Frequently asked questions in maintenance electrician interview

Frequently asked questions in maintenance electrician interview

Interview is to examine a person's working ability and comprehensive quality through written, face-to-face or online communication (video and telephone). Through the interview, you can preliminarily judge whether the candidate can integrate into your team. It is a recruitment activity carefully planned by the organizer. The following are the frequently asked questions of maintenance electrician interview that I sorted out. Welcome to share.

Maintenance electrician's question and answer

1. What are the advantages and disadvantages of belt drive?

Answer: Advantages: stable transmission and no noise; With overload protection function; Long transmission distance; Simple structure, convenient maintenance and low cost.

Disadvantages: the transmission ratio cannot be guaranteed; The structure is not compact enough; Short service life; Low transmission efficiency; Not suitable for high temperature, flammable and explosive occasions.

2. How to turn the three-phase asynchronous motor?

Answer: When three-phase alternating current flows into the three-phase stator winding, the first rotating magnetic field is generated in the stator cavity. Under the action of rotating magnetic field, the stationary rotor conductor before rotation is equivalent to the rotor conductor cutting the magnetic field lines relatively, thus generating induced current in the rotor conductor (electromagnetic induction principle). Because the conductors in the rotor are always symmetrically arranged, the electromagnetic force generated on the conductors is in the opposite direction, thus forming electromagnetic torque and making the rotor move. Because the current in the rotor conductor is generated by the rotating magnetic field of the stator, it is also an induction motor. Because the rotor speed is always lower than that of the stator rotating magnetic field, it is also called asynchronous motor.

3. What are the transmission modes between motor and machinery?

Answer: ① direct drive of copying wheel; ② Belt drive; ③ gear transmission; ④ Worm drive; ⑤ Chain drive; ⑥ Friction wheel drive.

4. In the single-phase bridge rectifier current, what happens if a diode is connected in the wrong direction? What happens if a diode is open internally?

Answer: In the single-phase bridge rectifier current, if a diode is wrongly connected, it will cause a short circuit of the power supply. If the diode is damaged internally, it will form a single-phase half-wave rectification.

5. What are the advantages and disadvantages of hydraulic transmission?

Answer: Advantages: stepless speed change; The movement is relatively stable; Melon has fast speed and small impact, and can start, brake and reverse at high speed; Can automatically prevent overload; Simple operation; Long service life; Small size, light weight and compact structure.

Disadvantages: easy to leak, high manufacturing accuracy of parts; The transmission efficiency is low.

6. What problems should I pay attention to when using the low-voltage test pen?

A: The following points should be noted:

(1), before the test, test the charged body to prove whether the test pen is in good condition.

(2) Wear insulated shoes when using.

(3) When testing under strong light, attention should be paid to avoiding light.

(4) When some equipment works, the shell will be charged due to induction, and the test result with a test pen will not be correct. It is necessary to use a multimeter to measure whether it is true charging or inductive charging.

(5) For charged bodies with voltage lower than 36V, the test pen is often invalid.

7. What is the main reason for the rubbing of the stator and rotor of the motor? What are the consequences of friction between stator and rotor?

Answer: The main reasons for the rubbing of the stator and rotor of the motor are: bearing damage; Bearing wear leads to rotor sinking; Bending and deformation of rotating shaft; There are cracks in the base and end cover; The end cover is not tightly closed; Dirty inside motor, etc. Rub-impact of stator and rotor will cause strong vibration and noise of motor, make the rubbing surface generate high temperature, even smoke and catch fire, make insulation brittle, thus burning the winding.

8. Briefly describe several electric braking principles of three-phase asynchronous motor?

Answer: (1) Reverse braking: The characteristic is that the motor runs in an electric state, but the electromagnetic torque is opposite to the motor rotation direction.

(2) Motor speed n >； during regenerative braking (feedback braking); When n 1, the cutting direction of rotor bar is determined by rotating magnetic field and electric state (n

9. What are the causes of nonlinear distortion of AC voltage amplifier circuit? What effect does static value have on distortion?

An inappropriate static value of the answer (1) will cause nonlinear distortion. In addition, temperature change and other interference signals (such as power supply voltage fluctuation, load change and external electromagnetic phenomena) will cause nonlinear distortion. (2) If the static value is not properly obtained and the AC signal is added, the transistor may enter the saturation state or the off state, that is, it will enter the nonlinear region of the transistor to work, which will cause nonlinear distortion.

10. The rated voltage of three-phase asynchronous motor is 380 volts. When the voltage of three-phase power supply line is 380 V, which connection should be used for stator winding? When the voltage of three-phase power line is 660V, which connection mode should be adopted?

Answer: When the voltage of the three-phase power supply line is 380V, the stator windings are connected in triangle; When the voltage of the three-phase power supply line is 660V, the stator winding should be connected in a star shape.

1 1. What are the common methods to reduce the speed of DC motor?

Answer: Take parallel motors as an example. There are three types.

① Reducing terminal voltage: This method is rarely used because it has the disadvantage of reducing terminal voltage and excitation current.

(2) Increase the excitation current: that is, increase the magnetic field, but this method is limited by magnetic saturation and has certain limitations.

(3) shunt resistor in armature circuit to reduce armature terminal voltage. This method is easy to implement, so it is often used.

12. what is the main reason for the rubbing of the stator and rotor of the motor? What are the consequences of friction between stator and rotor?

Answer: The main reasons for the rubbing of the stator and rotor of the motor are: bearing damage; Bearing wear leads to rotor sinking; Bending and deformation of rotating shaft; The end cover of the frame has cracks; The end cover is not tightly closed; Dirty inside motor, etc. Rub-impact of stator and rotor will cause strong vibration and noise of motor, make the rubbing surface generate high temperature, even smoke and fire, cause insulation to burn brittle, and even burn out windings.

13, why use closed-circuit shunt circuit in DC current measuring circuit of multimeter?

Answer: Because in the closed shunt circuit, the contact resistance of the transfer switch has nothing to do with the resistance of the shunt resistance, even if the contact is poor or the range is switched with load, it will not have any adverse effect on the instrument, which can ensure the accuracy of measurement and the safety of the instrument. If an open shunt circuit is used, its contact resistance will be connected in series with the shunt resistance, which will increase the error. Moreover, when the contact is poor or the range is switched with load, the large current will burn out directly through the instrument.

14, please explain the difference between magnetic field intensity and magnetic induction intensity?

Answer: The magnetic field intensity is expressed by H and the magnetic induction intensity by B, both of which can describe the intensity and direction of the magnetic field, and are related to the current and distribution of the excitation magnetic field. But h has nothing to do with magnetic field medium, and b has something to do with magnetic field medium. The unit of H is A/m (in meters) and the unit of B is T (Tesla). H is often used in the calculation of magnetic field, while B is often used in the qualitative description of magnetic field.

15, what problems should be paid special attention to in the use and maintenance of DC torque motor?

Answer: The armature current of DC torque motor should not exceed the peak current in operation, so as to avoid demagnetization of magnetic steel and reduce the torque. When the rotor is taken out, the stator must be protected by magnetic short circuit, otherwise it will also cause demagnetization of magnetic steel.

16, what are the general provisions for the installation of explosion-proof electrical equipment?

Answer: ① Power equipment and circuits, especially electrical equipment that generates sparks during operation, should be installed as far as possible outside explosive dangerous places. If it needs to be located in a dangerous place, it should be installed in a place with less danger and effective safety measures should be taken.

(2) Electrical equipment and wiring in explosive dangerous places should be protected from mechanical damage, and meet the requirements of environmental conditions such as corrosion protection, moisture-proof, sun protection, wind and rain protection and sand prevention.

(3) Electrical equipment in explosive dangerous places should have reliable protection such as overload and short circuit.

(4) In explosive dangerous places, sockets that must be installed should be installed in places where explosive mixing is not easy to accumulate; Local lighting lamps and lanterns should be installed in the position where gas is not easy to impact in the event of an accident.

17. What are the methods to adjust the average output DC voltage of chopper?

Answer: The output voltage of chopper is rectangular pulse wave voltage. There are three ways to adjust the average output voltage: keeping the pulse period t unchanged and changing the pulse width τ; Keep τ unchanged and change t; Change τ and t at the same time.

18. Why do AC contactors overheat during frequent operation?

Answer: When AC contactor is started, due to the large gap between iron core and armature and small reactance, the exciting current that can pass through the coil is very large, which is often more than ten times the working current. If it is started frequently, the excitation coil will pass a large starting current, which will lead to overheating of the coil and burn it out in severe cases.

19, how to disassemble and repair asynchronous motor correctly?

Answer: Before disassembling and repairing asynchronous motors, all preparations should be made, such as tools used, inspection and records before disassembling. To remove the motor:

(1) When disassembling the belt or coupling, mark it before disassembling the belt and coupling, derust it before installation, and clean it before resetting;

② Dismantle the end cover: first remove the bearing cover, then take the end cover, mark the front and rear covers, and reset them according to the marks during installation;

③ Dismantle the rotor: Wear-resistant thick paper should be padded between the stator and the rotor to prevent damage to the fixed-value winding. If the winding is heavy, check whether there are impurities in the stator when installing the rotor with lifting equipment, then install the shaft extension end cover first, and then install the rotor together with the fan and the back cover.

20, DC torque motor in use and maintenance should pay special attention to what problems?

Answer: The armature current of DC torque motor should not exceed the peak current during operation, so as to avoid demagnetization of magnetic steel and reduce the torque. When the rotor is taken out, the stator must be protected by magnetic short circuit, otherwise it will also cause demagnetization of magnetic steel.

2 1, what are the characteristics of screw drive?

Answer: It can change rotary motion into linear motion, with simple structure, stable transmission and low noise. A large reduction ratio can be obtained; Generating greater thrust; Self-locking can be realized. The disadvantage is low transmission efficiency.

22, in the asynchronous motor operation and maintenance work should pay attention to?

Answer: ① Keep the periphery of the motor clean;

(2) Check the changes of power supply voltage and current with instruments. Generally, the allowable voltage fluctuation of the motor is 5% of the voltage, the difference between the three-phase voltages is not more than 5%, and the unbalanced value of each phase current is not more than 10%. Pay attention to whether the motor runs out of phase.

(3) Regularly check the temperature rise of the motor, and commonly use a thermometer to measure the temperature rise. It should be noted that the temperature rise shall not exceed the maximum allowable value;

(4) Monitor the bearing for abnormal noise, ensure good sealing, and change the lubricating oil regularly. Generally, the oil change cycle of sliding bearing is 1000h, and that of rolling bearing is 500 h;

⑤ Pay attention to the sound, smell, vibration and transmission of the motor. During normal operation, the motor should have even sound without noise and special sound.

23, three-phase asynchronous motor rotor is how to turn up?

Answer: When symmetrical three-phase sinusoidal alternating current is fed into symmetrical three-phase stator winding, a rotating magnetic field is formed. When the rotating magnetic field cuts the rotor conductor, induced electromotive force and induced current are generated. When the induced current is influenced by the rotating magnetic field, electromagnetic torque is formed, and the rotor rotates gradually in the direction of the rotating magnetic field.

24, what is short circuit and short circuit fault? How to prevent the harm of short circuit fault?

Answer: Short circuit refers to the working state in which two points in the circuit are directly connected by a conductor with negligible resistance. Short circuit may occur at both ends of the load or anywhere on the line, or it may occur inside the power supply or load. If there is a short circuit at both ends of the power supply, there is only a small internal resistance of the power supply in the loop, which will form a large short-circuit current and cause circuit damage. Therefore, power short circuit is a serious fault and should be avoided as much as possible. However, the "local short circuit (short circuit)" used in the circuit to achieve a specific purpose cannot be said to be a fault. In order to prevent the harm of short circuit fault from expanding, fuses or automatic circuit breakers are usually connected to the circuit for protection.

25, thyristor controlled rectifier circuit trigger circuit must have which several basic links? What are the basic requirements?

Answer: The trigger circuit of thyristor must have three basic links: synchronous voltage formation, phase shift and trigger pulse formation and output. The requirements of thyristor for trigger circuit are:

(1) The trigger voltage must be synchronized with the anode voltage of the thyristor;

(2) The trigger voltage should meet the requirements of the phase shift range of the main circuit;

(3) The leading edge of the trigger voltage should be steep and the width should meet certain requirements;

(4) It has certain anti-interference ability;

(5) The trigger signal should have sufficient voltage and power.

26. Why does the electromagnet generate suction after being electrified?

Answer: After the excitation coil of the electromagnet is energized, a magnetic field is generated around the coil. When the iron core made of magnet material is put into the coil, the magnet is magnetized to produce magnetism. For electromagnets, the magnetic flux generated by energizing the excitation coil passes through the iron core and the armature to form a closed magnetic circuit, so that the armature is also magnetized, generating magnetic poles different from the iron core, thus generating attractive force.

27. Why can't the electromagnetic system measuring mechanism be directly used for the measurement of alternating current?

Answer: The direction of the magnetic field generated by the permanent magnet in the magnetoelectric measuring mechanism is constant. If alternating current is applied to the coil, the direction of rotating torque will also change due to the constant change of current direction. Its movable part has inertia, which makes the pointer almost motionless or slightly shaking in its original position, so it can't get the correct reading, so the electromagnetic measuring mechanism can't be directly used to measure alternating current.

28. Briefly describe the working principle and speed regulation method of three-phase AC commutator asynchronous motor?

Answer: A three-phase power supply is introduced into the primary winding of the rotor to generate a rotating magnetic field, which generates an induced electromotive force in the regulating winding and the secondary winding of the stator, and a current is generated in the secondary loop to form an electromagnetic torque, so that the rotor rotates. By changing the opening angle θ between in-phase brushes, the regulated electromotive force can be changed, and then the secondary loop current, current and electromagnetic torque can be changed, and then the motor speed can be changed.

29. What are the mechanical characteristics of DC motors? What is the artificial mechanical characteristic curve of series DC motor?

Answer: When the power supply voltage, excitation current and total resistance of armature circuit of DC motor are fixed, the relationship between speed and torque is called mechanical characteristics.

Answer: If a resistor is connected in series in the armature circuit of a series motor to change the speed characteristic curve, they are called artificial characteristic curves.

30. Why is the starting current of three-phase squirrel-cage asynchronous motor starting directly large? What are the adverse effects of excessive starting current?

Answer: At the moment when the three-phase asynchronous motor starts directly, the rotor speed is zero and the slip ratio is the largest, which makes the induced current in the rotor winding the largest, thus generating a larger starting current in the stator winding. Excessive starting current will cause voltage fluctuation of power grid and affect the normal operation of other electrical equipment. At the same time, the motor winding will be seriously heated, accelerating insulation aging and shortening service life.

3 1, what is eddy current? What are the advantages and disadvantages in production?

A: In an alternating magnetic field, a closed annular current is induced on the cross section perpendicular to the magnetic field lines, which is called eddy current. Eddy current principle can be used to make induction furnace to melt metal, and eddy current principle can be used to make magnetoelectric and inductive electrical instruments. The damper in the electric energy meter is also made by using eddy current principle. In the motor, transformer and other equipment, because of the existence of eddy current, it will produce additional consumption, and at the same time the magnetic field will be weakened, which will reduce the efficiency of electrical equipment and make the capacity of the equipment unable to be fully utilized.

32, how to change the steering of three-phase asynchronous motor?

Answer: The working principle of asynchronous motor shows that:

① The installation direction of asynchronous motor is consistent with the rotating direction of rotating magnetic field;

② The rotating direction of the rotating magnetic field depends on the phase sequence of the current in the stator winding;

③ As long as the phase sequence of the asynchronous three-phase power supply is changed, any direction of the rotating magnetic field of the motor can be changed, and the rotating direction of its rotor will also be changed.

33. What are the signal circuits of the substation?

Answer: The common operating mechanisms of circuit breakers are: ① electromagnetic operating mechanism; (2) spring operating mechanism; ③ Handle operating mechanism. There are two working power supplies: AC and DC.

34, motor core manufacturing should pay attention to what problems? What will happen if the quality is not good?

Answer: The manufacturing of motor iron core should pay attention to the selection of materials, the accuracy of dimensions, the regularity of shapes, the reliability of insulation and the firmness of assembly. Poor quality, increased motor excitation current, serious iron core heating, vibration, noise, bore sweeping and other phenomena will occur.

Extension: the responsibility of maintenance electrician

1 Under the leadership of shift supervisor and electrician monitor, be responsible for all electrical equipment and production safety operation in the workshop. The main task is to check, maintain and adjust all electrical equipment in the workshop according to regulations, report faults and hidden dangers in time and take measures to organize and deal with them in time, urge other posts to operate and maintain the electrical equipment in the workshop normally, and ensure the safe operation and production of the electrical equipment in the workshop.

2. Strictly abide by the company management system, workshop management system and team management system, and obey the command and scheduling of the kiln worker and the work arrangement of the superior leader.

3, seriously implement the safety operation procedures, work instructions, electrical equipment inspection system, lubrication system, to do a good job of post equipment management, maintenance and inspection. Strictly implement the system of "three good" (managing, using and maintaining well) and "four meetings" (using, maintaining, checking and troubleshooting) to ensure the safe operation of equipment and production.

4, strictly implement the system of succession, do a good job of succession, will be on duty post production, equipment operation and existing problems truthfully report to the next class, and make records. Do a good job of detailed handover and clear handover.

5. Strictly implement the workshop hygiene management system, and be responsible for the cleaning and cleaning of the environmental sanitation and equipment hygiene in the duty room and the health area of the division of labor, and try to keep it clean and tidy.

6. Constantly learn job knowledge, improve job skills, accept technical training and assessment of workshops and shift supervisors, accept three-level safety education, and strengthen safety prevention and safety inspection.

7. Complete other work assigned by the workshop.

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