Electrical Technology Q&A (1.1)

1. What is the basic content of Kirchhoff's law?

(1) Kirchhoff's first law, also called Kirchhoff's current law, namely KCL, is a law that studies the relationship between currents in various branches of the circuit. It states that: for any node in the circuit, the sum of the current flowing into the node is equal to the sum of the current flowing out of the node. Its mathematical expression is sigma I=0;

(2) Kirchhoff's second law is also called Kirchhoff voltage law, namely KVL, which studies the relationship between voltages in various parts of the circuit. It points out that: for any closed circuit in the circuit, the algebraic sum of voltages in each section is equal to zero. Its mathematical expression is: σ U=0;

2. What is skin effect?

When the alternating current passes through the conductor, the current distribution is not uniform at all places on the cross section of the conductor, with the smallest density at the center of the conductor, and the greater the density is closer to the surface of the conductor. This current distribution phenomenon tends to be along the surface of the conductor, which is called skin effect.

3. What are the concepts and characteristics of parallel resonance?

In the parallel circuit of inductor and capacitor, the phenomenon that the terminal voltage of the parallel circuit is in phase with the total current is called parallel resonance. Parallel resonance is characterized by: when the parallel resonance is achieved by changing the capacitance C, the total impedance of the circuit is maximum, so the total current of the circuit becomes minimum. But for each branch, its current may be much larger than the total current, so parallel resonance is also called current resonance. In addition, in parallel resonance, because the terminal voltage and total current are in the same phase, the power factor of the circuit reaches the maximum value, that is, cos is equal to 1, and parallel resonance will not produce resonant overvoltage endangering the safety of the equipment. Therefore, it provides us with an effective method to improve the power factor.

4. How are eddies created? What is eddy current loss?

An alternating current is passed through a coil with an iron core, which produces alternating flux and an induced potential. Under the action of this potential, the core will form a self-induced loop of current, called eddy current.

The energy loss caused by eddy current is called eddy current loss.

5.What is the transition process? What causes the transition process?

The transition process is a transient process, which is a process over a period of time from one stable state to another.

The transition process is due to the existence of energy storage elements. Energy storage components such as inductors and capacitors cannot jump in the circuit, that is, the current of the inductor and the voltage of the capacitor cannot mutate in the process of change. Therefore, there must be a process for the transition from a stable state to another state in the circuit.

6. What is the superposition principle?

In a linear circuit, the current (or voltage) of any branch is the algebraic sum of the current (or terminal voltage) generated in the branch when several power sources are acting simultaneously. When applying the superposition principle, the voltage source should be regarded as short-circuited and the current source as open-circuit.

7. What are the advantages of using three-phase generator and power supply equipment?

The volume of three-phase generator is smaller than that of single-phase generator when generating the same capacity of electricity. There are fewer three-phase transmission and distribution lines than single-phase transmission and distribution lines, which can save a lot of materials. In addition, three-phase motors have better performance than single-phase motors. Therefore, three-phase equipment is mostly used.

8. Why can three-phase three-wire system be used for the power supply of three-phase motors, while three-phase four-wire system must be used for lighting power supply?

Because the three-phase motor is three-phase symmetrical load, whether it is star connection or triangle connection, it only needs to connect the three phase lines of the three-phase motor to the three phase lines of the power supply, and does not need the fourth neutral line, so the three-phase three-line system power supply can be used. The load of lighting power supply is electric lamp, its rated voltage is phase voltage, one end must be connected with a phase line, one end is connected with the neutral line, so you can ensure that each phase voltage does not affect each other, so must use three phase four line system, but it is strictly prohibited to use one phase one ground lighting.

What are the main parameters of a transistor?

(1) amplification factor (β), is to measure the transistor amplification ability;

(2) reverse saturation current (Icbo), indicating the transistor junction quality;

(3) Penetration current (Iceo), illustrating triode performance.

10. What is transistor feedback? What are the types of feedback?

In a transistor amplifier, part of the voltage or current at the output is reversed back to the input by a method called feedback.

There are two kinds of feedback: after the introduction of feedback, the amplification factor of the amplifier increases is called positive feedback, and the amplification factor decreases is called negative feedback.

11. What are differential circuits and integral circuits?

Using the principle that the voltage between the two ends of the capacitor cannot be changed, the circuit that can change the rectangular wave into sharp pulse wave is called differential circuit, and the circuit that can change the rectangular wave into sawtooth wave is called integral circuit.

12. What is the role of the filter circuit?

Rectifiers convert alternating current to direct current, but the waveform after rectification contains a considerable amount of ALTERNATING current. Such direct current can only be used in equipment with low power requirements. Some devices, such as electronic instruments and automatic control circuits, require that the pulsation component of DC power supply is very small. Therefore, in order to improve the quality of rectifier voltage and improve the voltage waveform of rectifier circuit, filter circuit is often installed to filter out the AC component.

13. How to use silicon controlled rectifier?

In the rectifier circuit, the SCR in the time under the forward voltage, change the trigger pulse input time, that is, change the size of the control Angle, the load can get different values of DC voltage, thus controlling the size of the output voltage.