The working principle of the three-phase asynchronous motor is based on the principle of electromagnetic induction. When the stator winding passes through three-phase symmetrical alternating current, a rotating magnetic field is generated between the stator and the rotor. The rotating magnetic field cuts the rotor winding, and generates an induced electromotive force in the rotor circuit. The current, the current of the rotor conductor, is subjected to a force by the rotating magnetic field to rotate the rotor. Below, we analyze the generation of the rotating magnetic field, the rotation of the motor, the slip and the steering.
The precondition for a three-phase asynchronous motor to rotate is that it has a rotating magnetic field. The stator winding of a three-phase asynchronous motor is used to generate a rotating magnetic field. We know that the voltage between the phase and phase of the phase is phase-phase difference. At 120 degrees, the three windings of the stator winding of the three-phase asynchronous motor are not in the space position by 120 degrees. Thus, when the stator winding is connected to the three-phase power supply, the word winding generates a rotating magnetic field. The current changes by one cycle. The rotating magnetic field rotates in space for one week, that is, the rotational speed of the rotating magnetic field is synchronized with the current change. The rotational speed of the rotating magnetic field is n=60f/p where f is the power frequency and p is the pole pair of the magnetic field. The unit of n is: the speed per minute, the number of revolutions of the motor and the number of poles and the frequency of using the power supply. For this reason, there are two ways to control the speed of the motor: 1. Change the number of pole pairs of the motor. 2, frequency conversion method, many now use the frequency conversion technology to control the infinitely variable speed of the motor.
The direction of rotation of the rotating magnetic field is related to the phase sequence of the current in the winding. The phase sequence A, B, and C are arranged clockwise, and the magnetic field is rotated clockwise. If any two of the three power sources are reversed, for example, the phase B current is passed. In phase C, the phase C current flows into the B-phase winding, and the phase sequence is: C, B, and A, the magnetic field must rotate counterclockwise. With this characteristic, we can easily change the direction of the motor.
Under normal circumstances, the actual speed n of the motor is lower than the rotating magnetic field n1, because assuming n=n1, the rotor bar and the rotating magnetic field do not want to move, and the magnetic lines will not be cut, and no magnetic torque will be generated. Therefore, the rotational speed n of the rotor must be less than n1. For this reason, we call the three-phase motor an asynchronous motor.
Principle of rotating magnetic field generationIn the stator core of the three-phase asynchronous motor, the windings U, V, and W with the same three-phase structure are placed, and the phase windings are spatially different from each other by 120°, as shown in the following figure, and the three-phase windings are symmetrical. Three-phase AC power, as shown in Figure (b) (c). Below we take a two-pole motor as an example to illustrate the position of the magnetic field in space at different times.
As shown in the following figure (b), it is assumed that the instantaneous value of the current is flowing from the head end of each winding (indicated by a × in the middle of the )), the end is discharged (indicated by "⊙"), when the current is negative When, this is the opposite.
At the instant of ωt=0, i u =0, i v is a negative value, i w is a positive value, as shown in (c), the V phase current flows in from V 2 , V 1 flows out, and W phase current flows into the W 1, W 2 flows. The right-hand rule of Ampere can be used to determine the direction of the combined magnetic field generated by the three-phase current at ωt=0, as shown in Figure (d)1. It can be seen that the synthetic magnetic field at this time is a pair of magnetic poles, and the direction of the magnetic field is consistent with the direction of the longitudinal axis, the upper mode is the north pole, and the lower side is the south pole.
When ωt=π/2, after a quarter cycle, i u changes from zero to maximum value, current flows from the head end U1, and the end U2 flows out; i v is still negative, U phase current direction and (1 The same is true; i w also becomes negative, W phase current flows out from W1, W2 flows in, and the direction of the combined magnetic field is as shown in (d) 2, and it can be seen that the direction of the magnetic field has been turned clockwise when compared with ωt=0. 90°.
The same analytical method can be used to draw the synthetic magnetic field when ωt=π, ωt=2/3*π, ωt=2π, as shown in (d) 3 4 5 , respectively. The direction is gradually rotated clockwise, for a total of 360°, that is, rotated for one week.
- (a) Simplified three-phase winding profile
- (b) Connect the three-phase power supply by a star-connected three-phase winding
- (c) Three-phase symmetrical current waveform diagram
- (d) Rotating magnetic field of the two-pole winding
From this, it can be concluded that three-phase windings with completely different phase angles at 120 degrees in the spatial position are arranged on the stator of the three-phase AC motor, respectively connected to the three-phase symmetrical alternating current, which is generated between the stator and the rotor. The synthetic magnetic field is rotated along the inner circle of the stator, which we call a rotating magnetic field.
Direction of the rotating magnetic fieldAs can be seen from the above figure, the three-phase alternating current changes in the UVW phase sequence, and the generated rotating magnetic field spatially rotates in a clockwise direction.
If we arbitrarily adjust the current of the two-phase winding of the motor, such as the UWV phase sequence, theoretical analysis and practice prove that the generated rotating magnetic field rotates counterclockwise. It can be seen that the direction of rotation of the rotating magnetic field depends on the phase sequence of the three-phase AC power source that is passed into the winding, and the direction of the rotating magnetic field can be changed as long as the phase sequence of the motor is arbitrarily adjusted.
Speed ​​of rotating magnetic fieldThe above is a two-pronged example. If you want to obtain a four-pole rotating magnetic field, you should double the number of coils, as shown in Figure (a) (b). According to the above method, the schematic diagram of the synthesized magnetic field in space (C) is obtained.
We compare the rotational speed of the magnetic field in this figure and the one above, and it is not difficult to conclude that the speed of the rotating magnetic field is not only related to the frequency of the current, but also to the logarithm of the magnetic pole. In Figure (d) above, when the three-phase alternating current changes one week (that is, each phase passes through a 360° electrical angle), the rotating magnetic field generated by it also rotates exactly one revolution. Therefore, the rotational speed of the rotating magnetic field in the two-pole motor is equal to the changing speed of the three-phase alternating current.
That is, n1=60f1=3000 rpm
The speed of the rotating magnetic field is equal to half the speed of the three-phase alternating current, that is, n1 = (60/2) f1 = 1,500 rpm. Therefore, when the magnetic pole pair is doubled, the speed of the rotating magnetic field is reduced by half.
Similarly, through theoretical analysis, the rotational speed of the rotating magnetic field can be obtained as: n1=60f1/P, in the above formula
- N1: indicates the rotational speed of the rotating magnetic field, unit rpm;
- F1: indicates the frequency of the three-phase AC power supply, in units of Hz;
- P: indicates the number of magnetic pole pairs;
The rotational speed n1 of the rotating magnetic field is also referred to as synchronous rotational speed. China's three-phase AC frequency is specified at 50 Hz, so the two-pole rotation speed is 3000 rpm, the quadrupole is 1500 rpm, and the six poles are 1000 rpm.
The precondition for a three-phase asynchronous motor to rotate is that it has a rotating magnetic field. The stator winding of a three-phase asynchronous motor is used to generate a rotating magnetic field. We know that the voltage between the phase and phase of the phase is phase-phase difference. At 120 degrees, the three windings of the stator winding of the three-phase asynchronous motor are not in the space position by 120 degrees. Thus, when the stator winding is connected to the three-phase power supply, the word winding generates a rotating magnetic field. The current changes by one cycle. The rotating magnetic field rotates in space for one week, that is, the rotational speed of the rotating magnetic field is synchronized with the current change. The rotational speed of the rotating magnetic field is n=60f/p where f is the power frequency and p is the pole pair of the magnetic field. The unit of n is: the speed per minute, the number of revolutions of the motor and the number of poles and the frequency of using the power supply. For this reason, there are two ways to control the speed of the motor: 1. Change the number of pole pairs of the motor. 2, frequency conversion method, many now use the frequency conversion technology to control the infinitely variable speed of the motor.
The direction of rotation of the rotating magnetic field is related to the phase sequence of the current in the winding. The phase sequence A, B, and C are arranged clockwise, and the magnetic field is rotated clockwise. If any two of the three power sources are reversed, for example, the phase B current is passed. In phase C, the phase C current flows into the B-phase winding, and the phase sequence is: C, B, and A, the magnetic field must rotate counterclockwise. With this characteristic, we can easily change the direction of the motor.
Under normal circumstances, the actual speed n of the motor is lower than the rotating magnetic field n1, because assuming n=n1, the rotor bar and the rotating magnetic field do not want to move, and the magnetic lines will not be cut, and no magnetic torque will be generated. Therefore, the rotational speed n of the rotor must be less than n1. For this reason, we call the three-phase motor an asynchronous motor.
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