Electrician skill appraisal test question bank (selection)
First, yes and no questions
1. The indicating instrument can not only directly measure the electromagnetic quantity, but also can cooperate with various sensors to measure the non-electric quantity such as temperature, pressure and flow. (√)
2. In addition to the current meter and voltmeter, which can be used for both DC and high accuracy, the electric instrument can also be made into a power meter, a frequency meter and a phase meter. (√)
3. For instruments with an accuracy of 1.5, the basic error of measurement is ± 3%. (×)
4, to directly measure the average value of the current in the circuit, you can use the electromagnetic system. (×)
5, the additional resistance of the voltmeter can also expand the range, but also play a temperature compensation role. (√)
6. The secondary winding of the voltage transformer is not allowed to open, and the secondary winding of the current transformer is not allowed to be short-circuited. (×)
7. The DC ammeter can be used in AC circuits. (×)
8. The clamp ammeter can be made to measure both AC current and DC current. (√)
9. Use a multimeter to measure the resistance. You must adjust the pointer once every time you change the ohms. (√)
10. When measuring AC circuit and active energy, because it is AC, its voltage coil, current coil and each end can be connected to the line. (×)
11. When two-phase three-wire active load energy is measured by two single-phase electric energy meters, there is a table reversal, which is definitely a wiring error. (×)
12, the current coil of the electric power meter will reverse the deflection of the pointer, but if the voltage coil is also reversed, it can operate normally. (×)
13. The resistance of the electromagnetic system to the external magnetic field is stronger than that of the magnetoelectric system. (×)
14. The electric system phase meter has no hairspring that generates a reaction torque, so the pointer can stop at any position on the dial before the meter is connected to the circuit. (√)
15. According to the instrument's defense ability against electric field or external magnetic field, it is divided into four levels: I, II, III and IV. Class II meters are allowed to change their indication value by ± 1% under the influence of an external magnetic field or an external electric field. (√)
16. The larger the internal resistance of the ammeter that measures the current, the better. (×)
17. The multimeter's ohmmeter is not available to directly measure the internal resistance of the micro-ampere meter, galvanometer or standard battery. (√)
18, whether it is measuring direct current or alternating current, the illumination of the xenon lamp of the electroscope is the same. (×)
19. A low-voltage electroscope equipped with a neon bulb can distinguish between a live line (phase line) and a ground line, and can also detect alternating current or direct current. The digital display can detect the presence or absence of electricity in addition to the low-voltage electroscope. Line. (√)
20, wire stripper can be used to strip the insulation of plastic wire or rubber wire with a core area of ​​6mm2 or less, so there should be a cut of 6mm or less in diameter. (×)
21, the protection terminal of the soldering iron can be wired or not. (×)
22, manual hydraulic crimping pliers can be used to crimp copper and aluminum wires with a cross section of 16~240mm2. (√)
23, when installing the grounding wire, first install the three-phase line end, and then install the grounding end; when disassembling, reverse the grounding end first, then remove the three-phase line end. (×)
24, the length of the first and second wiring of the welding machine should not exceed 20m. (×)
25, AC current meter and voltmeter indicate all valid values. (√)
26, insulated boots can also be used as acid, alkali, oil resistant boots. (×)
27, copper has good electrical and thermal conductivity, high mechanical strength, but it is easy to be oxidized when the test is high, the melting time is short, it is suitable for fast melt, protect the transistor. (×)
28, tin and lead metal materials with low melting point and long melting time, suitable for high-pressure fuse melt. (×)
29, the contact for strong electricity and the contact for weak current, the performance requirements are the same, the materials used are also the same. (×)
30. AlNiCo materials used as sensors, speakers and microwave devices are permanent magnet materials that provide a constant magnetic field in a given space. (√)
31. When the aluminum wires are connected, the winding method or the splicing method cannot be used like the copper wires, just because the aluminum wires have poor mechanical strength. (×)
32. The safe current carrying capacity of the wire should have different values ​​at different ambient temperatures. The higher the ambient temperature, the greater the safe current carrying capacity. (×)
33. When the steel core aluminum stranded wire passes through the alternating current, the current actually flows only from the aluminum wire due to the skin effect of the alternating current, so the effective sectional area is only the area of ​​the aluminum wire. (√)
34. The thermal resistance coefficient of the soil will affect the heat dissipation of the buried cable, thus affecting the safe current carrying capacity of the cable. The greater the thermal resistance coefficient, the greater the safe current carrying capacity of the cable. (×)
35. The cable tube (TC) and the tube wall are thin, and the nominal diameter refers to the inner diameter. (×)
36. The bare wire should be laid at a height of 3.5m or more, and it should not be erected below 3.5m. (×)
37. The wire is laid in the ceiling or ceiling, and can be protected without pipe. (×)
38. The cable is short and the traction is small. The traction head can be used to make the wire core with tension. When the traction force is large, it can be pulled by the steel wire sleeve. (×)
39. For all pipe-passing lines, there shall be no more than one pipe joint. (×)
40, cable cores sometimes press round, semi-circular, fan-shaped and other shapes, this is to reduce the cable dimensions and save raw materials. (√)
41. The protective layer of the cable protects the cable core conductor. (×)
42. When the cable is moving, the cable reel should be placed flat on the car for fixing. (×)
43. In paper-insulated cables, the wrapped-type cable has a higher operating voltage than the split-phase lead-packed cable. (×)
44. Medium and low voltage PVC cables, polyethylene cables and cross-linked polyethylene cables, generally also have a completely sealed metal sheath, like paper insulated cables. (×)
45. When the cable is in operation, just monitor its load not to exceed the allowable value, and do not need to monitor the temperature of the cable, because both are consistent. (×)
46. ​​When the cable is sawn steel, the process is to tie the steel armor with a diameter of 2.0mm copper and tie it 3~4 ,. The winding direction of the copper wire should be opposite to that of the steel armor. (×)
47. When the substation is powered off, first isolate the switch and then cut off the circuit breaker. (×)
48. When the high-voltage isolating switch is in operation, if the surface of the insulator is severely discharged or the insulator is broken, the high-voltage isolating switch should be disconnected immediately and the operation should be terminated. (×)
49. The high-voltage load switch has an arc extinguishing device that can disconnect the short-circuit current. (×)
50. When replacing the quartz sand in the tube of the fuse, the quartz sand particles are the same size. (×)
51. The standby transformer should also be connected to the signal circuit in light gas relay. (√)
52. Unloaded voltage regulating transformer. After changing the tap switch, the DC resistance of each phase winding should be measured. The difference of DC resistance of each phase is not more than 10% of the minimum value of the three phases. (×)
53. Usually, the parallel capacitor bank is discharged by the voltage transformer or the discharge bulb after the circuit is cut off. Therefore, the substation can be repaired without manual discharge after power failure. (×)
54. After using the battery pack for a period of time, it is found that some battery voltages are already low, and most of the battery voltages are still high, so you can continue to use them. (×)
55. Electromagnetic overcurrent relay is instantaneous, and is often used for overcurrent protection or quick-break protection of lines and equipment. (√)
56. The operating time limit of the inductive overcurrent relay is proportional to the square of the current flowing. (×)
57. Gas (gas) protection can reflect various types of faults inside the transformer tank and also reflect some faults outside the tank. (×)
58. The primary current of the current transformer depends on the secondary current, the secondary current is large, and the primary current also becomes large. (×)
59. To replace the melt of the high-voltage fuse, copper or silver wire soldered with tin or shot ball, lead-tin alloy or silver wire, or melt made of lead-tin alloy or zinc. (×)
60. When the voltage transformer is in operation, its oil plug should be tightened. (×)
61. Vacuum circuit breakers are suitable for indoor and outdoor substations and industrial and mining enterprises of 35kV and below, where frequent operation is required and power distribution systems with many faults are particularly suitable for breaking capacitive load current. It has simple operation and maintenance and low noise. (√)
62. The shunt capacitor bank is allowed to operate at 1.1 times the rated voltage for a long time, allowing more than 30% of the rated current of the capacitor to operate for a long time. (√)
63. Inductive overcurrent relays combine the functions of electromagnetic current relays, time relays, signal relays and intermediate relays. It not only provides time-limited overcurrent protection, but also enables current quick-break protection. (√)
64. The general knife switch cannot cut off the fault current, nor can it withstand the electrodynamic and thermal effects caused by the fault current. (×)
65, low-voltage load switch (iron shell switch) can quickly open and close the knife switch, depending on the speed of the manual operating mechanism handle. (×)
66. When using a contactor (such as the CJ12 series) suitable for the long-term working system, it should be used for long-term work, and its capacity should be reduced to less than half of the rated capacity of the intermittent long-term working system. (√)
67. The contact resistance of the black oxide film formed by the breaking arc on the contactor silver and silver-based alloy contact surfaces is large and should be repaired. (×)
68. Motors that are frequently reversed and frequently switched on and off are suitable for thermal relay protection. (×)
69. Plastic shell type low voltage circuit breakers are widely used in switch cabinets for AC and DC distribution lines in industrial enterprises. Frame type low-voltage circuit breakers are often used to protect motors and lighting circuits with small capacity as control switches. (×)
70. In the flammable and explosive places, the lamps should be sealed or explosion-proof. In the dusty, humid and corrosive gas, the lamps should be waterproof and dustproof. (√)
71. For the lighting switch in dusty, humid places or outdoor places, the porcelain waterproof pull switch should be used. (√)
72. The rated capacity of the transformer is the apparent power of the transformer output. (√)
73. The power phase (fire) line can be directly connected to the luminaire, and the switch can control the ground. (×)
74. The safety voltage lighting transformer uses a double-turn transformer, and an autotransformer can also be used. (×)
75. The protective grounding end of the single-phase three-hole power socket (facing the uppermost end of the socket) and the zero-connecting end of the zero-connecting terminal can be connected by wires to share one wire. (×)
76, the power cord is connected to the socket or connected to the plug is the same. (×)
77. The phase (fire) line of the screw base should be connected to the tongue on the center of the lamp socket, and the neutral wire is connected to the threaded port. (√)
78. The rated voltage of the motor refers to the voltage of each phase input to the stator winding instead of the line voltage. (×)
79. The dynamic stability and thermal stability conditions at the start of the motor are reflected in the allowable starting conditions (direct or step-down) of the motor and the number of consecutive starts specified by the manufacturer. (√)
80. When the asynchronous motor adopts Y-Δ step-down starting, the stator windings are first connected by Δ, and then changed to Y-connected operation. (×)
81. The short-term duration specified by the “short-time operation†of the motor does not exceed 10 minutes. (×)
82. The insulation class of the motor, indicating the insulation material of the motor winding and the level at which the wire can withstand the temperature limit. For example, Class E insulation allows a maximum temperature of 120 degrees Celsius. (√)
83. The method of decompression starting of autotransformer is suitable for frequent starting of cage asynchronous motor with capacity below 320kW. (×)
84. The starting method of the wound rotor asynchronous motor is usually started by Y-Δ decompression. (×)
85. Wound rotor asynchronous motor should use frequent varistor starting when running at heavy load and low speed. (×)
86. The characteristic of starting the motor with frequent varistor is that the resistance of the frequent varistor can increase smoothly with the increase of the motor speed. (×)
87. When the rotor of the wound rotor is started by the rotor string resistor, the larger the string resistance, the larger the starting torque. (×)
88. Check the insulation resistance between the stator and rotor winding phases of the low-voltage motor and the winding to ground. When measuring with 500V insulation resistance, the value should not be lower than 0.5M Ω, otherwise it should be dried. (√)
89. For the operation of only a single operation, accident handling operation, pulling the grounding knife and removing the only set of grounding wire, it is not necessary to fill in the operation ticket, but it should be recorded in the operation record book. (√)
90. The operation ticket must also be written to the lock, hang or disassemble the operation handle at the beginning. (×)
91. Operation of electrical equipment must be carried out by two people, with the lower level of personnel as the guardian and the higher level of operation. (×)
92. In the operation of the substation, the items such as connecting or disassembling the ground wire, inspecting electricity, and installing and disassembling the fuse of the voltage transformer circuit may not fill in the operation ticket. (×)
93. The power failure operation of the substation can not be filled in the operation ticket after the “inspection†work after the circuit is cut off. (×)
94. Rescuing the electrocution victims, using the excitable respiratory center of corramin, Lobeline, or the adrenaline and other cardiac acupuncture to replace the manual breathing and extrathoracic cardiac compression. (×)
95. The power supply is from the secondary power distribution station in the plant introduced by the total step-down transformer in the factory. If the transformer capacity is below 500kVA, no special person can be on duty, and only patrol inspection is arranged. (√)
96. After the electrical equipment is powered off, do not touch the equipment or enter the equipment's barrier before disconnecting the power switch and taking safety measures to avoid personal electric shock. (√)
97. Test the electrolytic capacitor with a multimeter R × 1 Ω file. The black meter is connected to the positive pole of the capacitor. The red test lead is connected to the negative pole. The needle is slowly increased. If it stops at 10k Ω, the capacitor is good. (×)
98. The forward voltage drop between the base and emitter of the manifold is greater than the forward voltage drop of the silicon tube. (×)
99. For the butt joint of the thick plate opening, the first layer of welding should use a thicker electrode. (×)
100. For the butt welding of horizontally fixed pipe fittings, it can be welded by one-way clockwise or counterclockwise welding. (×)
Third, the judgment questions 1, DC motors generally use carbon-graphite brushes, only in low-voltage motors, only use brass graphite brushes or copper graphite brushes. (√) #p#分页头#e#
2. On the surface of the commutator, a layer of brown lustrous cuprous oxide film is usually produced. This film increases the contact resistance between the brush and the commutator. It has good lubrication and can be improved. to. (√)
3. A DC motor that is used for a short time and has no insulation. If one or two coils have a short-circuit fault, the short-circuit coil can be cut off during maintenance and the jumper is connected to the two commutator segments connected to it for continued use. (√)
4. For asynchronous motors, the increase of the number of probabilities in the stator will cause difficulty in weaving, waste copper wire, and increase the leakage reactance of the motor, thus reducing the maximum torque and starting torque. (√)
5, the stator winding of the three-phase asynchronous motor, whether it is a single layer or a double layer, its pitch must be a civilian integer. (√)
6. If the primary voltage of the transformer is lower than the rated voltage, its output power must be lower than the rated power regardless of the load, and the temperature rise must be less than the rated temperature rise. (×)
7. A single-phase transformer with a rated voltage of 380 V / 220 V. If used as a step-up transformer, it can be connected to a 380 V power supply on the secondary side and an output voltage of approximately 656 V on the primary side. (×)
8. For electric welding transformers with reactors, if the core air gap of the reactor is reduced, the leakage reactance increases and the welding current increases. (×)
9. The ratio of the ratio of the DC single bridge is chosen such that the number of comparison arm stages multiplied by the ratio level is approximately equal to the number of stages of the measured resistance. (√)
10. Changing the supply voltage value of the DC single-arm bridge will also affect the measurement accuracy of the resistor. (√)
11. When measuring the resistance with a DC double-arm bridge, the lead-out line of the bridge potential connector should be closer to the resistance to be measured than the lead-out line of the current connector. (√)
12, the electromagnetic instrument can measure the DC power, but also the AC power, and the scale when measuring AC is the same as the scale when the DC is measured. (√)
13. The shape of the load volt-ampere characteristic curve is only related to the characteristics of the load itself, regardless of the magnitude of the voltage actually applied to the load. (√)
14. In the RLC series circuit, the effective value of the total voltage will always be greater than the voltage rms value of each component. (×)
15ã€To compare the frequency and phase of the two voltages, only the two-wire system can be used. The single-line sand wave machine is hard to handle. (×)
16. For single-phase bridge rectifier circuits with capacitive filtering, the average value of the output voltage depends on the rms value of the secondary voltage of the transformer regardless of the magnitude of the load. (×)
17. Is the output resistance of the amplifier from the input of the amplifier? The equivalent resistance that goes in, it is a DC resistance. (×)
18. When any rectifier diode in a single-phase bridge rectifier circuit is short-circuited, the value of the output voltage will drop by half, and the circuit will become a half-wave rectifier circuit. (×)
19, the equivalent circuit of a single junction transistor is composed of a diode and two resistors, so the choice of a suitable diode and two resistors correctly connected, can be used to replace the single junction transistor. (×)
20, the amplification of the amplifier is for the amount of change, the amplification factor is the ratio of the amount of change in the output signal. (√)
21, DC amplifier can amplify the AC signal, but the AC amplifier can not amplify the DC signal. (√)
22, the production machinery requires that the ratio of the maximum speed and the minimum speed provided by the motor under no-load conditions is called the speed regulation range. (×)
23. If the disturbance is generated inside the system, it is called internal disturbance. If the disturbance comes from outside the system, it is called an external disturbance. Disturbance has an impact on the output of the system. (√)
24, closed-loop control system uses negative feedback control, in order to improve the mechanical characteristics of the system hardness, expand the speed range. (√)
25, the open loop system can not self-adjust the change of the speed caused by the load change, but it can self-adjust to the external disturbance. (×)
26, the electrical installation diagram of the substation must include: the primary system wiring diagram of the substation, the plan and section of the substation, the secondary system circuit diagram and installation diagram of the substation, and the large sample of non-standard components. Wait. Otherwise it is impossible to construct. (√)
27, when installing the drawer type power distribution cabinet, the mechanical interlock or electrical interlock of the drawer should be correct and the action is reliable. The correct judgment of the action is: the circuit breaker can be disclosed after the isolation contacts are separated. (×)
28. The peak current of a distribution line with multiple motors depends on the starting current of multiple motors. (×)
29, the impedance of the short-circuit current, can be calculated by ohmic value, is also available? ? Value calculation. (√)
30. Complete technical data means that the equipment has at least: a, nameplate and equipment technology two cards; b, calendar year test or inspection records; c, calendar year minor, minor repair and adjustment records; d, calendar accident record, relay protection The secondary equipment must also have drawings that conform to the field equipment. (√)
Fourth, question and answer questions 1, what is the effect of the collection?
Answer: When the alternating current passes through the wire, the current distribution is uneven on the cross section of the wire, and the current density at the center is small. The closer the current density is to the surface, the uneven current distribution is called the skin effect (also called the skin effect). Skin effect).
2. What is eddy current? What are the harmful effects of eddy current generation?
A: When alternating current is passed through the wire, an alternating magnetic field is generated around the wire. Inductive current is generated inside the monolithic conductor in the alternating magnetic field. Since this induced current is self-contained in the entire conductor, it is like a vortex of water, so it is called eddy current.
The eddy current will not only lose energy in vain, but also reduce the efficiency of using electrical equipment, and it will cause heating of electrical appliances (such as transformer core), which will affect the normal operation of the equipment.
3. Please write the formula for the starting torque of the asynchronous motor? And explain the relationship between starting torque and voltage, reactance, and rotor resistance?
A: The formula for starting torque is:
M = ( 1 / ω 1 ) × ( 3U 1 2 r 2 ' 2 ) / [(r 1 + r 2 ') 2 + (x δ 1 + x' δ 2 ) 2 ]
According to the formula, the starting torque M is proportional to the grid voltage U 1 2 . At a given voltage, the larger the reactance, the smaller the starting torque M. Winding asynchronous motor, the rotor circuit is connected to the resistor to increase the starting torque.
4. What is the torque characteristic or mechanical characteristic of the motor? What is a hard feature? What is a soft feature?
Answer: The relationship between the applied voltage of the motor and the change of the speed with the load is called the torque characteristic or the mechanical characteristic. If the load changes, the small change of the rotational speed is called the hard characteristic, and the change of the rotational speed is called the soft characteristic.
5. How to improve the hardness of the mechanical characteristics of the motor?
A: The hardness measure to improve the characteristics is to use feedback adjustment, especially the negative speed feedback or voltage negative feedback and positive current feedback can get harder torque characteristics, so the motor load can change very little from no load to full load speed.
6. Under what circumstances must the winding three-phase asynchronous motor be used? What is the role of the varistor in the rotor circuit?
Answer: In the case of equipment that requires starting torque or heavy load, such as a crane, a wire-wound three-phase asynchronous motor should be used. The function of the access resistor is: (1) reducing the starting current; (2) increasing the starting torque so that the motor can start relatively smoothly with the load.
As shown in the figure: 182 pages of Dagang press the start button QA, the contactor 1C is electrically connected, its normally open auxiliary contact is sucked to achieve self-holding, and the time appliance SJ is powered by the 2C contactor normally closed auxiliary contact. The motor rotor is serially connected to the frequency sensitive varistor to start. When the speed is close to the rated speed, the normally open contact is closed due to the time delay of the time relay SJ, the 2C contactor is turned on, and self-holding is achieved, and the normally open contact will be frequency sensitive. The varistor BP is shorted and the motor is put into normal operation.
7. What is the main difference between synchronous motor and asynchronous motor?
Answer: The main difference between the synchronous motor and the asynchronous motor is that the synchronous motor has a constant speed n constant for the synchronous speed and the load magnitude under the condition that the steady state and the grid frequency f1 are constant. Synchronous motors are classified by operating mode, frequency and structure type as follows:
According to the operation mode and power conversion direction, it is divided into three categories: generator, motor and compensation machine.
According to the structure, it is divided into two types: rotary armature type and rotary magnetic pole type.
In the rotating magnetic pole type, according to the shape of the magnetic pole, it can be divided into two types: a salient pole type and a hidden pole type.
8. What is the role of the AC contactor short circuit?
Answer: The function of the short circuit of the AC contactor is to maintain a certain suction between the moving and cooling cores when the alternating current crosses zero to remove the vibration between the moving and static iron cores.
9. What is feedback? How to classify according to their role?
A: Send some or all of the input power (voltage or current) back to its input and participate in the control output together with the input signal. This effect is called feedback.
According to the role of feedback, it is divided into negative feedback and positive feedback.
10. What problems should I pay attention to when dealing with faulty power capacitors?
Answer: When dealing with faulty capacitors, first open the power supply control switch of the capacitor bank. If fuse protection is used, the fuse tube should be removed. At this time, although the capacitor bank has been discharged by the discharge resistor, there is still some residual charge. Therefore, manual discharge must be performed until sparks and discharges are listed to avoid electric shock.
11, what is thyristor?
Answer: SCR is a new type of high-power rectifying component. The difference between it and the general silicon rectifying component is that the rectified voltage can be controlled. When the AC voltage supplied to the rectifying circuit is constant, the output voltage can be uniformly adjusted.
12. What effect does the air gap of the asynchronous motor have on the operation of the motor?
Answer: The air of the asynchronous motor is an important factor in determining the operation of the motor. If the air gap is too large, the magnetoresistance (the resistance of the air to the magnetic flux is called the magnetoresistance) will increase, so that the magnetostatic magnetic field will increase and the power factor will decrease. The performance of the motor deteriorates. If the air gap is too small, the core loss will increase, and the rotor shovel may touch the stator core during operation, and it is even difficult to start the squirrel cage rotor. Therefore, the air gap of the asynchronous motor must not be too large or too small. Generally, the air gap of small and medium-sized three-phase asynchronous motor is 0.2~1.0 mm, and the air gap of large three-phase asynchronous motor is 1.0~1.5 mm.
13. What is the leakage flux of the stator of an asynchronous motor?
Answer: According to the working principle of the three-phase asynchronous motor, when the stator winding is connected with three-phase alternating current, a rotating magnetic field is generated. In this rotating magnetic field, most of the magnetic flux passes through the air gap through the rotor, and is connected to the stator winding and the rotor winding, which is called the main magnetic flux. However, a very small part of the magnetic flux is only connected to the stator windings, which pass through the air gap into the rotor. This part of the magnetic flux is called the leakage flux of the stator winding.
In addition, the rotor winding cutting the rotating magnetic field also produces induced and induced currents. Similarly, the rotor current will also produce a flux that is only connected to the rotor windings. It is called the rotor leakage flux. The stator leakage flux and the rotor leakage flux are called leakage flux of an asynchronous motor.
14. What is the cause of the large imbalance of the no-load current of the asynchronous motor?
A: There is a large imbalance in the no-load current, which is usually caused by the following reasons:
1. The voltage of the three-phase power supply is not balanced too much;
2. Each phase winding of the motor has several branches connected in parallel, wherein the three-phase impedance is not equal due to the disconnection of one branch;
3. Faults such as one-phase open circuit in the motor winding or short-circuit between the turns in the phase winding, short-circuit of components;
4, the repaired motor, because of the carelessness of a coil or coil set connected;
5. The stator windings are connected in reverse, so that the phase current of the reverse phase is particularly large.
15. How to adjust the secondary voltage of the transformer?
Answer: In order to adjust the secondary voltage of the transformer, the usual method is to extract several taps on the coil on the primary side. By changing the tap, the effective number of turns on the primary side can be changed, thereby changing the turns ratio of the primary and secondary coils. , to achieve the purpose of adjusting the secondary side voltage.
16. What is the number of load transformers? What is its meaning?
Answer: The ratio of the actual load current I2 of the transformer to the rated load current I2N is called the load factor of the transformer. It represents the ratio of the actual load to the rated load. When the load factor is 1, it means that the transformer is fully loaded. If it is less than 1, it is underloaded; if it is greater than 1, it is overloaded.
17. What are the advantages of parallel operation of transformers?
A: The parallel operation of the transformer has the following advantages:
(1) Improve the reliability of power supply. If a transformer fails, it can be removed from the power grid for maintenance, and the power grid can continue to supply power;
(2) The number of participating transformers can be adjusted according to the size of the load to improve the operating efficiency;
(3) New transformers can be installed in batches to increase the reserve capacity.
18. What are the three conditions for obtaining a rotating magnetic field in a single-phase asynchronous motor?
A. To obtain a circular rotating magnetic field in a single-phase asynchronous motor, the following three conditions must be met:
(1) The motor has a two-phase winding with a 90 ° electrical angle difference in spatial position;
(2) a two-phase current with a phase difference of 90 ° is applied to the two-phase winding;
(3) The magnitude of the magnetic potential generated by the two-phase winding is equal.
19. What is the maximum torque and starting torque of an asynchronous motor?
Answer: In the mechanical characteristics of the asynchronous motor, starting from the rotor speed n=0, the electromagnetic torque T increases as n increases. When the critical speed nc is reached, the electromagnetic torque T has a maximum value, and the torque is called the maximum torque. As shown in the figure (motor principle on page 52, Figure 21), Tmax. The electromagnetic torque of the asynchronous motor at the start of power-on is called the starting torque, as shown by Tst in the figure.
20, what is the synchronization step of the synchronous motor?
A: The relationship between the electromagnetic torque and the power angle of a synchronous motor is approximately a sinusoid, as shown in Figure 22 (Figure 22 of the Electrical Engineering Principles). At low loads, the power angle is small; as the load increases, the power angle also increases; when the load torque exceeds the maximum torque, the motor can no longer maintain synchronous speed operation. This phenomenon is called out of step.
21. What is the role of the excitation system of the synchronous motor?
Answer: There are two functions of the synchronous motor excitation system: one is to supply DC power to the excitation winding of the synchronous motor; the other is to automatically adjust the excitation current according to the running condition of the synchronous motor. The former is called the excitation power system; the latter is called the excitation regulation system.
22. What are the main requirements for the excitation system of synchronous motors?
A: The main requirements for synchronous motor excitation systems are as follows:
(1) For the synchronous motor with heavy load starting, when starting the starting, the excitation winding is connected to the closed circuit through a resistor until the speed rises to about 95% of the synchronous speed, the resistor is cut off, and the excitation is input;
(2) For synchronous motors with large power, excitation adjustment should be performed according to a constant power factor;
(3) When the grid voltage is too low or the synchronous motor is overloaded, it should be able to perform strong excitation; #p#åˆ†é¡µæ ‡é¢˜#e#
(4) Automatically de-excitation after shutdown.
23ã€When starting the synchronous motor, why should I wait for the rotor to approach the synchronous speed before putting the excitation?
Answer: When the synchronous motor stator winding is connected to the three-phase power supply, a rotating magnetic field is generated. The rotor is subjected to an alternating traction torque of frequency f1, the average torque is zero, and the motor cannot start. When the external force is used to rotate the rotor in the direction of the rotating magnetic field of the stator, the frequency of the alternating traction torque on the rotor is reduced to f2 = sf1 (s is the slip). When the rotor is close to the synchronous speed, the frequency of the alternating traction torque on the rotor is already low, so that the rotor can be accelerated to the synchronous speed within a torque alternation period. Therefore, when the synchronous motor is started, the excitation is started when the rotational speed is close to the synchronous rotational speed.
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