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Disadvantages: the transmission ratio can not be guaranteed; the structure is not compact enough; the service life is short; the transmission efficiency is low; it is not suitable for high temperature, flammable and explosive occasions.

Answer: When the three-phase AC power is passed into the three-phase stator winding, a first-turn magnetic field is generated in the stator cavity. The rotor conductor that is stationary before the rotation is equivalent to the magnetic field line of the rotor conductor that relatively cuts the magnetic field under the action of the rotating magnetic field, thereby generating an induced current (electromagnetic induction principle) in the rotor conductor. Since the inner conductors of the rotor are always symmetrically arranged, the electromagnetic forces generated on the conductors are opposite in direction, thereby forming an electromagnetic torque that causes the rotor to move. Since the current in the rotor conductor is generated by the rotating magnetic field of the stator, it is also an induction motor. Since the rotational speed of the rotor is always lower than the rotational speed of the rotating magnetic field of the stator, it is also called an asynchronous motor.

4. In a single-phase bridge rectifier current, what happens if a diode is connected in reverse? What happens if there is a diode inside that has been broken?

A: In a single-phase bridge rectifier current, if a diode is connected in reverse, it will cause a short circuit. If a diode is internally broken, a single-phase half-wave rectification is formed.

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

Answer: Advantages: Infinitely variable speed can be carried out; the movement is relatively stable; the melon is fast, the impact is small, the high-speed start, brake and reversing can be achieved; the overload can be automatically prevented; the operation is simple; the service life is long; the volume is small, the weight is light, and the structure is compact.

Disadvantages: easy to leak, no component manufacturing precision requirements; low transmission efficiency.

6. What problems should I pay attention to when using a low-voltage electroscope?
A: The following points should be noted:
(1) Test on the electrified body with electricity confirmed before testing to prove whether the electroscope is good.
(2) Wear insulated shoes when using.
(3) Tested in bright light, care should be taken to avoid light.
(4) When some equipments are working, the casing will be charged by induction. If the electroscope is used, the test results will not be correct. It is necessary to use a multimeter to measure whether it is actually energized or inductively charged.
(5) For charged bodies with voltages below 36V, the electroscope is often ineffective.
7. What are the main reasons for causing the motor to fix the rotor and the rotor? What are the consequences of stator and rotor rubbing?
Answer: The main causes of motor stator and rotor rubbing are: bearing damage; bearing wear causes the rotor to sink; the shaft is bent and deformed; the base and end cap are cracked; the end cap is not tight; the motor is dirty inside. Fixed and rotor phase rubbing will cause strong vibration and noise of the motor, causing high temperature on the surface of the phase rubbing, and even smoke and fire, causing the insulating charring to become brittle and burn the winding.
8. Briefly describe several electric braking principles for three-phase asynchronous motors?
Answer: (1) Reverse brake: The characteristic is that the motor runs in the electric state, but the electromagnetic torque is opposite to the motor rotation direction.
(2) Regenerative power generation braking (feedback braking) When the motor speed n>n1, the direction in which the rotor bar is cut by the rotating magnetic field is exactly opposite to that in the motoring state (n<n1), and thus the direction of the induced electromotive force in the rotor conductor When changing, the rotor current direction also becomes opposite to that in the motoring state, and the torque direction generated by the motor at this time is opposite to that in the motoring state.
9. What are the causes of nonlinear distortion in the AC voltage amplifying circuit? What effect does a static value have on distortion?
Answer: 1 If the static value is not properly obtained, it will cause nonlinear distortion. In addition, temperature changes and other annoying signals (such as power supply voltage fluctuation, load change, external electromagnetic phenomenon) will cause nonlinear distortion;

2 If the static value is not suitable, after the AC signal is added, the transistor may enter a saturated state or an off state, that is, enter the nonlinear region of the transistor, which may cause nonlinear distortion.

10. The rated voltage of a three-phase asynchronous motor is 380V. When the voltage of the three-phase power line is 380V, what kind of connection should the stator winding use? When the three-phase power line voltage is 660V, which connection method should I use?
Answer: When the three-phase power line voltage is 380V, the stator windings are connected by delta; when the three-phase power line voltage is 660V, the stator windings should be connected by star.
11. What are the common methods for DC motor to reduce the speed?
Answer: Take the shunt motor as an example, there are three types.
1 Lowering the terminal voltage: This method is rarely used because of the drawback that the terminal voltage is lowered and the field current is lowered.
2 increase the excitation current: that is, increase the magnetic field, but this method is limited by magnetic saturation, which has certain limitations.
3 Parallel resistors in the armature circuit to reduce the armature terminal voltage. This method is easy to implement, so it is a commonly used method.
12. What are the main reasons for the motor and rotor rubbing? What are the consequences of stator and rotor rubbing?
Answer: The main causes of motor stator and rotor rubbing are: bearing damage; bearing wear causes the rotor to sink; the shaft is bent and deformed; the end cap of the base is cracked; the end cap is not tight; the motor is dirty inside. Fixed and rotor phase rubbing will cause strong vibration and noise of the motor, causing high temperature on the surface of the phase rubbing, and even smoke and fire, causing the insulating charring to become brittle and burn the winding.

13. Why do you use closed-circuit shunt circuits in the DC current measurement circuit of the multimeter?
Answer: Because the contact resistance of the transfer switch is independent of the resistance of the shunt resistor in the closed shunt circuit, even if the contact is poor or the load is converted, the meter will not have any adverse effect on the meter, which can ensure the accuracy of the meter and the meter. Safety, if an open-circuit shunt circuit is used, the contact resistance will be shunted into the shunt resistor to increase the error, and when the contact is not good or the load is converted, the large current will directly pass through the meter and burn out the meter.

14. What is the difference between magnetic field strength and magnetic induction?
Answer: The magnetic field strength 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 are related to the current and distribution of the excitation magnetic field. However, H is independent of the magnetic field medium and B is related to the magnetic field medium. The unit of H is A/m (by /m), and the unit of B is T (Tesla). Use H more in the calculation of the magnetic field and B in the qualitative description of the magnetic field.
15. What problems should the DC torque motor pay special attention to during use and maintenance?

Answer DC arm motor should not exceed the peak current during operation to avoid demagnetization and torque drop. When the rotor is taken out, the stator must be magnetically short-circuited with environmentally-friendly magnetic materials, otherwise the magnetic steel may be demagnetized.

17. What are the methods for adjusting the average DC voltage output of the chopper?
Answer: The output voltage of the chopper is a rectangular pulse wave voltage. There are three ways to adjust the average value of the output voltage: keep the pulse period T unchanged and change the pulse width τ; keep τ unchanged and change T; change τ and T at the same time

18. Why is the AC contactor overheated when it is frequently operated?
Answer: When the AC contactor is started, due to the large gap between the iron core and the armature, the reactance is small, and the excitation current that can pass through the coil is very large, often more than ten times the working current. If it is frequently started, the excitation coil passes through a large The starting current causes the coil to overheat and, in severe cases, burns the coil.
19. How to properly disassemble the asynchronous motor?
A: All preparations should be done before dismantling the asynchronous motor, such as the tools used, the inspection work before the disassembly and the recording work.
Steps to remove the motor:
1 Remove the belt or coupling. Mark the belt and coupling before removing the belt. Before installation, remove the rust before cleaning.
2 Remove the end cap: first remove the bearing cap, then take the end cap, and mark the front and rear covers. The installation should be reset according to the mark;
3 Disassemble the rotor: Wear thick and thick paper between the stator and rotor to prevent damage to the fixed value winding. If the winding is very heavy, use the lifting equipment to check whether there is any impurity inside the stator when installing the rotor, and then first extend the shaft end. Cover and install the rotor together with the fan and the back cover.

21. What are the characteristics of the screw drive?
Answer: The rotary motion can be changed into linear motion, and the structure is simple, the transmission is stable, the noise is small, a large reduction ratio can be obtained, a large thrust is generated, and self-locking can be realized. In the case of disadvantages, the transmission efficiency is low.
22. What should I pay attention to in the operation and maintenance of asynchronous motors?
Answer: 1 Keep the motor clean; 2 Check the change of the power supply voltage and current with the meter. Generally, the allowable voltage fluctuation of the motor is set to ±5% of the voltage. The difference between the three-phase voltage must not exceed 5%. More than 10%, and should pay attention to whether the phase is running. 3 Regularly check the temperature rise of the motor, commonly used thermometer to measure the temperature rise, should pay attention to the temperature rise must not exceed the maximum allowable value; 4 monitor bearing with abnormal noise, seal should be good, and regular replacement of lubricating oil, general sliding bearing oil change cycle is 1000h, rolling bearing 500h; 5 pay attention to motor sound, smell, vibration and transmission. During normal operation, the motor should be sound even, with no noise or special squeaking.

23. How does the rotor of a three-phase asynchronous motor rotate?
Answer: A symmetrical three-phase sinusoidal alternating current is passed into a symmetrical three-phase stator winding to form a rotating magnetic field. The rotating magnetic field cuts the rotor conductor to generate an induced electromotive force and an induced current. The induced current is subjected to a rotating magnetic field to form an electromagnetic torque, and the rotor is gradually rotated along the turning of the rotating magnetic field.
24. What is short circuit and short circuit fault? How to prevent the danger of short circuit fault?
Answer: Short circuit refers to the working state that a certain point in the circuit is directly connected by a conductor whose negligible value is negligible. Short circuits can occur at either end of the load or anywhere on the line, or they can occur inside the power supply or load. If a short circuit occurs at both ends of the power supply, there is only a small internal resistance of the power supply in the circuit, which will cause a large short-circuit current, causing damage to the circuit. Therefore, short circuit of the power supply is a serious fault and should be avoided as much as possible. However, the "partial short circuit (short circuit)" used in the circuit for a specific purpose cannot be said to be a malfunction. In order to prevent the danger of short-circuit faults from expanding, fuses or automatic circuit breakers are usually inserted in the circuit for protection.
25. What are the basic links of the trigger circuit of the thyristor controllable rectifier circuit? What are the basic requirements?
Answer: The trigger circuit of the thyristor must have three basic steps: synchronous voltage formation, phase shifting and trigger pulse formation and output. Thyristors have requirements for trigger circuits:
(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 must meet certain requirements;
(4) has a certain anti-interference ability;
(5) The trigger signal should have sufficient voltage and power.
26. Why does the suction force occur when the electromagnet is energized?
Answer: After the excitation coil of the electromagnet is energized, a magnetic field is generated around the coil. When an iron core made of a magnet material is placed in the coil, the magnet is magnetized to generate magnetism. In the case of an electromagnet, the magnetic flux generated after energization of the exciting coil forms a closed magnetic path through the iron core and the armature, so that the armature is also magnetized, and an opposite magnetic pole different from the iron core is generated, thereby generating suction.
27. Why can't the electromagnetic measuring mechanism be used directly for AC measurement?
Answer: The direction of the magnetic field generated by the permanent magnet in the magnetoelectric measuring mechanism should be constant. If the alternating current is applied to the coil, the direction of the rotating torque will change due to the continuous change of the current direction. The movable part has inertia, so that the pointer is almost motionless or slightly shaken in the original position, and the correct reading is not obtained, so the electromagnetic measuring mechanism cannot be directly used for the measurement of the alternating current.
28. Briefly describe the working principle and speed control method of three-phase AC commutator asynchronous motor?
Answer: The primary winding of the rotor introduces a three-phase power supply to generate a rotating magnetic field, generates an induced electromotive force in the secondary winding of the regulating winding and the stator, generates a current in the secondary circuit, forms an electromagnetic torque, and the rotor rotates. By changing the opening angle θ between the in-phase brushes, the adjustment electromotive force can be changed to change the secondary circuit current and current and the electromagnetic torque, thereby changing the motor speed.
29. What is the mechanical characteristics of a DC motor? What is the artificial mechanical characteristic curve of the series-excited DC motor?
Answer: When the power supply voltage, excitation current and total armature circuit resistance of the DC motor are equal to constant, the relationship between the speed and the torque is called the mechanical characteristic.
If a resistor is connected in the armature circuit of the series-excited motor, the speed characteristic curve is changed, and they are called artificial characteristic curves.

0. Why does the three-phase cage asynchronous motor start directly? What is the adverse effect of excessive starting current?
Answer: When the three-phase asynchronous motor is directly started, the rotor speed is zero, the slip is the largest, and the induced current in the rotor winding is the largest, so that a large starting current is generated in the stator winding. Excessive starting current will cause grid voltage fluctuations, affecting the normal operation of other electrical equipment, while the motor's own windings are severely heated, accelerating insulation aging and shortening service life.
31. What is eddy current? What are the pros and cons in production?
A: The inside of the conductor in the alternating magnetic field will induce a closed annular current in the section perpendicular to the direction of the magnetic field lines, called eddy current. The eddy current principle can be used to make induction furnace to smelt metal, and eddy current can be used to make magnetoelectric and inductive electric instrument. The damper in electric energy meter is also made by eddy current principle; in electric motor, transformer and other equipment, due to eddy current Existence, additional consumption will occur, and at the same time, the magnetic field will be weakened, resulting in a decrease in the efficiency of the electrical equipment, so that the capacity of the equipment cannot be fully utilized.

32. How to change the steering of three-phase asynchronous motor?
A: The working principle of asynchronous motor can be known:
1 The direction of installation of the asynchronous motor is consistent with the direction of rotation of the rotating magnetic field;
2 while the direction of rotation of the rotating magnetic field depends on the phase sequence of the current in the stator windings;

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