Principle and structure of three-phase asynchronous motor
abstract:
As a motor, the rotor speed of a three-phase asynchronous motor is lower than the rotating magnetic field. Because of the relative movement between the rotor winding and the magnetic field, the rotor winding induces electromotive force and current, which interact with the magnetic field to generate electromagnetic torque and realize energy conversion. Compared with single-phase asynchronous motor, three-phase asynchronous motor has good running performance and can save various materials. According to the different rotor structure, three-phase asynchronous motor can be divided into cage type and wound type.
Three-phase asynchronous motor; Basic structure; Working principle; Select employment/use
Basic structure of single-phase and three-phase asynchronous motors
1, stator (static part)
(1) stator core
Function: a part of the magnetic circuit of the motor and stator winding is placed on it.
Structure: The stator core is generally made of silicon steel sheet with a thickness of 0.35~0.5 mm, with insulating layer on the surface, and evenly distributed slots are punched in the inner circle of the core for embedding the stator winding.
Stator core slot types are as follows: semi-closed slot, semi-open slot and open slot.
(2) Stator winding
Function: it is the circuit part of the motor, which is connected with three-phase alternating current to generate rotating magnetic field.
Structure: It consists of three windings with the same structure, with an interval of 120 electrical angle, arranged in a straight line. Each coil of these windings is embedded in each slot of the stator according to certain rules.
(3) Framework
Function: Fix the stator core and front and rear end covers, support the rotor, and play the role of protection and heat dissipation.
2. Rotor (rotating part)
(1) rotor core of three-phase asynchronous motor;
Function: As a part of the motor magnetic circuit, the rotor winding is placed in the iron core slot.
(2) Rotor winding of three-phase asynchronous motor
Function: the rotating magnetic field of the cutting stator produces induced electromotive force and current, forming electromagnetic torque to make the motor rotate.
Structure: divided into squirrel cage rotor and wound rotor.
3. Other accessories of three-phase asynchronous motor
End cap: Support function.
Bearing: connecting the rotating part and the stationary part.
Bearing end cover: protects the bearing.
Fan: cooling motor
Working principle of two-phase and three-phase asynchronous motors
After the stator winding is connected to the three-phase power supply, the motor will produce a rotating magnetic field. The so-called rotating magnetic field refers to the magnetic field that is distributed on the circumference of the air gap between the stator and the rotor in the motor according to the sinusoidal law and can continuously rotate around the motor in space. There is relative motion between the rotor and the rotating magnetic field. The rotor bar is cut by the magnetic force of the rotating magnetic field to produce induced electromotive force, which induces current in the rotor winding, and the interaction between them produces electromagnetic torque, which makes the rotor rotate. So as to convert electric energy into mechanical energy of the rotating shaft.
When three-phase symmetrical alternating current is applied to the three-phase stator winding of the motor (the electrical angle difference of each phase is 120 degrees), a rotating magnetic field will be generated, which will cut the rotor winding, thus generating an induced current in the rotor winding (the rotor winding is a closed path), and the current carrier conductor will generate electromagnetic force under the action of the stator rotating magnetic field, thus forming electromagnetic torque on the motor rotating shaft to drive the motor to rotate, and the motor rotating direction is the same as that of the rotating magnetic field.
Selection of three-phase asynchronous motor
Three-phase asynchronous motor is widely used as the main power supply. Here, it is particularly important to choose the rated power of the motor reasonably. If the rated power is too large, it will not only increase the equipment investment cost, but also make the motor run at the point of low efficiency and low power factor for a long time, which is unreasonable and uneconomical.
1 and the selection points of three-phase asynchronous motor
(1) According to the comprehensive indicators such as mechanical load characteristics, production technology, power grid requirements, construction cost and operation cost. , reasonably choose the type of motor.
(2) According to the overload capacity, starting torque, working system and working conditions required by the mechanical load, reasonably select the power of the motor, make the power match reasonable, and have appropriate standby power, so as to ensure safe, reliable and economical operation.
(3) According to the environment of the place of use, choose the protection grade and structural form of the motor.
(4) According to the highest mechanical speed of the production machinery and the requirements of the transmission speed control system, select the speed of the motor.
(5) According to the requirements of environmental temperature, convenient maintenance and inspection, safety and reliability, choose the insulation grade and installation mode of the motor.
(6) Select the rated voltage and frequency of the motor according to the grid voltage and frequency.
2, three-phase asynchronous motor selection steps:
Select the motor type → select the motor capacity → check the maximum starting torque → check the equivalent heating → check the comprehensive economic indicators → compare the mechanical characteristics and load characteristics of the motor → determine the motor voltage level and frequency.
Iron core → comparison of mechanical characteristics and load characteristics of motor → voltage level and frequency of motor → determination.
3, three-phase asynchronous motor maintenance
Preparation and inspection before startup
(1) Check whether the grounding of motor and starting equipment is reliable and complete, and whether the wiring is correct and good.
(2) Check whether the rated voltage and rated frequency on the motor nameplate are consistent with the power supply voltage and frequency.
(3) For newly installed or long-term disabled motors (disabled for more than three months), check the insulation resistance of windings to ground before starting. (measured by 1000 volt megohmmeter). Insulation resistance should be greater than 0.5 megohm. If it is lower than this value, the winding should be dried.
(4) For the wound rotor, check whether the brush and the brush lifting device on the slip ring can work normally and whether the pressure of the brush can meet the requirements. Brush pressure 1.5N/cm-2.5 N/cm.
(5) Check that the motor rotor rotates flexibly and reliably, and the oil in the sliding bearing reaches the specified oil level.
(6) Check whether the rated current of the fuse used in the motor meets the requirements.
(7) Check whether the fastening bolts and mounting bolts of the motor are firm and meet the requirements.
4, fault handling in operation
(1) failed to start.
When the circuit breaker or automatic switch is closed, the motor does not rotate, only hears buzzing, or cannot rotate at full speed. The reasons for the failure may be:
If the stator circuit is open in one phase, such as the fuse of the low-voltage motor is blown, or the short circuit of the high-voltage motor is in poor contact with the isolating switch, the three-phase rotating magnetic field cannot be formed.
The rotor circuit is broken or the contact is poor, so that the rotor winding has no current or the current is reduced, and the motor does not turn or turns very slowly.
In the transmission machinery, there is a mechanical jam phenomenon. In severe cases, the motor does not turn and the noise is abnormal.
Too low voltage will reduce the torque of the motor, making it difficult or impossible to start.
The stator and rotor core of the motor rub against each other, which increases the load and makes the rotation difficult.
When the operator finds the above faults, the circuit breaker and isolating switch of the high-voltage motor should be opened immediately to check the stator and rotor circuits.
(2) Stator winding single-phase grounding fault.
Due to the erosion of various factors, the insulation level of motor windings is reduced. In addition, due to the long-term overload operation of the motor, the insulation of the winding will become brittle or fall off due to long-term overheating. This will lead to single-phase grounding of the motor stator winding.
(3) Single-phase operation fault of the three-phase motor During the operation of the three-phase motor, if the single-phase fuse is blown or poorly contacted, the single-phase contact of the disconnector, fuse, cable head and conductor is loose, and the stator winding is broken, it will cause the single-phase operation of the motor.
When the operator confirms that the motor is in single-phase operation according to the abnormal phenomenon generated by the motor, the power supply should be cut off and the machine should be stopped. Measure the resistance of stator circuit with megger. If the resistance value is large or infinite, it means that the phase is broken. Then check whether the fuses, circuit breakers, disconnectors, cable heads and wiring contacts in the junction box in the stator circuit are intact.
Nameplate of four-phase or three-phase asynchronous motor
There is a nameplate on the shell of each motor, indicating the specifications, performance and conditions of use of the motor, which is the basis for us to use the motor correctly. The main technical parameters on the nameplate are introduced as follows.
1, model In order to meet the needs of different uses and working environments, three-phase asynchronous motors are made into different series and models, and the technical parameters such as frame length, center height and speed of different models of motors are different. When using or purchasing, you should pay attention to the model or consult the corresponding product catalogue and technical manual as needed.
2. Under the operating conditions specified in the nameplate, the output power (kw) of the power motor during normal operation.
3. The rated line voltage of the stator winding of the voltage motor (V).
4. When the current motor is running under rated working conditions, the line current flowing into the stator winding (A).
5. When the speed regulating motor runs under rated working conditions, the revolutions per minute (r/min) of the rotor.
6. The junction box connecting the motor has six terminals. When the current direction of the rotor needs to be changed, the direction of the motor can be changed only by switching any two of the three power lines of the motor.
Conclusion:
Practice has proved that in industrial and agricultural production, scientific selection of three-phase asynchronous motor according to actual needs can improve production efficiency and receive good economic benefits. Scientific maintenance of the running motor can keep the motor in a very good technical state for a long time, prolong its service life and improve the efficiency of industrial and agricultural production. Very necessary.
References:
[1] Cai Jiagang. Motor use and maintenance technology. Beijing: Water Resources and Hydropower Press, 1998.
[2] Fu. Practice and technology of electrical engineering. Beijing Chemical Industry Press 2003
[3] Zhang Zengchang. Quick connection of motor windings. Beijing: Machinery Industry Press, 1996.
[4] pine and cypress. Self-study instruction of three-phase motor repair. Beijing: Beijing Science and Technology Press, 200 1