Advanced maintenance electrician knowledge appraisal test questions and answers - Chinahourly

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1. What is the first law of the magnetic circuit Kirchhoff? It explains what the problem is.
Answer: The first law of magnetic path Kirchhoff shows that for any node in the magnetic circuit, the magnetic flux algebra and zero passing through the node are the embodiment of flux continuity. The second law shows that in any loop in the magnetic circuit, the algebraic number of the magnetic flux and the algebraic sum equal to the magnetic pressure of each segment illustrate the relationship between the magnetomotive force and the magnetic pressure in any loop of the magnetic circuit.
2. What is a non-inductive resistance? Why can you reduce the inductance of components?
Answer: When manufacturing some resistance components, the resistance wire should be wound on the insulation shaft. Due to the large number of turns, the resistance element also becomes the inductance coil, which affects its accuracy. In order to minimize the inductance, Usually, the two wires are wound together, and the current flows out from the other end due to the inflow of the terminal. The resistance thus formed is called a non-inductive resistor. Because the currents passing through the coils are equal in magnitude and opposite in direction, the magnetic flux they produce is exactly offset, so there is almost no magnetic flux in the insulating shaft, so it is called non-inductive.
3. When testing the breakdown characteristics of a transistor with a JT-1 type diagram, why does it generally not damage the tube under test?
Answer: It is because the voltage applied to the collector of the tube under test is a sinusoidal half-wave voltage, not a DC voltage. When testing the breakdown characteristics, the peak value of the sinusoidal half-wave voltage is gradually increased, so that the breakdown voltage occurs in the sine. The highest point of the half-wave voltage, so that the voltage and current applied to the collector of the tube under test are instantaneous, and the tube will not break down due to over-current, but the instantaneous voltage and current may be due to the afterglow of the oscilloscope tube. And the residual effect of the human eye forms a curve on the screen. Note that it is also possible to break the tube if it is not properly adjusted during the test.
4. What is the role of the oscilloscope probe? Under what circumstances do I need to use a 10:1 partial pressure probe?
A: The oscilloscope's probe (also called probe) is used to connect the oscilloscope to the signal under test. It has a high input impedance. When it is used to test signals with higher frequency and smaller amplitude, the signal can be transmitted without distortion. Go inside the oscilloscope. The probe is used in the following cases: (1) The signal source under test is isolated from the oscilloscope. (2) Increase the input impedance of the oscilloscope. (3) Reduce the impact of external interference. (4) Measurement of signals with higher voltage (but generally should not exceed 500V)
5. What are the main features and functions of the oscilloscope?
A: It is characterized by the use of a single beam oscilloscope and the basic display principle of the oscilloscope. It can display electrical signals visually and intuitively, and directly observe the waveform of electrical signals. It can test the amplitude, period, frequency and phase of the electrical signal, observe the nonlinear distortion, test the parameters of the modulated signal, especially the test of the pulse signal, which occupies a very important position.
6. What is a dual trace oscilloscope? What is the difference between it and a two-wire (double beam) oscilloscope?
A: The dual-trace oscilloscope is a general-purpose oscilloscope. It uses an ordinary oscilloscope tube and adds an electronic switch. It alternately applies A, B and C signals to the deflection board, so that the screen displays two waveforms. The time division principle is made, and the two waveforms displayed are not present at the same time. The two-wire oscilloscope is not a general-purpose oscilloscope. It uses a dual-beam oscilloscope that can simultaneously emit two electrons. Each electron beam has its own Y-deflection plate, and two signals, A and B, are controlled one by one. Can display two signals at the same time. The main difference between the two oscilloscopes is that the oscilloscope structure is different and the display principle is different.
7. What is the impact of transformer leakage on the thyristor rectifier circuit? How to reduce the adverse effects caused by leakage?
Answer: Due to the existence of transformer leakage resistance, there is a gap in the phase voltage waveform. If the rectifier device has a large capacity, it will cause distortion of the power grid waveform, and interfere with the normal operation of the rectifier device itself and other devices on the power grid. At the same time, the power factor of the rectifier device is lowered, and the voltage ripple coefficient is increased. The method of reducing the adverse effects caused by the leakage reactance is to add a power supply filtering device or a phase shifting link in the device, such as an AC synchronous voltage input circuit of the trigger circuit to increase the resistance-capacitance phase shift or capacitance filtering.
8. Under what circumstances is the use of thyristors in series? What protective measures should be used in series?
Answer: When the working voltage exceeds the limit of the voltage of a single thyristor, two or more thyristors of the same type can be used in series. When connected in series, due to the different leakage currents of the thyristors, the voltages of the thyristors operating in series in the blocking state are not uniform, so the voltage equalization is required at both ends of the device. In addition, there is a dynamic voltage equalization problem. When the circuit is turned off, the turn-off time of each thyristor is different, and the first one that is turned off will be separately subjected to breakdown voltage to cause breakdown. Therefore, in each thyristor, parallel resistance and voltage equalization must be connected, and the instantaneous reverse voltage of each tube can be made uniform by the characteristics that the voltage across the capacitor cannot be abrupt. Although the above voltage equalization measures are adopted, the voltages on the thyristors are not completely equal. Therefore, the reverse peak voltage of the thyristors should be reduced when used, and the coefficient is generally multiplied (0.8-0.9).
9. Under what circumstances is the use of thyristors in parallel? What protective measures should be taken when connecting in parallel?
Answer: When the on-state average current of a single thyristor cannot meet the needs of the circuit, two or more thyristors of the same type and specification can be used in parallel. Since the tube characteristics cannot be completely consistent, the tube with low trigger voltage is triggered when the conduction is triggered. It will be turned on first, and the tube with small internal resistance will flow a large current, which will cause the tube to over-current and damage, so the current sharing measure is needed. In small power devices, current sharing resistors are often used. In high power devices, due to the large current, a current collector with a center tap is generally used. After the current sharing measure, the current distribution will not be completely equal, and the rated current should be reduced when used. Usually multiplied by the coefficient (0.8-0.9).
10. What are the main features of the three-phase double-reverse star-controlled rectifier circuit with balanced reactor?
Answer: (1) In the circuit, two sets of three-phase half-wave circuits work in parallel at the same time. The waveform of the rectified voltage is the same as the waveform of the six-phase half-wave rectification, and the pulsation degree of the waveform is much smaller than that of the three-phase half-wave. (2) Each of the two sets of stars connected to the transformer is electrically conductive, the DC magnetic potentials cancel each other, the magnetic circuit of the transformer is balanced, and there is no DC magnetization problem. (3) The load current is simultaneously borne by two thyristors and two windings. The maximum conduction time is doubled compared with the six-phase half-wave rectification, and the current flowing through the thyristor and the secondary winding of the transformer becomes smaller, so the optional rated current is smaller. Components. Compared with the six-phase half-wave circuit, the utilization of the transformer is doubled, so that a transformer with a smaller capacity can be used.
11. What issues should I pay attention to when using MOS circuits?
Answer: (1) The input terminal cannot be suspended. The MOS circuit is a high input impedance device. If the input terminal is suspended, the charge generated by the electrostatic induction is not discharged, and it is easy to accumulate and high voltage occurs. Although measures are taken in the circuit, However, there is still the danger of breaking through the grid. In addition, the voltage generated by the electrostatic induction causes the circuit to be disturbed, which is liable to cause logic confusion. Therefore, the redundant end should be connected to the high level according to the logic function, or connected to the low level, and cannot be left floating. (2) When the integrated MOS circuit is saved, each pin should be shorted. When soldering, the soldering iron must be grounded to prevent the static induced voltage generated by the soldering iron from damaging the component.
12. When the load of the three-phase asynchronous motor increases, the rotor current increases and the stator current increases.
Answer: When a three-phase alternating current is passed through the stator winding, a rotating magnetic field is generated, which cuts the stator winding and generates an induced electromotive force e1, which is opposite to the applied voltage, and also cuts the rotor winding, and generates an induced electromotive force e2, and a rotor current. I2, which interacts with the rotating magnetic field, turns on electromagnetic torque to rotate the motor. The resultant magnetic potential in the magnetic circuit of the motor is synthesized by the magnetic potential of the stator and the magnetic potential of the rotor. From the magnetic circuit analysis, the magnetic flux of the rotating magnetic field is determined by the applied voltage. As long as the applied voltage is constant, the magnetic flux can be considered unchanged regardless of the load variation. Therefore, when the motor is loaded with load, the resultant magnetic potential must be empty. The magnetic potential is phase balanced. Therefore, the stator current increases as the rotor current increases as the load increases.
13. Under steady operating conditions, when the load torque increases, why does the electromagnetic torque generated by the asynchronous motor increase accordingly? What happens to the motor when the load torque is greater than the maximum torque of the asynchronous motor?
Answer: When the motor is running with load, if the load torque changes within the allowable range, the motor can automatically adapt to the load requirements without manual adjustment. When the load torque increases, the rotational speed is slowed down because the electromagnetic torque is less than the load torque, so that the relative speed of the rotating magnetic field to the rotor conductor is increased, and the rotor current is increased, so the electromagnetic torque is also increased accordingly. Until the load torque is equal, the motor runs at a new steady state with a lower speed than before. In the stable working area of ​​the mechanical characteristics, when the speed decreases, the electromagnetic torque of the motor increases. Adapted to this operational requirement. When the load torque increases beyond the maximum torque of the motor, the motor will operate in an unstable region of mechanical characteristics. At this time, the electromagnetic torque of the motor will not increase, but will drop sharply until the motor stops.
14. What is the role of the servo motor in the automatic control system? Explain the structural characteristics and types of DC servo motors.
A: The role of the servo motor in the automatic control system is to convert the electrical signal into an angular displacement or angular velocity on the motor shaft. The DC servo motor and the general DC motor are basically the same in structure. The stator has a main magnetic pole and the rotor has an armature winding and a commutator. The difference is that the armature current of the servo motor is small, the commutation is not difficult, so the commutating pole is not installed, and the rotor is made slender and the air gap is small. The DC servo motor is divided into two types: electromagnetic (excitation type) and permanent magnet type. The former's field winding and armature winding are powered by two separate power sources. The latter is further divided into an armature control type and a magnetic field control type.
15. Explain how to check the short-circuit fault point of the stator winding of the three-phase asynchronous motor by voltage drop method.
Answer: When there is a short-circuit fault in the stator winding, the resistance of the winding is reduced. When the current is applied, the voltage on the winding with short-circuit fault is lower. Therefore, the short-circuit fault can be checked by the voltage drop method. The insulation of the connecting line between each pole group is stripped, and appropriate low-voltage alternating current or direct current is taken from the lead line, and then the voltage across the pole sets is measured by a voltmeter, and the short reading is short in the group with the smaller reading. . Further, the low-voltage power supply is connected to both ends of the pole phase group with short-circuit faults, and the voltage on each coil is measured by pinning the two ends of each coil, wherein the coil with the lowest voltage is a coil with a short-circuit fault.
16. Briefly describe the characteristics and working principle of the inductive synchronizer?
A: The inductive synchronizer is manufactured by the principle that the mutual inductance of the two planar printed circuit windings changes with its position, and is a sensor for detecting displacement. When a sine and cosine excitation voltage is applied to the slider of the inductive synchronizer, the fixed-length winding induces an electromotive force, which is repeated with the winding pitch as a period. In the pitch range, the induced electromotive force is opposite to the fixed slider. The mechanical displacement changes as a sinusoidal function. This electromotive force is processed by a circuit to obtain a mechanical displacement amount. Therefore, the characteristics of the inductive synchronizer are that the electromagnetic induction relationship is used as the basis for the work.
17. What are the servo drive components commonly used in CNC machine tools?
Answer: The servo drive components commonly used in CNC machine tools are: (1) Stepper motors: usually divided into three-phase or five-phase, working in three-phase three-shot, three-phase six-shot and five-phase ten-shot. (2) Electro-hydraulic pulse motor: Because the capacity of the stepping motor is limited. When driving a high-torque load, hydraulic power is used, and a small stepping motor and an oil motor are combined into an electro-hydraulic pulse motor, and the driving property is similar to that of a stepping motor. The driving device with the stepping motor as the control and the oil motor as the power is called the electro-hydraulic pulse motor. (3) Electro-hydraulic servo valve. (4) Servo motor.
18. In the automatic control system, what is the static difference and no static adjustment system?
A: There is a static adjustment system that works by the deviation value. The input signal to the amplifier (ie, the regulator) is the difference between the control signal and the actual value (ie, feedback) signal. The smaller the deviation value, the higher the speed regulation accuracy and the smaller the static difference. When the amplifier's magnification is large enough, the offset can be small, but it always exists. This means that the setpoint of the governor system always deviates from the actual value of the output, and the static difference cannot be zero. This is the meaning of static differential speed regulation. The difference between the static differential speed control system and the static differential speed control system is that the static differential degree of the speed control system can be zero, the set value of the system is completely equal to the actual value of the output, and there is no deviation, which is determined by the system. Proportional integral regulator to achieve. When the regulator is in steady state, the magnification is close to infinity, which means that the deviation value is infinitely small and is actually zero. There is a regulator in the static differential speed control system, the amplification factor can not be too large, otherwise it will generate oscillation, and the non-restroom speed control system uses a proportional integral regulator, the amplification factor is extremely large in steady state, in dynamic time Since the action of the integral link can eliminate the oscillation, it can be designed as a static-free speed control system. The difference between using a regulator is the difference between the two in the circuit.
19. In the B2012A type planer, in order to expand the speed range and improve the characteristics, what links are adopted in the main consignment system.
A: In order to expand the system adjustment range and improve the system characteristics, the main drag system uses voltage negative feedback, current positive feedback, current cutoff negative feedback and stability link. The function of voltage negative feedback is: (1) Maintain the generator end The voltage is basically constant, reducing the motor speed drop, improving the hardness of the characteristic (2) reducing the remanence voltage of the motor amplifier and generator, expanding the speed range of the system, preventing the table from crawling (3) speeding up the transition process, improving System dynamics.
The function of positive current feedback is to (1) compensate the internal voltage drop of the generator and the motor, so that the motor speed remains unchanged (2) to speed up the transition process.
The function of the current cutoff negative feedback is to limit the main loop current and improve the performance of the transition process to make it fast and stable.
The role of the stabilization link is to eliminate system oscillations and improve system stability.
20. Explain the role and difference between random access memory RAM and read only memory ROM?
A: Random access memory RAM: This kind of memory can read data or write data to store programs and data running in the computer. The disadvantage is that data and programs will be lost when the computer is powered off.
Read-only memory ROM: Programs and data in the memory cannot be changed once they are written, so their contents are fixed and can only be read but not written. They are used to store fixed programs and data tables, such as monitoring programs. , operating system firmware. There is also a UV erasable programmable EPROM memory, and an electrically erasable rewritable E2PROM memory.
21. What is the main role of the interface circuit in the microcomputer?
A: The circuit that connects a microcomputer to various peripheral devices or external devices is called an interface circuit. Its main functions: (1) provide the computer with status information of peripheral devices and external devices. Such as the device "ready", "busy" or "free", and so on. (2) Coordinating the difference between the "timing" or data processing speed of the external devices of the computer and peripheral devices. (3) Converting information transmitted between the computer and peripheral devices or external devices into a compatible format, such as parallel and serial conversion of grain data. Digital to analog or analog to digital conversion. (4) Level conversion function.
22. What is a Flexible Manufacturing System (FMS)?
A: FMS consists of three basic components, namely the processing system, the storage and handling system, and the control system. The processing system refers to the processing directly performed on the product parts, and the required hard equipment such as welding machinery, painting equipment, cleaning washing machine, CNC machine tool group, etc. The storage and handling system transfers parts from one location to another, transferring parts between the processing unit and the flat warehouse or warehouse. The handling system can be a conveyor belt, an unmanned trolley, an industrial robot, and the like. A control system, or operation management system, implements control management of the entire flexible manufacturing system. This is a hierarchical control network consisting of a central control computer and control devices for each device, which constitutes the information flow. #p#分页头#e#
23. What is computer-aided design and assisted manufacturing?
A: In the process of mechanical production, two forms of widely used computers are Computer Aided Design (CAD) and Computer Aided Manufacturing (CAM). They represent the direction in which application computers automate mechanical production. Computers are characterized by high speed, high accuracy, strict logic and huge information storage capabilities. Infiltrate the computer into the design, manufacturing and control process, so that people and computers can be combined to form a whole, to develop their own specialties, and to complete the design and manufacturing tasks. At this time, the storage of programs and data, high-speed computing and judgment, is done by the computer: the work that is creative or impossible for the computer can be borne by the person.
24. What is active inverter and passive inverter?
A: The inverter is a device that turns DC power into cool power. There are two types: one is called active inverter, which inverts direct current into power frequency alternating current and can feed energy back to the AC grid. The other is called passive inverter (also called inverter), which inverts DC power into AC power whose frequency is adjustable within a certain range, and supplies this AC power to the load instead of feeding back to the grid.
25. What are the methods for commutation in thyristor inverters?
A: Three commutation methods are usually used: (1) Load resonant commutation: it uses the oscillation characteristics of the capacitor and the inductor in the load loop to commutate. In this type of load circuit, the current has an automatic zero-crossing characteristic. As long as the load current leads the voltage longer than the thyristor turn-off time, the thyristor of the inverter can be turned off and turned off during this time, and restored. Positive blocking characteristics. (2) Forced commutation: Separate the commutation loop from the load. When commutating, the auxiliary thyristor is turned on, causing the commutation loop to generate a pulse, forcing the originally turned on the thyristor to be turned off due to the current interruption, and to withstand a section. The back pressure of time restores its positive blocking. This commutation is also called pulse commutation. (3) Switching with a turn-off thyristor or high-power transistor: it can eliminate additional commutation links, improve the economic indicators of the equipment, improve work efficiency, and reduce equipment volume.
26. What is the point-by-point comparison interpolation method for CNC machine tools?
A: The point-by-point comparison interpolation method means that each step of the machine tool is compared with the coordinates on a given trajectory once, to see where the actual machining point is at a given trajectory, above or below, or at a given trajectory. The outside is still inside, which determines the direction of the next feed. Its basic working method can be summarized into four "work beats": the first step: position discrimination. The positional deviation between the machining point and the specified part pattern size is determined to determine the direction of the tool. The second step: when processing. According to the position difference result, in order to reduce the deviation, a feed pulse is sent in the appropriate direction, and the control tool or the table is fed one step in the corresponding feed direction. The third step: deviation calculation. After the feed step, a new machining deviation is calculated for the new machining point as the basis for the next position determination. The fourth step: the end point discrimination. If the machining requirements are met, the end point coordinates are reached and the interpolation device automatically sends a stop signal. On the other hand, if the result of the discrimination has not reached the end point coordinate, the four beats are continued, and the stop signal is issued until the end point is reached.