Introduction
A threephase circuit consists of a threephase source, a threephase load, and a threephase transmission line. The most basic characteristic of this circuit is that it has one or more groups of power supplies. Each group consists of three sinusoidal power supplies with the same amplitude, the same frequency, 120° phase difference, and the power supply and the load are connected in a specific way. Threephase circuits are widely used in power systems such as power generation, transmission, distribution, and highpower electrical equipment.
What does 3 phase mean?
Catalog
1.1 Threephase Circuit Characterized 
Ⅰ Threephase Circuit Basics
The three phases could be supplied over six wires, with two wires reserved for the exclusive use of each phase. However, they are generally supplied over only three wires, and the phase or line voltages are the voltages between the three possible pairs of wires. The phase or line currents are the currents in each wire. Voltages and currents are usually expressed as rms or effective values, as in singlephase analysis.
1.1 Threephase Circuit Characterized
Special power supply
Special load
Special connection
Special solution
1.2 Threephase Circuit Terms
1) End wire (fire wire)
2) Neutral line
3) Line current
4) Line voltage
5) Phase current
6) Phase voltage
7) Threephase threewire system and threephase fourwire system
1.3 Threephase Voltage & Current
 Star Connection
Summery: Line Voltage vs Phase Voltage
1) The line current is equal to the corresponding phase current.
2) If the phase voltage is symmetrical, the line voltage is also symmetrical.
3) The line voltage is equal to √3 times the phase voltage.
4) The phase of the line voltage leads the corresponding phase voltage by 30°.
 Delta Connection
Summery: Line Current vs Phase Current
1) The line voltage is equal to the corresponding phase voltage.
2) If the phase currents are symmetrical, the line currents are also symmetrical.
3) The line current is equal to √3 times the phase voltage.
4) The phase of the line current lags behind the corresponding phase voltage by 30°.
1.4 Threephase Circuit Advantages
Power generation: Threephase power is increased by 50% compared to singlephase power.
Transmission: 25% less material than singlephase circuit transmission. That is, under certain conditions, transmitting a certain amount of power by threephase only requires 75% of the copper of singlephase transmission.
Power distribution: More economical than singlephase transformers and easier to connect to the load.
Transportation: simple structure, low cost, reliable operation, convenient maintenance.
In addition, three wires are usually seen in highvoltage transmission lines, whether on towers or poles, with pin or suspension insulators. Some highvoltage lines are now DC, since solid state devices make it easier to convert to and from AC. The DC lines are free of the problems created by phase, as well as eliminating the skin effect that reduces the effective area of the conductors. It is not nearly as easy to manage longdistance electrical transmission as might be thought.
Ⅱ Symmetrical vs Asymmetrical
2.1 Symmetrical Threephase Circuit
A symmetrical threephase power source is usually generated by a threephase synchronous generator, as shown in Figure (a). Among them, the threephase windings differ by 120° in space. When the rotor rotates at a uniform angular velocity ω, an induced voltage is generated in the threephase winding, thereby forming a symmetrical threephase power supply as shown in Figure (b). Among them, the three ends of A, B, and C are called the start end, and the three ends of X, Y, and Z are called the end. When you connect a load to the three wires, it should be done in such a way that it does not destroy the symmetry.
Instantaneous Voltage Calculation of Threephase Power
In the formula, take the phase A voltage u_{A} as the reference sine quantity. The threephase voltage waveform diagram is shown in Figure (a).
The key to understanding threephase is to understand the phasor diagram for the voltages or currents. The phasor of the threephase power supply can be represented by the Figure (b).
The characteristics of the symmetrical threephase power supply can be derived from the above formula:
From the above formula, the sum of the instantaneous value of the threephase power supply and the sum of the phasor are always zero.
The sequence in which each phase of the threephase power passes through the same value (such as the maximum value) is called the phase sequence of the threephase power, and the phase sequence of the abovementioned threephase voltage is called the positive sequence. Conversely, if phase B exceeds 120° of phase A and phase C exceeds 120° of phase B, this phase sequence is called reverse sequence. If there is no special instructions, it will generally default to positive order.
2.2 Threephase Asymmetry
1) In a threephase circuit, as long as there is asymmetrical part, it is called a threephase asymmetry.
2) The complex power absorbed by the threephase load is equal to the sum of various complex powers.
3) The instantaneous power of a threephase circuit is the sum of the instantaneous power of each phase load.
4) In a threephase threewire circuit, whether symmetrical or not, two power meters can be used to measure threephase power.
When the power supply voltage in the threephase circuit is asymmetrical or the parameters in the circuit are asymmetrical, the current in the circuit is generally asymmetrical. This kind of circuit is called threephase asymmetry. There are a lot of asymmetry parts in threephase circuits, and the causes are different. For example, there are many lowpower singlephase loads in a threephase circuit, it is difficult to make them into a completely symmetrical circuit. When a threephase circuit is broken or shortcircuited, it is also a threephase asymmetry circuit. In addition, some electrical equipment and instruments formally use threephase asymmetry to work.
For example, the most common lowvoltage threephase fourwire system. Due to the large number of singlephase loads in the lowvoltage system, the equivalent impedances Z_{A}, Z_{B}, and Z_{C} of the three phases circuit are generally different from each other, and the power supply voltage can generally be considered symmetrical. In this way, a symmetrical threephase power supply converts to an asymmetrical threephase load.
The circuit shown in the figure has two nodes, and the voltage between the two nodes can be directly calculated according to the node voltage method.
Although the power supply voltage in the above formula is symmetrical, the voltage between the neutral point of the power supply and the neutral point of load is not zero due to the load asymmetry, that is, U_{NN}≠0. According to Kirchhoff's voltage law, the phase voltage of the load can be obtained as:
The phasor diagram of each voltage corresponding to the above formula is as follows:
Ⅲ Power in Three Phase Circuit Formulas
1. Average Power
Suppose the power absorbed by a phase load in a symmetrical threephase circuit is equal to P_{p}=U_{p}I_{p}cosφ, where U_{p} is the phase voltage and I_{p} is the phase current of the load. Then the total threephase power is: P=3U_{p}I_{p}cosφ
Pay Attention To
1) φ in the above formula is the phase difference angle (impedance angle) of phase voltage and phase current.
2) cosφ is the power factor of each phase, in a symmetrical threephase system:
cosφA=cosφB=cosφC=cosφ
3) The formula calculates the circuit power (or the power absorbed by the load).
When the load is in a star connection, the line voltage and line current at the load end are substituted into the above formula:
When the load is in a delta connection, the line voltage and line current at the load end are substituted into the above formula:
2. Reactive power
The reactive power absorbed by the load in a symmetrical threephase circuit is equal to the sum of the reactive power of each phase:
3. Apparent Power
4. Instantaneous Power
Suppose the voltage and current of phase A of the threephase load are:
Then the instantaneous power of each phase is:
It can be proved that their sum is
The above formula shows that the instantaneous power of a symmetrical threephase circuit is a constant, and is equal to the average power. This is one of the advantages of a symmetrical circuit. For example, on a threephase motor, a balanced electromagnetic torque is obtained and mechanical vibration is avoided, which is not available in singlephase motors.
Ⅳ Frequently Asked Questions about Threephase Circuit
1. What is a 3 phase circuit?
Threephase power is a threewire ac power circuit with each phase ac signal 120 electrical degrees apart. ... threephase is that a threephase power supply better accommodates higher loads. Singlephase power supplies are most commonly used when typical loads are lighting or heating, rather than large electric motors.
2. How many wires are in a 3 phase?
four wires
The threephase system has four wires. Three are conductors and one is neutral.
3. What is the 3 phase power formula?
3Phase Calculations. For 3phase systems, we use the following equation: kW = (V × I × PF × 1.732) ÷ 1,000.
4. What is the advantage of threephase system?
A threephase circuit provides greater power density than a onephase circuit at the same amperage, keeping wiring size and costs lower. In addition, threephase power makes it easier to balance loads, minimizing harmonic currents and the need for large neutral wires.
5. What is meant by 3 phase balanced load?
A balanced threephase voltage or current is one in which the size of each phase is the same, and the phase angles of the three phases differ from each other by 120 degrees. ... With such a balanced load, if a balanced threephase supply is applied, the currents will also be balanced.
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