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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 certain environments, due to the usage of high-power electrical equipment, three-phase electricity becomes essential. It serves as a form of electric energy transmission, often simply referred to as three-phase electricity. A three-phase AC power supply consists of three alternating current potentials of the same frequency, equal amplitude, and a phase difference of 120°.
**Characteristics of Three-Phase Power**
1. In three-phase electricity, there is one live wire, and touching it alone isn't harmful. The voltage between two live wires is 380V, while the voltage between each live wire and the neutral wire is 220V, which is what we commonly use.
2. Three-phase power can drive a motor. Connecting three live wires to a motor simultaneously can result in either star or delta connections.
3. The voltage between two live wires is 380V, not 220V.
4. You can form 220V by using one live wire and the neutral wire.
5. The neutral wire is generally used to handle unbalanced currents when the three-phase load is uneven. The neutral wire originates from the neutral point of the transformer and forms a loop with the phase wires to supply power to electrical equipment. Under normal conditions, the neutral wire is repeatedly grounded at the neutral point of the transformer and the ground wire, serving as a double protection against voltage differences. It is also the equipotential body that serves as the reference point for circuit potential.
**Mathematical Equation for Three-Phase AC Voltage**
\[ V1 = Asin(2\pi ft) = Asin(\omega t) \]
\[ V2 = Asin(2\pi ft - 2\pi/3) = Asin(\omega t - 2\pi/3) \]
\[ V3 = Asin(2\pi ft + 2\pi/3) = Asin(\omega t + 2\pi/3) \]
Where \( A \) 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 and two-phase electricity lies in the number of live wires. Three-phase electricity has three live wires, whereas two-phase electricity has only two. Additionally, the voltage levels differ—three-phase electricity operates at 380V, while two-phase electricity operates at 220V. Three-phase electricity is typically used in industrial settings and factories for powering large equipment like motors and pumps. Two-phase electricity, on the other hand, is more common in households, schools, hotels, and other places for household appliances. Both single-phase and two-phase electricity fall under the category of low-voltage three-phase four-wire (380V/220V) systems. A single phase represents the neutral wire and any phase wire (A, B, C), with a voltage of 220V. Two phases represent any two phase wires (AB, BC, AC) with a voltage of 380V. Three-phase electricity refers to three wires (A, B, C) spaced 120 degrees apart, operating at 3Φ380V.
**Difference Between Three-Phase and Single-Phase Electricity**
The difference between three-phase and single-phase electricity is that single-phase electricity is 220V, representing the voltage between the phase wire and the neutral wire. Three-phase electricity, however, refers to the 380V phase-to-phase voltage between A, B, and C. Appliances powered by three-phase electricity include 380V motors or equipment. The electricity generated by generators 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. It's important to note that in AC circuits, it's incorrect to refer to positive or negative; instead, it should be called the phase wire (commonly known as the live wire in civilian power) and the neutral wire (commonly known as the neutral line in civilian power).
**Principle of Three-Phase Electricity**
An analogy helps explain the principle of three-phase electricity. Imagine three people standing at the vertices of an equilateral triangle. One person starts pushing the triangle, followed by another, and then the third, causing the triangle to rotate continuously. If one person pushes harder, the forces become unbalanced, causing the triangle to wobble, and a reaction force occurs at the support point (the neutral point has current passing through). If there’s no resistance, the triangle could fly off, akin to a short circuit in three-phase electricity.
**Composition of Three-Phase Circuits**
The three phases are generally AC, with a phase deviation, and the current naturally flows from high voltage to low voltage. For instance, an electric welder might be connected to one of the three phases, labeled U and V, at 380V. At some point, when U is at high voltage, V will be at low voltage, creating a potential difference between the two. When this difference is significant, current flows through, forming a loop. Of course, there are instances when V is at high voltage!
**Principle of a Motor Connected to Three-Phase Electricity**
Each waveform of three-phase electricity is a single sine wave, but the phase angles are 120 degrees 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**
A three-phase power supply consists of three AC voltages of the same frequency and similar amplitude. Each AC voltage "phase" is 120° apart from the others. This can be depicted graphically using waveforms and vector diagrams.
**Reasons for Using a Three-Phase System**
1. A three-phase voltage can generate a rotating magnetic field in a motor, enabling it to start without additional windings.
2. A three-phase system requires only half the amount of copper connections compared to other methods, reducing both the number of cables and transmission losses.
**Comparison of Y-Joint (Star Connection) and Delta Connection**
A Y-connection is commonly used to power single-phase devices in homes and offices. A single-phase load is connected between a line and the neutral. The load for each phase is evenly distributed to balance the main three-phase power supply.
The Y-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 available, as shown by the black vector.
The most common case of delta connection is to power higher power three-phase industrial loads. 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 US, a 240V delta system can provide two 120V supplies via split or center-tapped coils.
**Power Measurement**
In an AC system, power is measured using a power meter. Modern digital sampling power meters multiply the instantaneous samples of multiple voltages and currents to calculate the instantaneous power, then take 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., across a wide range of waveforms, frequencies, and power factors. Proper wiring configuration and correct connection of the power analyzer are necessary for accurate results.
**Single-Phase Power Meter Connection**
Only one power meter is required, as shown. The connection between the system and the power meter voltage terminal and current terminal 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.
**Single Phase Three Phase Connection**
In this system, as shown, 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.
**Verification of Three-Phase Y-Shaped System**
The instantaneous power measured by the power meter is the product of the instantaneous voltage and current samples.
**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.
**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.