC motors was the first electrical change in railway vehicles. For higher power densities AC motors reduced dimensions and weight, increasing efficiency and power densities, lowering the operation costs, and reducing maintenance since they don’t have brushes. Nowadays, DC motors are used in special applications with lower power requirements since the controlling cost (power electronics), is lower.

16. Electrical vehicles

Electric vehicles use DC power and their batteries can be charged using DC power in a small fraction of the time needed for charging using AC power. Fast EV chargers are always DC.

17. Extremely long-distance transmission

Transmission by ac replaced dc transmission because of the ease and efficiency with which voltages can be transformed using transformers. However, high voltage dc transmission (HVDC) has the advantage that there are no reactive components of current and therefore no losses in the lines due to such currents and there is no need for synchronization. Direct current is normally used only for long-distance transmission because the equipment used for conversion is expensive. It is also used for the interconnection of two systems with very short lines so the phases of the systems need not be synchronized and for underwater power cables because of the charging current limitations in ac cables.

In conclusion, understanding the differences between alternating current (AC) and direct current (DC) is crucial in choosing the appropriate current for your project or application. The direction of current, waveform, history, safety, transmission and distribution, conversion, generation, and efficiency are the primary differentiating factors between the two types of currents. AC is more commonly used across the world in high-power applications because it is possible to build electric generators, motors, and power distribution systems that are far more efficient than DC. It is essential to know which current best suits your needs to ensure safety, efficiency, and effectiveness in your project.

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