# Electromotive force and U of the hottest three-pha

2022-08-24
• Detail

Electromotive force and U

electromotive force of three-phase AC and u-v-w phase sequence introduction

in actual production, three-phase AC is widely used, which is sent by three-phase generator and transmitted to users by three-phase power transmission system. A three-phase AC circuit is formed by correctly connecting the three-phase AC power supply and the load with a transmission line

three phase alternating current has the advantages of convenient and economical power transmission, and three-phase power is more volatile than single-phase power. Therefore, it has been widely used. Three phase AC circuit generally refers to the existence of three sinusoidal AC electromotive forces with the same maximum value, the same frequency and the phase difference of 120 at the same time in the circuit. The part of the circuit composed of each electromotive force is called one phase

the analytical expression of three-phase AC EMF is as follows:

eu=emsin t

ev=emsin (T-120)

ew=emsin (t+120)

its waveform and phasor diagram are shown in the right figure above

we call the three-phase sinusoidal AC electromotive force with equal amplitude, same frequency and 120 phase difference from each other as symmetrical electromotive force

the three-phase electromotive force mentioned below refers to symmetrical electromotive force unless otherwise specified

The characteristic of symmetrical electromotive force is that the sum of the instantaneous values of three-phase electromotive force is always zero

in practical application, the order of u-v-w is commonly used to represent the phase sequence of three-phase electromotive force. The so-called phase sequence refers to the sequence in which the three-phase electromotive force passes through the maximum value and zero value, that is, u is 120 ahead of V-phase electromotive force; V is 120 ahead of W phase electromotive force; W is 120 ahead of u phase electromotive force

among the three windings, it doesn't matter which one is located in U phase, but once u phase is set, V and W are set together in sequence. That is, V lags behind u by 120, and W lags behind V by 120. This order cannot be confused. Therefore, the phase sequence of u-v-w is called coherent phase sequence

connection of three-phase star power supply, difference between zero line/ground line/phase line/live line and phase voltage/line voltage

if the three-phase alternating current is used, then the stress is further increased until each phase of the sample tear current power supply is connected with two wires and loads, forming three unrelated circuits, as shown in the following figure. This connection requires six conductors to transmit electricity, which is very uneconomical

And the material is relatively cheap

therefore, in fact, star (y) or triangle (△) connection is used

zero line/phase line, phase voltage/line voltage

zero line/ground wire, phase line/live wire

connect the ends of the three coils (U2, V2, W2) of the three-phase power supply together, and lead out the wires from the three starting ends (U1, V2, W3) respectively. This connection method is called star connection, as shown in the right figure. The connection point of the three ends is called the neutral point

the conductor led out by the neutral point is called the neutral line (i.e. line n in the figure). When the neutral point is grounded, the line led out by the neutral point is called the zero line (commonly known as the bottom line). The three wires from the beginning of the coil (U1, V1, W1) are called phase wires (commonly known as live wires)

There are two kinds of voltages between conductors in the connection mode of phase voltage and line voltage: phase voltage and line voltage. The voltage of each phase line and neutral line is called phase voltage, and their effective values are expressed in UU, UV and UW respectively; The voltage between the phase lines is called line voltage, and their effective values are expressed by UUV, UVW and uwu respectively

relationship between phase voltage and line voltage

when the three-phase power supply is star (y) connected, the phasor diagram of voltage is shown in the following figure on the right. From the phasor diagram, it can be seen that the numerical relationship between line voltage and phase voltage can be obtained from isosceles triangle. u. Line voltage between V:

similarly, line voltage between V and W: uvw= (3) UV; w. Line voltage between u: uwu= (3) uw

that is:

in the above formula:

ul: represents line voltage, unit voltage (V for short)

u: represents phase voltage, unit voltage (V for short)

therefore, when the three-phase power supply is star connected, line voltage UL is (3) or 1.73 times of phase voltage U (root 3 equals 1.73)

so you sometimes hear about 380V voltage. How does it come from? We all know that the voltage of ordinary household is 220V, which is the phase voltage (that is, the voltage between the live line and the zero line). When we connect the live line with another live line (not the zero line), we can output a voltage of 380V

triangle connection (△ connection) of three-phase power supply and the relationship between line voltage and phase voltage

connect the three coils of three-phase power supply in sequence with the end of one coil and the beginning of the adjacent one phase coil to form a triangle circuit, and then lead three wires from the three connection points to connect with the load, as shown in the following figure

it can be seen from the figure that when the power supply is connected into a triangle, the relationship between line voltage and phase voltage is: line voltage UL is equal to phase voltage U (what is line voltage and phase voltage), that is, ul=u. The line voltage in the star connection we learned in the last lesson is 1.73 times the phase voltage

usually, the generator is connected as a star (y). For users, three-phase transformers are also equivalent to power supplies. They are connected into stars or triangles

star connection of three-phase circuit load and the role of neutral wire, phase/line voltage and current relationship

three-phase circuit with star connection (y) can be divided into three-phase four wire system (with neutral wire, as shown in the figure below, u, V, w points are phase wire, n electricity is neutral wire) and three-phase three wire system (without neutral wire)

phase/line voltage current relationship

the current in each phase line in the circuit is called line current, which is represented by IU, IV and IW respectively. It can be seen from the figure that when the load is star connected, the current (phase current) I in each phase load is also the line current IL. The line voltage UL on the load is three times the phase voltage U (root 3 1.73)

function of neutral line

when the three-phase loads are symmetrical (i.e. the load size of each phase is equal, and the impedance angle is equal), it can be seen from the right figure that at any moment, the synthesis of the three line currents is zero, that is, the current passing through the neutral line is zero. Since there is no current on the neutral line, the neutral line is useless at this time, which is redundant and can be omitted. For example, when supplying power to three-phase motors or three-phase electric furnaces, the three-phase three wire system can be used

however, we often use the three-phase four wire power supply mode with neutral line, because in addition to the symmetrical three-phase load, there are also single-phase loads (such as single-phase motor, single-phase electric furnace, lighting, etc.). When in use, if the load of each phase is asymmetric, the current of each phase is different, which requires the neutral line as the common circuit of each phase load, so there is current on the neutral line

the function of neutral wire is to make the voltage at both ends of each load change slightly when the load changes, and the phase voltage of each phase load is basically symmetrical

neutral line is very important in asymmetric three-phase power supply system. The neutral line is not allowed to be disconnected, and the fuse or switch is not allowed to be connected to the neutral line. If the neutral line is disconnected, each phase load will cause three-phase phase voltage asymmetry, some phase voltage increases significantly, and some phase voltage decreases, which is easy to damage electrical equipment

if there is current on the neutral line, it will have a certain voltage to the ground. Therefore, for the sake of safety, the neutral line is often used for grounding. And the neutral line is re grounded at the user load, which is called repeated grounding

triangular connection of three-phase circuit load and phase/line voltage and current relationship

three-phase circuit with triangular (△) connection of load, as shown in the above figure. It is generally used when the three-phase load is symmetrical, such as the motor with triangular winding, and the three-phase circuit with triangular connection is connected with the primary or secondary winding of the transformer

in triangle connected three-phase circuit, the phase voltage U of the load is equal to the line voltage UL on the line

when the load connected in a triangle is symmetrical, the phasor diagram of the phase current I in the three-phase load and the line neutral current IL is shown in the right figure below. It can be seen from the figure that the magnitude relationship between line current IL and phase current I

similarly: iv= 3 IVW, iw= 3 iwv

i.e.: il= 3 i

therefore, in a three-phase circuit with a symmetrical load connected in a triangle, the line current (IL) is equal to three times the phase current (I) (root sign 31.73). The line voltage (UL) is equal to the phase voltage (U)

star connection and triangle connection? Selection of three-phase electrical load connection method

if there are some doubts in this article that are difficult to understand, you may need to understand the respective characteristics of the star connection and triangle connection of the load

in symmetrical three-phase circuit, for the connection method of load, is star connection method or triangle connection method adopted

we should decide according to the rated voltage of each phase load and the current of the power line, which is mainly based on two points:

if the rated voltage of each phase load is equal to 1/3 of the power line voltage (13, that is, the line voltage is equal to 3 times the root of the rated voltage), the load connection should adopt the star connection method

for example, when the voltage on the nameplate of the three-phase electric furnace is 220V, the power line voltage is 380V, and the rated voltage is 1/3 of the three-phase power line voltage (root number 3=1.73), star connection is adopted

if the rated voltage of each phase load is equal to the line voltage of the power supply, the load should be connected in triangle. If there is a motor (three-phase motor), the rated voltage on its nameplate is 380V, which is equal to the line voltage of the power supply, so triangle connection should be adopted

for three-phase loads, it is very important to choose the connection method, which must not be connected wrong. If the wrong connection method is selected, the following faults may occur

the star connection method should be used and the wrong triangle connection is used. As a result, the phase voltage of each phase load is 3 times higher than the rated voltage, which leads to the increase of line current and electric power by 3 times, and the load will be burnt out

when the delta connection method should be adopted and the star connection is wrongly selected, the phase voltage of each phase load is only 1/3 of the rated voltage, and the current and electromotive force will also be reduced, and the load will cause accidents due to insufficient power

electric power of three-phase AC circuit when star and triangle are connected

in three-phase AC circuit, the power consumed by the total load is equal to the sum of the active power consumed by each phase load, that is,

p=pu+pv+pw

when the three-phase loads are symmetrical, whether it is star connection (y) or triangle connection (△), u, I and COS (power factor) in the three phases are the same, so the power in each phase is equal, Therefore, the three-phase power is three times of the single-phase power, that is, the three-phase power p=3p, and the expression is:

because it is convenient to measure the line voltage and line current in the circuit, the line voltage and line current values are often used to calculate the three powers

when the load is star connected and the load of each phase is balanced, il=i

when the load is triangle connected and the load of each phase is balanced, ul=u

therefore, whether the load is star or triangle, substitute the above relationship into p=3u I cos:

the same principle, Three phase symmetrical negative, so the reactive power of the dominant load with certification and selection in the use of export products is:

apparent power

note: the words UL and IL in the above formula are line voltage and line current