The problem of step down start up of a squirrel cage 3 phase asynchronous motor

Here's how I understand it.

Q1The stator phase current of the motor is synthesized by two components: the excitation current and the rotor current (this relationship can be applied to the transformer, the stator current is the current on the power side of the transformer, and the rotor current is the current on the load side of the transformer).

At the time of start-up, due to the low equivalent impedance of the rotor (similar to the short circuit on the load side of the transformer), the rotor current increases, and the stator current increases, so "in order to limit the starting current" is indeed to limit the "current" in the rotor winding or copper strip, but at the same time, it is also to limit the stator phase current of the motor, the two cannot exist independently, but in the application of estimation, detection and discussion, the stator phase current is simpler than the rotor current, so the limitation of the starting current is based on the stator current. Then the voltage drop caused by the large current generated by the rotor is related to the entire power grid, because the stator current comes directly from the power grid, the stator current becomes larger, the current on the load side of the power grid transformer increases, and the impedance of the transformer itself reduces the voltage on the load side, affecting the operation of other equipment.

Q2It is certainly not correct to say that the electromagnetic torque is proportional to the square of the stator winding voltage, for example, when the constant torque load is running stably, the frequency does not change, and the u decreases, but the load torque does not change, so that the speed decreases, the slip rate rises, and the electromagnetic torque does not change. It can be seen that to discuss the electromagnetic torque is proportional to the square of the stator winding voltage, firstly, the motor parameters cannot be changed, secondly, the frequency of the power supply does not change, and the slip rate does not change again, the first two conditions are easy to meet, and the slip can be approximated constant in three cases:

No load, maximum torque, starting torque.

When there is no load, the slip rate is approximately 0, and the electromagnetic torque is approximately 0; When discussing the maximum torque, it is possible to derive a constant slip rate; When finding the starting torque, the slip rate is 1. In all three cases, the electromagnetic torque can be considered proportional to the square of the voltage, but the first one is meaningless. The formula is not given, as long as the relationship is known in practical application.

Q3The reason why it is necessary to check the starting current, in fact, it has been discussed in question 1, that is, the starting current is required to be less than the requirements of the power grid, no matter how the starting current is adjusted, the starting torque must change, and the starting torque is generally required to be greater than the double load torque, so it needs to be checked at the same time.

Q4The number of poles refers to the sum of the number of poles of the rotating magnetic field n,s in the air gap. The number of stator and rotor windings or the number of copper strips is determined by the number of magnetic poles, the number of slots, the winding method, etc.

Q5Line voltage = root 3 phase voltage, which means that the effective value of the line voltage is 3 times the root value of the phase voltage. Symmetrical three-phase is necessary to derive the formula.

Phase voltage refers to the voltage of each phase uan, ubn, ucn. Line voltage refers to the voltage difference between each phase, e.g. UAB = UAB-UBN.

It is easy to conclude from the vector method that the UAB vector UAB amplitude is 3 times that of UAN, UBN because the amplitudes of UAN and UBN are equal and the difference is 120 degrees. UBC, UAC are similar.

The "in order to limit the starting current" in Q1 is to limit the winding starting current, because the direct starting current of the motor is 4-7 times of the rated current. Direct start-up will affect the operation of other electrical appliances due to the large input power, and the output voltage of the transformer will also fluctuate greatly. It even affects the security of the entire power grid.

Therefore, buck start-up should be used.

Excuse me2 Electromagnetic torque is proportional to the square of the voltage of the stator winding How is this derived, what is the formula? This is not very clear, so it is not easy to talk nonsense.

Q3 When choosing the starting method of phase asynchronous motor, the starting current should be checked at the same time, why should the starting current be checked? It is necessary to check whether the power of the power supply can meet the input power needs of the motor. The starting current has been mentioned above.

The number of poles is the number of pole pairs generated by the winding when the power supply current enters the motor, that is, the number of S poles and N poles. An S pole and an N pole are a pair of magnetic poles.

Synchronous speed 3000 rpm. Two S poles, two N poles, two pairs of magnetic poles. The same speed is 1500 rpm.

When the stator windings are connected to a star, the line voltage = the phase voltage 3 times the root number. This is an axiom that does not need to be discussed.

The problem smacks of a "dead end". 1. Theory Book knowledge is enough to be thorough. Second, the actual operation according to the requirements of the power distribution materials can be (generally more than 10kw should use the star triangle, step-down start).

If you have an egg, you must blame the chicken who laid it, and most people can't answer it, but they can suggest how to eat it. Therefore, it is recommended that you put more effort into how to equip the right electrical materials according to the requirements.

1. Of course, the stator (winding) current is limited, because only the current of the stator winding will have an impact on the power grid.

2. The electromagnetic torque is proportional to the square of the terminal voltage, which is well understood. According to the laws of electromagnetism and the law of conservation of energy, the input power is U or U2 R, and the output is torque. That's the way it is.

3. To check the starting current, one should consider the impact and impact of the starting current on the power grid, and the other should consider the protection of the motor. The focus here should be on the impact on the power grid.

I've seen some things before, and they're not comprehensive:

1. Limiting current is the current of the winding, the starting current of the motor is often several times of the normal working current, the current increases, the power increases, and the power increases, under the premise of "the power supply capacity is not large enough", the instantaneous power of the motor may exceed the power supply capacity, so the entire power grid will be affected.

2 I don't know how to calculate this.

3. Check the starting current - because the starting current of the motor has a certain range, it will not work if it is low, and it will not work if it is high, so it will be supplemented; In the asynchronous motor, the magnetic pole controls the speed of the motor, such as 380V 50Hz, the two poles are 3000 revolutions per minute, and the four poles are 1500 revolutions per minute (it will vary with the frequency of the power supply).

I don't know about the rest either.

The step-down starting mode of the asynchronous motor is as follows:

The commonly used methods for step-down start-up of three-phase cage asynchronous electric pulse liquid-pneumatic motor are resistance step-down start-up, auto-transformer step-down start-up, Y-start, and step-down start. Ji is blind.

1.Three-phase asynchronous motor.

The rotational speed of the buried finger is lower than the rotational speed of the rotating magnetic field, and there is a relative motion between the rotor winding and the magnetic field.

and an electromotive force is generated.

and electric current, and interact with the magnetic field to generate electromagnetic torque to achieve energy conversion.

2.Compared with single-phase asynchronous motors, three-phase asynchronous motors have good running performance and can save various materials. According to the different rotor structures, three-phase asynchronous motors can be divided into cage type and winding type.

The asynchronous motor of the cage rotor has a simple structure, reliable operation, light weight and low cost, and has been widely used, but its main disadvantage is that it is difficult to regulate the speed.

The rotor of the wound three-phase asynchronous motor, like the stator, is also provided with three-phase windings and passes through slip rings and brushes.

Connection with an external rheostat. Adjusting the rheostat resistance can improve the starting performance of the motor and adjust the rotational speed of the motor.

The working principle of a three-phase asynchronous squirrel cage motor is:

When the three-phase stator windings of the motor (each with a difference of 120 degrees of electrical angle), after the three-phase symmetrical alternating current is introduced, a rotating magnetic field will be generated, which cuts the rotor winding, thereby generating an induced current in the rotor winding.

The current-carrying rotor conductor will generate electromagnetic force under the action of the stator rotating magnetic field, thereby forming an electromagnetic torque on the motor shaft, driving the electric car to rotate, and the motor rotation direction is the same as the direction of the rotating magnetic field.

1. Star wiring, which helps to reduce the voltage of the winding (220V) and reduce the insulation level. The starting current is reduced, and most of the low-power motors below 4 kW are connected by the star connection method.

The star connection method is a wiring method between the three-phase AC power supply and the three-phase electrical appliances. Connect the ends of the three windings of the three-phase power supply, X, Y, and the finger nucleus Z together to become a common point O, and lead out three end wires from the beginning A, B, and C. It is a system in which three sinusoidal power supplies with the same frequency, equal amplitude, and 120° phase difference are connected in a certain way to supply power to the outside.

2. Triangle wiring, the motor of the triangle connection method is started by Y- conversion method when starting at light load to reduce the starting current, and the triangle connection method is used for those greater than 4kw.

The triangle connection method is to connect each phase power supply or load end to end in turn, and lead each connected point out as the three phase wires of three-phase electricity. The wiring is shaped like a triangle.

Summary. The general circuit faults and troubleshooting steps of the three-phase squirrel cage asynchronous motor Y- step-down starting control circuit are as follows: 1. The reasons for not being able to start are as follows:

1. The power supply is not connected 2, the stator winding is faulty 3, the load is too large or the transmission machinery is stuck 4, the wiring error of the control equipment The elimination method for the above 4 reasons that cannot be started is as follows: 1. Check the switch, fuse, each pair of contacts and the lead wire, and find out the fault. 2. Professionally check whether there is a winding open circuit, short circuit, grounding, etc., and find out the repair.

3. Choose a higher power motor or reduce the load, if the transmission machinery is stuck, check the machinery and eliminate obstacles. 4. Correct the wiring. There are two reasons why the speed of the two-phase asynchronous motor is lower than the rated value when the three-phase asynchronous motor is running with load

1. The power supply voltage is too low 2, and the load is too largeThe above two reasons for the above two reasons that cause the motor to run with load is lower than the rated value are as follows: 1. Use a voltmeter and a multimeter to check the power supply voltage at the input end of the motor. 2. Choose a higher power motor to reduce the load.

Three-phase squirrel cage asynchronous motor Y- step-down starting control circuit general circuit fault and troubleshooting steps.

Three-phase squirrel cage asynchronous motor Y- step-down starting control circuit front mold general circuit fault and fault rental repair steps are as follows: 1. The reasons for not being able to start are as follows: 1. The power supply is not open 2, the stator winding is faulty 3, the load is too large or the transmission machinery is stuck 4, and the wiring error of the control equipment is as follows

1. Check the switch, fuse, each pair of contacts and lead-out wires to find out the fault. 2. Professionally check whether there is a winding open circuit, short circuit, grounding, etc., and find out the repair. 3. Choose a higher power motor or reduce the load, if the transmission machinery is stuck, check the machinery and eliminate obstacles.

4. Correction wiring is regretful. Second, the speed of the three-phase asynchronous motor running with load is lower than the rated value, resulting in the motor running with load speed is lower than the rated value, there are two reasons: 1, the power supply voltage is too low 2, the load is too large, and the above two reasons for the motor running with load are lower than the rated value

1. Use a voltmeter and a multimeter to check the power supply voltage at the input end of the motor. 2. Choose a higher power motor to reduce the load.

Common causes and countermeasures of the motor itself: 1There is a mistake in the winding connection, and the star is mistakenly connected into a triangle or vice versa.

2.The short circuit or grounding between turns or phases of the stator winding increases the current and copper loss. If the fault is not serious, only the insulation needs to be repacked, and the winding should be replaced if the grinding spine bush is serious.

3.The stator one-phase winding is broken, or a branch of the parallel winding is broken, causing the three-phase current to be unbalanced and the winding is overheated. 4.

Rotor breaks. The copper rotor should be welded or replaced, and the cast aluminum rotor should be replaced. 5.

The stator and rotor rub each other. It can check whether the bearing is loose, and whether the stator and rotor are poorly assembled. 6.

The ambient temperature is high, there is a lot of dirt on the surface of the motor, or the Yebi ventilation duct is blocked; 7.Faulty motor fan, poor ventilation;

Buck-down starting method commonly used in cage asynchronous motors

The methods for step-down starting of three-phase cage asynchronous motor are: stator winding string resistance (reactance) starting; Manual control of y-buck start-up; Autotransformer step-down start.

1. Auto-decompression Sun Slide starts

Auto-decompression start is one of the starting methods of cage induction motor (also known as asynchronous motor). It has the advantages of compact circuit structure, not limited by the motor winding wiring mode, and can also choose different transformer voltage taps according to the allowable starting current and the required starting torque, so it is suitable for motors with large capacity.

2. Manual control Y-step-down start

Y- Buck start-up is characterized by a simple and economical method. Its starting current is 1 3 when it starts directly, so it is only suitable for the motor to start under no-load or light load conditions.

3. The stator winding series resistance (reactance) is rented or started

When the motor starts, the resistance is connected in series in the stator winding of the motor, due to the voltage drop on the resistance, the voltage added to the motor winding is lower than the power supply voltage, and after starting, the resistance is shorted to make the motor run under the rated voltage to achieve the purpose of safe start.

The difference between cage type and wound motor:

1. Structure. A cage motor is a simple motor with a rotor consisting of a metal cage structure filled with conductor material. The conductor material in the cage structure is usually aluminum, and they are connected to the external power supply by end rings and axial channels.

The rotor of the wound motor, on the other hand, is made of many coils, which are connected to the end ring, which is connected to an external power source.

2. Working principle.

The working principle of the cage motor is based on the principle of induction, when the current passes through the stator windings, a rotating magnetic field will be generated in the stator, and the rotating magnetic field will induce eddy currents in the rotor, and the magnetic field generated by the eddy current will interact with the rotating magnetic field to produce torque, which will rotate the rotor.

The working principle of the wound motor is based on the principle of electromagnetic induction, when the current passes through the stator winding, a magnetic field will be generated in the stator, and the magnetic field will interact with the magnetic field in the rotor to produce torque, which will make the rotor rotate.

3. Features. Cage motors have the characteristics of simple structure, low manufacturing cost and easy maintenance, so they are widely used in industry, agriculture and other fields. However, due to the particularity of its rotor structure, its starting performance and speed regulation performance are poor, and it cannot meet the application requirements of high precision, high speed and high load.

The winding motor has the characteristics of good starting performance and speed regulation performance, stable operation and high output power, so it is widely used in some high-demand application fields, such as machine tools, wind power generators and other fields.