Most popular electrician review question 6

2022-08-06
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Electrician review question (6)

II. Technician question

1. Explain the difference between magnetic field intensity and magnetic induction intensity

answer: the magnetic field intensity is represented by H, and the magnetic induction intensity is represented by B. both can describe the strength and direction of the magnetic field, and both are related to the current exciting the magnetic field and its distribution. However, h has nothing to do with the magnetic field medium, while B has something to do with the magnetic field medium. H is in a/m (ampere/meter) and B is in t (Tesla). H is often used in the calculation of magnetic field, and B is often used in the qualitative description of magnetic field

2. What is eddy current? What are the advantages and disadvantages in production

answer: the interior of the conductor in the alternating magnetic field will feel on the section perpendicular to the direction of the magnetic line of force. China's plastic machinery enterprises have expanded the development of emerging markets for extruders in recent years, which should produce a closed ring current, called eddy current. The eddy current principle can be used to make induction furnace to smelt metal. The eddy current can be used to make magnetoelectric and induction electrical instruments. The damper in the electric energy meter is also made by using the eddy current principle; In the motor, transformer and other equipment, due to the existence of eddy current, additional losses will occur. At the same time, the magnetic field will weaken, resulting in the reduction of the efficiency of electrical equipment, so that the capacity of the equipment can not be fully utilized

3. What can be achieved by applying graphic method to analyze amplification circuit

answer: ① judge whether the q-point setting is appropriate ② analyze the waveform distortion ③ determine the maximum output range of the amplifier without distortion ④ estimate the multiple of the voltage amplifier

4. What is a DC amplifier? What is the zero drift of DC amplifier? How does the zero drift affect the operation of the amplifier

answer: an amplifier that can amplify a slowly changing DC signal is called a DC amplifier. When the input signal of the DC converter is zero, the output signal cannot be kept at zero, but will move up and down from the zero. This phenomenon is called zero drift. If the amplifier produces zero drift during operation, because the signal changes slowly, the drift and useful signal can not be distinguished at the output of the amplifier, and even the circuit can not work normally

5. What is binary number? Why use binary counting in digital circuits

answer: binary numbers are counted according to the rule of two into one. Since the binary number has only two states of "0" and "1", it is easy to realize with electronic components. Binary numbers are easy to operate and can be easily converted into octal and hexadecimal numbers. It can also be converted into decimal numbers. Therefore, digital circuits generally use binary counting

6. Briefly describe the steps for debugging single-phase controllable rectifier circuit

answer: the general debugging steps are to adjust the control circuit first, and then debug the main circuit. First, use an oscilloscope to observe the waveforms of the three basic links of synchronous voltage formation, phase shift, pulse formation and output in the trigger circuit, adjust the given value potentiometer to change the given signal, and observe the phase shift of the trigger pulse. If the waveforms of each part are normal, the pulse can shift smoothly, the phase shift range meets the requirements, and the pulse amplitude is sufficient, the control circuit commissioning is completed. The commissioning steps of the main circuit are as follows: firstly, add a mortar tensile testing machine to the main circuit with a voltage regulator, connect the trigger circuit with a low voltage (10~20 V), and observe the voltage change between the positive and negative poles of the thyristor with an oscilloscope. If a part of the waveform is a horizontal line, it means that the thyristor has been turned on. The length of the horizontal line can be changed. A and B are load ports, indicating that the conduction angle of the thyristor is adjustable. During commissioning, pay attention to whether the current changes of output and input circuits correspond to each other, and whether there is local open circuit and heating

7. How is electricity classified

answer: it can be divided into power transmission and distribution according to its functions; The scope of regular power supply can be divided into regional power supply and local power supply; The periodic structure can be divided into open type and closed type

8. How is the power load classified

answer: it can be divided into three levels according to the requirements of power load on power supply reliability and the loss and impact caused by power interruption: sudden power failure of class I load will cause personal death, or cause major political and economic losses. Such as power load of important transportation and communication hubs, major equipment and continuous production lines in key enterprises, political and foreign affairs activity centers, etc. Sudden power failure of secondary load will cause great economic losses, or cause adverse political effects. For example, sudden power failure will cause major equipment damage, a large number of products scrapped or a large number of production reduced factory power loads, traffic and communication hub power loads, public places with a large number of people, etc. Class III load does not belong to class I and class II load. The power supply department will determine the power supply mode according to the load level of the user

9. What kind of power system does the small grounding short-circuit current system refer to? What kind of power system does the large grounding short-circuit current system refer to

answer: the small grounding breaking current system refers to the power system (such as it system) in which the neutral point of the power supply is not grounded and grounded through the arc suppression coil; Large ground breaking current system refers to the power system (such as TT, TN System) in which the neutral point of the power supply is directly grounded

10. What is the impact on the operation of the system when a single-phase grounding fault occurs in the power system with ungrounded neutral

answer: the three-phase electrical equipment in normal operation in the system is not affected, because the line voltage of the line has not changed regardless of its phase and amplitude. However, this line is not allowed to operate for a long time in case of single-phase grounding fault, because in case of single-phase grounding, the voltage to ground of other two phases will increase by √ 3 times, which is easy to cause damage to phase insulation, thus forming two-phase and three-phase open circuit, causing accidents in the power system and affecting the safe use of electricity. In addition, when a single-phase grounding fault occurs, the system grounding current (fault phase grounding current) increases to 3 times of the original, which will cause an arc at the grounding point, which is very dangerous. If the grounding is poor, the so-called intermittent arc may also appear at the grounding. Intermittent arc often causes over-voltage (usually 2.5~3 times of phase voltage) which threatens the safe operation of power system. For this reason, the power system dispatching regulations stipulate that the single-phase ground fault can be effectively contained during operation, and the interval shall generally not exceed 2H

11. In 110kV and above EHV systems and 220/380v low-voltage distribution systems in China, the neutral point of power supply usually adopts the operation mode of direct grounding. What are the reasons why two systems with different properties adopt the same medium operation mode

answer: the EHV system adopts the operation mode of direct grounding of the neutral point of the power supply. When a single-phase grounding fault occurs in the system, due to the large single-phase short-circuit current IK, the relay protection device acts immediately, the circuit breaker is disconnected, and the grounded line is cut off, so there will be no intermittent arc. At the same time, because the neutral point potential is fixed by the grounding body, in case of single-phase grounding fault, non

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