Electrician small common sense summary

Update date:2018-04-07 Source:MAXGE

Electrician small common sense summary

What is a three-phase circuit?

Generators that can generate equal amplitude, equal frequency, and phase difference of 120° are called three-phase generators; three-phase generators are used as power supplies, which are called three-phase power supplies; circuits that are powered by three-phase power supplies are called three-phase power supplies. Phase circuit.

What is the three-phase three-wire power supply? What is the three-phase four-wire power supply?

Three-phase three-wire system is a connection method of three-phase AC power supply. Three wires are drawn from the ends of three coils, and the other three coil ends are connected together, which is also called a star connection. Commonly used symbol “Y” means This method of drawing three wires to supply power is called three-phase three-wire system. If one wire is drawn from the connection point of the three coil ends, the power supply for drawing the four wires is called the three-phase four-wire system.

What is the phase line (or FireWire) and what is the midline (or zero line)?

The connection point at the three ends is called the midpoint, and the wire from the midpoint is called the neutral or neutral line. The lead from the three ends is called the phase or fire wire.

What is phase voltage, line voltage, phase current, line current?

The voltage across the coil of each phase is called the phase voltage. Usually expressed by UA, UB, UC. The voltage between the end line and the end line is called the line voltage. Usually expressed by UAB, UBC, UCA. The current flowing through each phase coil is called the phase current, and the current flowing through the terminal line is called the line current.

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What are the advantages of three-phase AC and single-phase AC?

Three-phase alternating current has obvious advantages over single-phase alternating current in power generation, transmission and distribution, and electrical energy conversion to mechanical energy. For example, the manufacture of three-phase generators and transformers is more economical than manufacturing single-phase materials, and the construction is simple and the performance is excellent. At the same time, three-phase transmission has less power loss than single-phase transmission.

What is the star and delta connection for three-phase power and load?

Connect one end of the three-phase load to A, B, and C of the three-phase power supply, and connect the other end to the midpoint. The left picture shows the star connection. The end of one phase winding and the beginning of the adjacent phase winding are connected in sequence to form a triangle circuit. Three end wires are drawn from the three connection points to supply the load connected to the triangle, thus forming a delta-connected three-phase circuit.

Why three-phase four-wire system is widely used in low-voltage power grids?

With a star-connected three-phase four-wire system, two voltage values can be provided at the same time, namely line voltage and phase voltage. It can provide three-phase power load and single-phase lighting. For example, the commonly used low voltage 380/220V, can not only provide the use of three-phase AC motor with a power supply voltage of 380V, but also provide a single-phase 220V lighting power supply.

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What is a pure resistance circuit?

Through the electric resistance, the electric energy is transformed into the heat energy of the alternating current circuit in a thermal effect manner, which is called a pure resistance circuit, as shown in the following figure. For example, incandescent lamps, electric furnaces, electric irons and other AC circuits are purely resistive circuits.

What is a pure inductor circuit?

When the resistance of the coil is negligible, when an AC voltage is connected across the coil, there will be an AC current passing through the coil, thus generating a self-induction electromotive force in the coil against the current change, which is a purely inductive circuit. If the coil is connected to direct current, the current will not change, so the inductive reactance is zero and the current in the coil is infinitely large to form a short circuit. Therefore, care must be taken that the inductor current cannot be used in a DC circuit.

What is a pure capacitor circuit?

When the dielectric loss of the capacitor is neglected, an AC power supply is connected to both ends of the capacitor, and the capacitor is continuously charged and discharged. This is a purely capacitive circuit. Experiments have shown that it presents a certain resistance to alternating current, called capacitive reactance, and is represented by XC. The higher the AC frequency, the smaller the capacitive reactance. When DC is connected, the capacitive reactance is infinite, indicating that the capacitor has a DC blocking effect. If you access high-frequency power, it is equivalent to a short circuit.

What is active power, reactive power, apparent power?

Active power: This part of the power that converts electrical energy into other forms of energy.

Reactive power: Does not participate in electrical energy conversion and only participates in alternating magnetic field throughput conversion power.

Apparent power: The total power supplied by the power supply.

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What is a power factor?

The ratio of the active power consumed by the circuit to the total power supplied is called the power factor. The higher the power factor, the less power is transferred between the power supply and the circuit. Therefore, in power engineering, it is desirable to have a higher power factor.

How to calculate the power factor?

Its instantaneous value can be directly read by the power factor meter. If there is no power factor meter, according to the readings of the voltmeter, ammeter and power meter at the same time, the following formula is used: cosφ=P/(1.73*U*I) where P, U, I are the power meter, respectively. Voltmeter and ammeter readings.

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