It can be said without ambiguity: the current electrician team is mixed, and there are not a few electricians who take up their jobs after a little study, and more electricians value experience and practical operation, and sometimes even ignore the consolidation of theoretical knowledge of electrical engineering and I have learned so much that I have forgotten even the basic knowledge of electricians. The road of electricians is not easy to walk. Is it true technology or just messing around? This question is also worth thinking about!
1. What are the advantages and disadvantages of the three operating modes of the neutral point of the power system?
1. The advantages of the neutral point ungrounded system:
When the single-phase grounding occurs in this system, the three-phase electrical equipment can work normally, and it is allowed to temporarily continue to operate within two hours, so the reliability is high;
Disadvantage: When single-phase grounding occurs in this system, the other two intact phase-to-ground voltages rise to the line voltage, which is √3 times that of normal, so the insulation requirements are high and the insulation cost is increased.
2. Advantages of neutral point grounding system via arc suppression coil:
In addition to the advantages of the neutral point ungrounded system, it can also reduce the grounding current;
Its disadvantages: similar to the neutral point ungrounded system.
3. The advantages of the neutral point direct grounding system:
When a single-phase grounding occurs, the other two intact phase-to-ground voltages do not rise, so the insulation cost can be reduced;
Disadvantages: When a single-phase grounding short circuit occurs, the short-circuit current is large, and the faulty part must be quickly removed, resulting in poor power supply reliability.
2. What are the methods for speed regulation of separately excited DC motors? What are the characteristics of various speed regulation methods?
There are three speed regulation methods for separately excited DC motors:
1. Reduce the armature voltage for speed regulation.
2, the armature circuit series resistance speed regulation.
3. Weak magnetic speed regulation.
Features of various speed-adjusting methods:
1. Reduce the armature voltage for speed regulation: the armature circuit must have a voltage-adjustable DC power supply, the resistance of the armature circuit and the excitation circuit is as small as possible, the voltage is reduced and the speed is reduced, the hardness of the artificial characteristics remains unchanged, the running speed is stable, and the stepless operation is possible. speed.
2. Armature circuit string resistance speed regulation: the larger the string resistance, the softer the mechanical properties and the more unstable the rotational speed. At low speed, the string resistance is large, the energy loss is also more, and the efficiency becomes lower. The range of speed regulation is affected by the size of the load, the range of speed regulation is wide when the load is large, and the range of speed regulation is small when the load is light.
3. Weakening speed regulation: In general DC motors, in order to avoid over-saturation of the magnetic circuit, only weak magnetic field can not be strong magnetic, the armature voltage maintains the rated value, the armature circuit series resistance is minimized, the excitation circuit resistance Rf is increased, and the excitation current And the magnetic flux decreases, the motor speed increases immediately, and the mechanical properties become soft.
When the speed increases, if the load torque is still at the rated value, the motor power will exceed the rated power, and the motor is overloaded, which is not allowed, so when the field weakening speed is adjusted, as the motor speed increases, the load torque Correspondingly reduced, it belongs to constant power speed regulation. In order to prevent the rotor winding of the motor from being damaged due to excessive centrifugal force, it should be noted that the motor speed does not exceed the allowable limit during field weakening speed regulation.
3. What is the difference between the shunt-excited DC motor and the series-excited DC motor? What load is each suitable for?
The shunt DC motor has hard mechanical characteristics, the speed changes little with the load, the magnetic flux is a constant value, and the torque changes proportionally with the armature current. Under the same conditions, the starting torque is smaller than that of the series motor, which is suitable for speed requirements. Stable, and loads with no special requirements for starting torque.
The series-excited DC motor has soft mechanical characteristics, the speed varies greatly with the load, the speed is fast when the load is light, and the speed is slow when the load is heavy, the torque is approximately proportional to the square of the armature current, and the starting torque is larger than that of the shunt motor. It is suitable for transporting and dragging machinery that requires a particularly large starting torque but does not require the stability of the rotational speed.
4. What method is usually used to start a wound three-phase asynchronous motor? What are the advantages and disadvantages of each method?
There are usually two ways to start a wound asynchronous motor:
1. Three-phase symmetrical variable resistance starting of rotor circuit series
This method can not only limit the starting current, but also increase the starting torque. If the value of the series resistance is properly obtained, it can also make the starting torque close to the maximum torque for starting, and appropriately increase the power of the series resistance, so that the starting resistance can also be used for speed regulation. Resistors are used for two purposes. They are suitable for loads requiring large starting torque and speed regulation. Disadvantages: The multi-level adjustment control circuit is more complicated, and the resistance consumes a lot of energy.
2. The rotor circuit is connected in series with a frequency sensitive rheostat to start
At the beginning of starting, the rotor circuit frequency is high, the equivalent resistance and inductive reactance of the frequency sensitive varistor increase, limiting the starting current also increases the starting torque, as the speed increases, the frequency of the rotor circuit decreases, and the equivalent impedance also decreases automatically. , After starting, remove the frequency sensitive varistor. Advantages: simple structure, economical and cheap, no need for manual adjustment in the middle of starting, convenient management, and heavy-load starting.
5. Commonly used step-down starting methods for cage-type three-phase asynchronous motors: What is the difference between Y-△ switching starting and autotransformer step-down starting?
1. Y-△ switch start
For the cage-type three-phase asynchronous motor connected to △ in normal operation, change the connection to star shape when starting, so that the armature voltage is reduced to 1/√3 of the rated voltage. When the speed is close to the rated value, change it to △ connection, and the full voltage of the motor is normal. run. The actual starting current and starting torque of Y-△ switch are reduced to 1/3 of those of direct starting, and only light-load starting is possible.
Advantages: The starting equipment is simple in structure, economical and cheap, and should be used first;
Disadvantages: low starting torque, only suitable for normal operation △ connected to the motor.
2. Autotransformer step-down start (also known as compensation start)
When starting, use the autotransformer to reduce the power supply voltage and add it to the stator winding of the motor to reduce the starting current. When the speed is close to the rated value, cut off the autotransformer and run with full voltage. The torque is 2 times that of full pressure starting (W2/W1).
Advantages: It is not limited by the connection method of the motor winding, and can obtain a larger starting torque than the Y-△ switch; there are 2-3 sets of plugs on the secondary side of the autotransformer, which can be selected by the user, suitable for large capacity and requiring starting Motors with high torque.
6. What parameters can be measured to determine the working state of the transistor in the circuit?
The simplest can be determined by measuring the Vce value of the triode:
That is: if Vce ≈ 0, the tube works in a saturated conduction state.
If Vbe ∠ Vce ∠ Ec, it can be considered that the work is in an enlarged state.
If Vce ≈ VEc, the transistor works in the cut-off region. Here (Ec is the supply voltage).
7. What are the commonly used materials for busbars? What are the advantages and disadvantages of each?
Common materials for busbars are aluminum, steel and copper.
The resistivity of aluminum busbar is slightly larger than that of copper, its electrical conductivity is inferior to that of copper, its mechanical strength is smaller than that of copper, and it is easy to corrode and oxidize, but it is cheap and light in weight.
Copper busbars have good electrical conductivity, low resistivity, high mechanical strength, and good anti-corrosion performance, but they are expensive.
The steel busbar has poor electrical conductivity and is easy to corrode, but it is cheap and has high mechanical strength.
8. What is the general selection principle of automatic air switch?
1. The rated voltage of the automatic air switch ≥ the rated voltage of the line.
2. The rated current of the automatic air switch ≥ the load current calculated by the circuit.
3. The setting current of the thermal release = the rated current of the controlled load.
4. The instantaneous trip setting current of the electromagnetic release is ≥ the peak current when the load circuit is working normally.
5. The rated voltage of the undervoltage release of the automatic air switch = the rated voltage of the line.
9. What is your understanding of cos Φ? How does cos Φ affect the power system? What is the reason for the low cos Φ? How to improve the user's cos Φ?
The understanding of cosΦ: in the DC circuit, P=UI; in the AC circuit, P=UIcosΦ, where U and I are the rms value of the voltage and current, so in the AC circuit, the effective power of the load is not only the rms value of the voltage and current It is proportional to cos Φ, and cos Φ is a unitless factor that determines power, so it is called power factor.
cos Φ has the following effects on the power system:
(1) Low cos Φ increases the voltage loss and power loss of the line.
(2) The low cos Φ makes the power generation equipment not fully utilized, that is, the utilization rate is low.
From the influence of the above two aspects, it can be seen that low cos Φ is unfavorable to the national economy, so the power supply department attaches great importance to this parameter.
It is known from the formula ψ =tg -1 that it is determined by the load factor. The capacitive load is the least used load, and even no capacitive load is used. The inductive load is widely used in the industry, and the XL is very large, such as electric motors and electric welding machines. , induction furnaces, transformers, etc. are all inductive loads. Because XL is very large and large, cosΦ is very low.
Therefore, the main reason for the low cosΦ is the large use of inductive loads in the industry. The method to improve the user's power factor is to connect a capacitor in parallel at the user's incoming line or at the user's load.
