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Construction of three-phase asynchronous motor

Update time:2019-02-05 19:36:00Clicks:1753
The construction of three-phase asynchronous motor, the operation and maintenance of motor are the problems that we often face in our daily work. Today, we will discuss this with you.
The construction of three-phase asynchronous motor, the operation and maintenance of motor are the problems that we often face in our daily work. Today, we will discuss this with you.

Construction of 1-Phase AC asynchronous motor
Three-phase AC asynchronous motor is one of the most widely used motors in industrial and agricultural production. Asynchronous motor, also known as induction motor, is a kind of AC motor.
The structure of three-phase AC asynchronous motor can be divided into two types according to the rotor structure: one is squirrel cage type, the other is winding type. This time we mainly introduce squirrel cage three-phase AC asynchronous motor.
Motor profile
The structure of asynchronous motor mainly consists of stator (fixed part) and rotor (rotating part).
The stator of three-phase AC asynchronous motor is composed of iron core and three-phase winding, which is used to generate rotating magnetic field.
The rotor of three-phase AC asynchronous motor (squirrel cage type) has a copper bar in each groove of the rotor core, and two conductive end rings at the groove opening at both ends of the core connect the copper bar in the groove separately to form a short circuit for the output of mechanical energy.
Motor decomposition diagram
The rotor of three-phase AC asynchronous motor is placed in the stator, and the bearings at both ends of the rotor shaft are placed in the cover grooves at both ends of the stator, so that there is a layer of air gap between the stator and the rotor, which is about 0.2-1.0 mm.
Rotor of three-phase AC asynchronous motor (squirrel cage type)
Working Principle of 2-Phase AC Asynchronous Motor
How does the rotating magnetic field in the stator of the motor come into being?
The three windings in the stator of a phase asynchronous motor differ by 120 degrees in spatial orientation. Thus, when three-phase AC power is connected to the stator winding, a rotating magnetic field will be generated, and the process of its generation is shown in the figure. There are three moments in the graph to describe the generation process of rotating magnetic field. The rotating magnetic field rotates in space for one cycle with the change of current. That is to say, the rotating speed of the rotating magnetic field is synchronous with the change of current.
When the phase of the three-phase current changes from zero to 600, the direction of the synthetic magnetic field generated by them rotates 600 in space.
When the same current changes to 1200, 1200 is rotated by the synthetic magnetic field. When the current changes to 1800, 1800 is rotated by the synthetic magnetic field.
Rotating magnetic field produced by three-phase alternating current
When the current completes a periodic change, the direction of the synthetic magnetic field generated by them also rotates for a period in space. Obviously, the three-phase AC current changes over time, and the synthetic magnetic field generated by the three-phase current rotates continuously in space. In this way, the rotating magnetic field needed by the asynchronous motor is obtained.
Rotation and Speed of Rotating Magnetic Field
If the three-phase windings of the stator of the motor are connected to the phase sequence of the three-phase AC power supply respectively, then the phase sequence of the current in the three-phase windings of the stator is arranged clockwise. As can be seen from the above figure, the rotating magnetic field also rotates clockwise at this time. That is to say, the direction of the rotating magnetic field in the stator is determined by the phase sequence of the three-phase current in the three-phase winding of the stator.
If the direction of the rotating magnetic field in the stator (that is, the direction of the motor rotation) is to be changed arbitrarily, the two power supply phases connected can be changed arbitrarily.
Moreover, the rotating speed of the rotating magnetic field is proportional to the frequency of the power supply and inversely proportional to the logarithm of the magnetic poles.
Rotational Speed of Asynchronous Motor (Rotor Speed)
The speed of asynchronous motor (rotor speed) is the same as the synchronous speed of rotating magnetic field, but their speed is different.
Because the induced current generated in the rotor conductor is caused by the relative motion between the rotor conductor and the rotating magnetic field, which cuts the magnetic line, the rotor conductor rotates with the rotating magnetic field only under the action of the electromagnetic moment. Therefore, the speed of the motor (rotor speed) is not equal to the synchronous speed, and always less than the synchronous speed, the two speed is not synchronous, which is the origin of the name of the asynchronous motor.
Starting of 3-phase AC asynchronous motor
Motor Starting
The stator three-phase winding is connected to three-phase AC power supply to start up. When the stator three-phase winding is connected to the three-phase AC power supply, the rotor is stationary and the speed of the motor (rotor speed) is zero n = 0. At this time, the rotating magnetic field has the largest relative speed to the stationary rotor, at this time, the slip rate is the largest and the induced current in the rotor conductor is the largest.
The stator current of the motor varies with the rotor current. When the motor starts, the maximum current is about 5-7 times of the rated current. Such a large starting current will bring a lot of harm to the power grid and equipment, so it is necessary to limit the starting current of the motor and take some necessary measures.
Starting Method of Squirrel Cage Three-phase AC Asynchronous Motor
At present, there are many starting methods of squirrel cage three-phase AC asynchronous motor: direct starting, step-down starting, frequency conversion starting, soft starting and so on.
Direct starting: that is to connect the three-phase windings of the motor directly to the power grid to start. Conditions: Direct starting is allowed when the capacity of power supply transformer is much larger than that of direct starting motor (motor capacity < transformer capacity 20%-30%).
Step-down starting: Reduce the voltage added to the stator winding when starting. At present, the commonly used starting methods are as follows:
Auto-decompression starting
Star-triangle starting (Delta connection motor)
Delta Starter with Extended Edge (Delta Connection Motor)
(3) Controlled starting by frequency converter: Smooth starting can be achieved by changing the frequency of power supply with frequency converter.
Soft Starter Starting: Smooth Starting by Using Soft Starter to Change Power Voltage
Three-phase AC asynchronous motor
(Edit:admin)