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From the principle to the connection method to fully understand the three-phase electricity: the difference between three-phase electricity and two-phase electricity, single-phase electricity
Three-phase alternating current is a common presence in our daily lives. In environments where high-power electrical equipment is frequently used, three-phase electricity is often required. This type of electricity serves as a form of energy transmission and is commonly referred to as three-phase electricity. A three-phase AC power supply consists of three alternating current potentials with the same frequency, equal amplitude, and a phase difference of 120°.
**Characteristics of Three-Phase Power**
1. In a three-phase system, there are three live wires. Touching any one of them alone won't result in a shock, but the voltage between any two live wires is 380V, while the voltage between each live wire and the neutral wire is 220V. This is the electricity we use every day.
2. Three-phase power can be connected to motors. When three-phase power is applied to a motor, it can be connected in either a star or delta configuration.
3. The voltage between two live wires is 380V, not 220V.
4. You can use one live wire and the neutral wire to form a 220V connection.
5. The neutral wire is typically used to handle unbalanced currents when the three-phase load is uneven. The neutral wire is derived from the neutral point of the transformer and forms a loop with the live wires to supply power to electrical equipment. Normally, the neutral wire is grounded at the neutral point of the transformer and the earth line, providing double protection against voltage differences. The neutral wire serves as the reference point for the circuit's potential.
**Mathematical Equation for Three-Phase AC Voltage**
\[ V1 = A\sin(2\pi ft) = A\sin(\omega t) \]
\[ V2 = A\sin(2\pi ft - 2\pi/3) = A\sin(\omega t - 2\pi/3) \]
\[ V3 = A\sin(2\pi ft + 2\pi/3) = A\sin(\omega t + 2\pi/3) \]
Here, \( A \) is the peak voltage, and \( f \) is the frequency of the AC voltage.
**Difference Between Three-Phase and Two-Phase Electricity**
The difference between three-phase power and two-phase power lies in the number of live wires. Three-phase power has three live wires, whereas two-phase power has only two. Additionally, the voltage levels differ; three-phase power operates at 380V, while two-phase power operates at 220V. Three-phase power is commonly used in industrial settings such as powering large equipment like motors and pumps. Two-phase power is mostly used for household appliances in places like homes, schools, and hotels. Both types of power are part of the low-voltage three-phase four-wire (380V/220V) power supply system. A single phase refers to the neutral wire and any one phase wire (A, B, C), with a voltage of 220V. Two phases refer to any two phase wires (AB, BC, AC) with a voltage of 380V. Three-phase power represents a combination of three phase lines (A, B, C) spaced 120° apart, denoted as 3Φ380V. Electrical loads can be connected in two ways: star and delta configurations.
**Difference Between Three-Phase and Single-Phase Electricity**
The difference between three-phase power and single-phase power lies in their voltage levels. Single-phase power operates at 220V, which is the voltage between the live wire and the neutral wire. Three-phase power operates at 380V, which is the phase-to-phase voltage between A, B, and C. Three-phase power is typically used for heavy-duty equipment like 380V motors. The power generated by a generator is three-phase. Each phase and the neutral point of the three-phase power supply can form a single-phase loop to provide power to users. In AC circuits, it is incorrect to refer to positive or negative; instead, it should be referred to as the live wire (commonly called the hot wire in household electricity) and the neutral wire (commonly called the neutral wire in household electricity).
**Principle of Three-Phase Electricity**
Let’s consider an example to understand the principle of three-phase electricity. Imagine three people standing at the vertices of a regular triangle. One person pushes the wheel first, followed by another, and then the third, causing the wheel to rotate continuously. If one person exerts more force than the others, the support point will sway, creating a reaction force (the neutral point, or N line, will have current passing through). If there is no resistance in the wheel, it could result in a phenomenon akin to a "flying car," which is similar to a short circuit in electrical terms. This analogy helps illustrate how three-phase power works.
**Composition of a Three-Phase Circuit**
In a three-phase circuit, the three phases are AC, and there is a phase deviation. Current naturally flows from high voltage to low voltage. For instance, when an electric welder is connected to one of the three phases, let’s call it U and V, the voltage is 380V. At certain moments, when the U phase is at high voltage, the V phase might be at low voltage, creating a potential difference between them. This difference allows current to flow, forming a circuit. Sometimes, the situation might reverse, with V being at high voltage!
**Connection Principle of Motors to Three-Phase Electricity**
Each waveform of three-phase power is a single sine wave, but the phase angles are 120° apart. Three-phase electricity is the smallest number of phases that can produce a fixed starting torque in the rotor of a motor.
**Connection of Three-Phase Electricity**
Three-phase power consists of three AC voltages of the same frequency and similar amplitude. Each AC voltage, or "phase," is 120° out of phase with the other AC voltages. This can be visually represented using waveforms and vector graphics.
**Reasons for Using a Three-Phase System**
1. A three-vector voltage can generate a rotating magnetic field in a motor, allowing it to start without additional windings.
2. A three-phase system requires half the amount of copper connections compared to other methods, reducing transmission losses.
Consider three single-phase systems, each providing 100W of power to one load (total load = 300W). To provide power, 1 amp of current flows through 6 lines, resulting in 6 units of loss. Alternatively, the three power supplies can be connected to a common bus, as shown in Figure 4. When the load current in each phase is the same, the load is balanced, and the three currents are shifted by 120° from each other. At any given moment, the sum of the currents is zero, meaning there is no current in the return line.
**Star Connection (Y-Joint)**
A three-phase system with a common connection is generally shown in the schematic diagram as a "Y" or "star" connection. A common point is called a neutral point. For safety, this point is usually grounded on the power supply. In practice, the load is rarely perfectly balanced, requiring a fourth "neutral" line for current transmission. Neutral conductors may be much smaller than the three main conductors if permitted by local regulations and standards.
**Delta Connection**
The three single-phase power sources discussed earlier can also be connected in series. At any point in time, the sum of the three 120° phase-shifted voltages is zero. If the sum is zero, both endpoints are at the same potential and can be joined together. This connection is shown in the schematic diagram as a delta connection.
**Comparison of Star and Delta Connections**
The star connection is used to power everyday single-phase devices in homes and offices. A single-phase load is connected between one leg of the star connection and the neutral. The total load for each phase is shared as much as possible to provide a balanced load for the main three-phase power supply.
The star connection also provides single or three-phase power for higher power loads at higher voltages. The single-phase voltage is the phase-to-neutral voltage. A higher phase-to-phase voltage is also provided, as shown by the black vector in Figure 8.
The most common case of delta connection is to power a three-phase industrial load with higher power. By connecting or "stripping" along the transformer coil, different voltage combinations can be obtained from the three-phase delta power supply. For example, in the United States, a 240V delta system can have split or center tap coils that provide two 120V supplies.
**Power Measurement**
In an AC system, power is measured using a power meter. The modern digital sampling power meter multiplies the instantaneous samples of multiple voltages and currents to calculate the instantaneous power, and then takes the average of the instantaneous power in one cycle to indicate the active power. The power meter accurately measures active power, apparent power, reactive load, power factor, harmonics, etc., over a wide range of waveforms, frequencies, and power factors. To ensure accurate results, the wiring configuration must be properly identified and the power analyzer correctly connected.
**Connection of Single-Phase Power Meter**
Only one power meter is required, as shown in Figure 10. The connection between the system and the power meter voltage terminals and current terminals is straightforward. The voltage terminals of the power meter are connected in parallel through the load, and the current is input through a current terminal connected in series with the load.
**Connection of Single-Phase and Three-Phase Systems**
In this system, as shown in Figure 11, a voltage is generated from a centrally tapped transformer coil, all voltages being in phase. This is common in residential applications in North America, where a 240V power supply and two 120V power supplies are provided, with different loads on each leg line. To measure total power and other quantities, connect two power meters as shown in Figure 11.
**Number of Power Meters Required**
In a single-phase system, there are only two wires. Power is measured using a power meter. In a three-wire system, two power meters are required.
**Verification of the Three-Phase Star System**
The instantaneous power measured by the power meter is the product of the instantaneous voltage and current samples.
Power meter 1 reading = i1 (v1 - v3)
Power meter 2 reading = i2 (v2 - v3)
The sum of readings W1 + W2 = i1v1 - i1v3 + i2v2 - i2v3
= i1v1 + i2v2 - (i1 + i2) v3
(According to Kirchhoff’s law, i1 + i2 + i3 = 0, so i1 + i2 = -i3)
2 readings W1 + W2 = i1v1 + i2v2 + i3v3 = total instantaneous power.
**Three-Phase Three-Wire Connection - Two Power Meter Methods**
When there are three wires, two power meters are required to measure the total power. Connect the two phases to the voltage terminals of the power meter according to the method shown in the figure.
**Three-Phase Three-Wire Connection - Three Power Meter Methods**
Although only two power meters are required to measure the total power in a three-wire system, it is sometimes convenient to use three power meters. In the connection shown in the figure, a false neutral is created by connecting the low voltage terminals of all three power meters together.
**Three-Phase Four-Wire Connection**
Measuring the total power in a four-wire system requires three power meters. The measured voltage is the true phase voltage. By using vector math operations, the phase-to-phase voltage can be accurately calculated from the amplitude and phase of the phase voltage. Modern power analyzers also use Kirchhoff’s law to calculate the current flowing through the neutral line.