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Индиядагы электр кыймылдаткычтарын өндүрүүчүлөрдүн 3 фазалуу индукциялык мотор тизмеси

Индиядагы электр кыймылдаткычтарын өндүрүүчүлөрдүн 3 фазалуу индукциялык мотор тизмеси

As far as the induction motor itself is concerned, direct starting is allowed, i.e. starting with rated voltage.

Because the capacity of the motor does not match the capacity of the power supply it is connected to, the induction motor may not start up due to too low line terminal voltage drop and insufficient starting torque. In order to solve this problem and reduce the impact on other electrical equipment with the same bus, some motors with large capacity must adopt starting equipment to limit the starting current and its impact.

Whether starting equipment is required or not depends on the comparison of power supply capacity and motor capacity. The larger the power plant or power grid capacity, the larger the motor capacity allowed to be started directly. Therefore, in the newly-built medium and large power plants, almost all induction motors except the wound type are started directly. Only in the old and small power plants, motors started by various starting equipment can be seen.

For squirrel cage motors, the purpose of using starting equipment is to reduce the starting voltage, so as to reduce the starting current. According to different depressurization methods, the starting method is (1) y/ △ conversion starting method. During normal operation, the motor whose stator winding is connected into a delta shape is connected into a Y shape at startup, and then changed into a delta connection after startup. (2) Start with autotransformer. (3) Start with reactor.

5. the three-phase winding of the motor is connected reversely from end to end. What happens when starting? How do I find it?

Answer: the three-phase winding and one-phase winding of the motor are connected reversely, so when starting:

(1) Difficult to start.

(2) One phase current is large.

(3) Vibration may occur and cause a loud sound.

The general search method is:

(1) Carefully check the three-phase winding head and tail marks.

(2) Check the polarity sequence of three-phase winding. If n and s are not staggered, it means that one phase winding is connected reversely.

6. why can't one phase of stator winding of induction motor start up when it is disconnected?

Answer: for three-phase star connected stator winding, when one phase is disconnected, the motor will be at the line voltage with only two phase lines connected to the power supply, forming a series circuit and becoming a single-phase operation.

During single-phase operation, there will be the following phenomena: the original stopped electric motor can not start, and "does not" make a sound. Maybe it can rotate slowly by manually pulling the rotor shaft. The rotating motor turns slower, the current increases, and the motor heats up or even burns out.

Индиядагы электр кыймылдаткычтарын өндүрүүчүлөрдүн 3 фазалуу индукциялык мотор тизмеси

1. how is the temperature resistance of insulating materials divided?

A: China is now divided into six levels, namely a, e, B, F, h and C.

(1) The maximum allowable working temperature of class a insulating material is 105 ℃

(2) The maximum allowable working temperature of class E insulating material is 120 ℃

(3) The maximum allowable working temperature of class B insulating material is 130 ℃

(4) The maximum allowable working temperature of class F insulating material is 155 ℃

(5) The maximum allowable working temperature of class H insulating material is 180 ℃

(6) The maximum allowable working temperature of class C insulating material is above 180 ℃.

2. briefly describe the structure and working principle of induction motor.

Answer: the working principle of the induction motor is as follows: when the three-phase stator winding passes through the three-phase symmetrical AC current, a rotating magnetic field is generated. The rotating magnetic field rotates in the stator bore. Its magnetic line of force cuts the wire on the rotor and induces current in the rotor wire. Because the interaction between the stator magnetic field and the rotor current produces electromagnetic torque, the stator rotating magnetic field pulls the rotor with current carrying wires to rotate.

3. why is the current high when the induction motor is started? And the current will decrease after startup?

Answer: when the induction motor is in the stopped state, from an electromagnetic point of view, it is like a transformer. The stator winding connected to the power supply is equivalent to the primary coil of the transformer, and the closed rotor winding is equivalent to the secondary coil of the transformer which is short circuited; There is no electrical connection between stator winding and rotor winding, only magnetic connection. The magnetic flux is closed through stator, air gap and rotor core. At the moment of closing, the rotor has not turned up due to inertia, and the rotating magnetic field cuts the rotor winding at the maximum cutting speed - synchronous speed, so that the rotor winding induces the highest possible potential. Therefore, a large current flows through the rotor conductor, which generates magnetic energy to offset the stator magnetic field, just as the secondary magnetic flux of the transformer offsets the primary magnetic flux.

In order to maintain the original magnetic flux corresponding to the current supply voltage, the stator automatically increases the current. At this time, the rotor current is very large, so the stator current also increases greatly, even up to 4~7 times the rated current, which is the reason for the large starting current.

Why it is small after startup: as the motor speed increases, the speed at which the stator magnetic field cuts the rotor conductor decreases, the induced electromotive force in the rotor conductor decreases, and the current in the rotor conductor also decreases, so the part of the stator current used to counteract the influence of the magnetic flux generated by the rotor current also decreases, so the stator current increases from large to small until it is normal.

4. is there any danger of large starting current? Why do some induction motors need starting equipment?

Answer: Generally speaking, because the starting process is not long, large current flows in a short time, and the heating is not too severe, the motor can withstand it. However, if the normal starting conditions are damaged, for example, the motor starting with light load is required to start with heavy load, and the speed cannot be increased normally, or when the voltage is low, the motor fails to reach the rated speed for a long time, and the motor is started repeatedly, the motor winding may overheat and burn out.

Индиядагы электр кыймылдаткычтарын өндүрүүчүлөрдүн 3 фазалуу индукциялык мотор тизмеси

Large starting current of the motor will affect other electric equipment on the same power bus. This is because of the large starting current supplied to the motor and the great voltage drop of the power supply line, which greatly reduces the voltage of the bus connected to the motor and affects the normal operation of other electrical equipment, such as the electric light is not on, other motors cannot be started, and the electromagnet is automatically released.

7. what is the abnormal phenomenon of rotor bar breaking during the operation of squirrel cage induction motor?

Answer: when the rotor bar of squirrel cage induction motor is broken during operation, the motor speed will slow down, the stator current fluctuates periodically, and the body vibrates, which may produce a rhythmic "buzzing" sound.

8. what are the abnormal phenomena of single-phase grounding during the operation of stator winding of induction motor?

Answer: for 380V low-voltage motor, when it is connected to the neutral point grounding system, when single-phase grounding occurs, the current of the grounding phase increases significantly, the motor vibrates and makes abnormal noise, and the motor heats up, which may fuse the fuse of the phase at the beginning, or damage the winding group due to overheating.

9. what is the effect of frequency variation on the operation of induction motor?

Answer: when the frequency deviation exceeds ± 1% of the rated current, the operation of the motor will deteriorate, affecting the normal operation of the motor.

When the operating voltage of the motor is constant, the magnetic flux is inversely proportional to the frequency, so the change of frequency will affect the magnetic flux of the motor.

The starting torque of the motor is inversely proportional to the cube of frequency, the maximum torque is inversely proportional to the square of frequency, and the maximum torque is inversely proportional to the square of frequency. Therefore, the change of frequency also has an impact on the motor torque.

The change of frequency will also affect the speed and output of the motor.

When the frequency increases, the stator current usually increases. When the voltage decreases, the frequency decreases and the reactive power absorbed by the motor decreases.

Due to the change of frequency, it will also affect the normal operation of the motor and make it hot.

10. under what conditions will the induction motor be over-voltage?

Answer: the operating induction motor is prone to operating overvoltage of inductive load at the moment of switching off. In some cases, it can also generate operating overvoltage when closing. If the rotor of a wound motor with a voltage of more than 3000 V is open circuit, the magnetic flux will change suddenly at the moment of closing when starting, which will also produce overvoltage.   

 

11. what is the effect of voltage variation on the operation of induction motor?

Answer: the following describes the impact on motor operation when the voltage deviates from the rated value. For simplicity, when discussing voltage changes, it is assumed that the frequency of the power supply and the load torque of the motor are constant.

(1) Effect on magnetic flux

The magnitude of the magnetic flux in the motor core depends on the magnitude of the electric potential. On the premise of neglecting the pressure drop of stator winding leakage resistance, the potential is equal to the voltage of the motor. As the electric potential changes in direct proportion to the magnetic flux, when the voltage increases, the magnetic flux increases in direct proportion; As the voltage decreases, the magnetic flux decreases proportionally.

 

Индиядагы электр кыймылдаткычтарын өндүрүүчүлөрдүн 3 фазалуу индукциялык мотор тизмеси

(2) Effect on moment

Whether it is the starting torque, the operating torque or the maximum torque, it is proportional to the square of the voltage. The lower the voltage, the smaller the torque. As the voltage decreases, the starting torque decreases, which will increase the starting time. For example, when the voltage decreases by 20%, the starting time will increase by 3.75 times. It should be noted that when the voltage drops to a certain value, the maximum torque of the motor is less than the resistance torque, so the motor will stop. In some cases (such as when the load is a water pump and there is water pressure), the motor will reverse.

(3) Effect on speed

The change of voltage has little effect on the speed. But the general trend is that the voltage decreases and the speed also decreases, because the voltage decreases and the electromagnetic torque decreases. For example, for a motor with a rated slip of 2% and a maximum torque of twice the rated torque, when the voltage is reduced by 20%, the speed is reduced by only 1.6%.

(4) Influence on output

Output is the shaft output power. Its relationship with voltage is similar to the relationship between speed and voltage. The voltage change has little effect on the output, but the output also decreases with the decrease of voltage.

(5) Influence on stator current

Stator current is the vector sum of no-load current and load current. The load current actually corresponds to the rotor current. The change trend of load current is opposite to that of voltage, that is, when the voltage increases, the load current decreases, the voltage decreases, and the load current increases. The change trend of no-load current (or excitation current) is the same as that of voltage, that is, when the voltage increases, the no-load current also increases, because the no-load current increases with the increase of magnetic flux.

When the voltage decreases, the electromagnetic torque decreases, the slip increases, the rotor current and the load current in the stator increase, and the no-load current decreases. Usually, the former is dominant, so when the voltage decreases, the stator current usually increases.

When the voltage increases, the electromagnetic torque increases, the slip decreases, the load current decreases, and the no-load current increases. But there are two cases here: when the voltage deviates from the rated value little and the magnetic flux does not increase much, the iron core is not saturated, and the increase of no-load current is proportional to the voltage. At this time, the decrease of load current is dominant and the stator current is reduced; When the voltage deviates from the rated value greatly and the magnetic flux increases a lot, the no-load current rises rapidly due to the saturation of the iron core, so that its increase takes advantage. At this time, the stator current increases. Therefore, when the voltage increases, the stator current begins to decrease slightly and then increases. At this time, the power factor becomes worse.

 

Индиядагы электр кыймылдаткычтарын өндүрүүчүлөрдүн 3 фазалуу индукциялык мотор тизмеси

(6) Influence on absorbed reactive power

The reactive power absorbed by the motor is the leakage reactive power and the magnetization reactive power. The former establishes the leakage magnetic field, and the latter establishes the main magnetic field for electromagnetic energy conversion between stator and rotor.

The leakage reactive power varies inversely with the square of the voltage, while the magnetization power varies proportionally with the square of the voltage. However, due to the influence of iron core saturation, the magnetization power may not change in proportion to the square of the voltage. Therefore, when the voltage decreases, the total reactive power absorbed from the system does not change much and may decrease.

(7) Impact on efficiency

If the voltage is reduced, the mechanical loss is virtually unchanged, and the iron loss is almost proportional to the square of the voltage; Loss of the rotor winding increases in direct proportion to square of the rotor current; The loss of stator winding depends on the increase or decrease of stator current, and the stator current depends on the relationship between load current and no-load current. In general, the efficiency of the motor increases a little when the load is small (≤ 40%), and then starts to decrease rapidly.

(8) Effect on fever

When the voltage variation range is small, the stator current increases due to the decrease of voltage; As the voltage increases, the stator current decreases. Within a certain range, iron loss and copper loss can compensate each other, and the temperature is kept within the allowable range. Therefore, when the voltage changes within ± 5% of the rated value, the capacity of the motor can still remain unchanged. However, when the voltage drops by more than 5% of the rated value, the output of the motor shall be limited, otherwise the stator winding may overheat, because the stator current may have risen to a higher value at this time. When the voltage rises by more than 10%, the stator winding temperature will exceed the allowable value due to the increase of magnetic flux density, iron loss and stator current.

 Редукторлор жана электр кыймылдаткычтары өндүрүүчүсү

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