Is there a big difference between the maintenance of Hisense inverter air conditioner and Midea inverter air conditioner?

DC motor is a kind of rotating machine that converts DC energy into mechanical energy. It consists of stator, rotor and commutator, as shown in Figure 3.

Fig. 3 Structure of Brushed DC Motor

The stator (main magnetic pole) is fixed on the fan bracket and is the non-rotating part of the motor.

There are more than two groups of coils in the rotor, which are made of enameled wires and called windings. When current passes through the winding, a magnetic field is generated, which acts as a force together with the magnetic field of the stator. Because the stator is fixed, the rotor rotates under the force.

Commutator is a special device of DC motor, which is composed of many commutator segments, and there is an insulating segment between every two adjacent commutator segments. The fixed brush is pressed by the spring on the commutator surface, so that the rotating armature winding can be connected with the external circuit. When the rotor rotates for a certain angle, the commutator connects the power supply voltage to another pair of windings and continues to generate magnetic fields in the windings. It can be seen that due to the commutator, the electromagnetic torque in the armature coil remains unchanged, and the armature rotates under the action of this electromagnetic torque, as shown in Figure 4.

Fig. 7 schematic diagram of brushless DC motor

The rotor realizes the dynamic fit between the bearing and the shell. The blades of the fan are fixed on the rotor, so when the rotor rotates, the blades will rotate with the rotor. Ordinary fans generally use ball bearings (as shown in Figure 5), while high-grade fans generally use more advanced liquid bearings (as shown in Figure 6) in order to improve the stability of operation and prolong the service life.

Figure 5 Ball bearing

Fig. 6 Structure of Liquid Bearing

Second, the brush motor and brushless motor

As mentioned earlier, DC motors use carbon brushes to realize commutation. Because of the friction of carbon brushes, the life of brushes and even motors is shortened. At the same time, the brush will generate sparks in the process of high-speed operation, and it will also interfere with the surrounding electronic circuits. For this reason, people invented a DC motor without carbon brush, which is also commonly known as brushless motor.

Brushless motor takes the winding as the stator and the permanent magnet as the rotor (as shown in Figure 7), which is just the opposite of brushed motor in structure. Brushless motor uses electronic circuit to switch the energization sequence of windings, which generates rotating magnetic field and drives the rotor to rotate.

Fig. 7 schematic diagram of brushless DC motor

Brushless motor has no carbon brush, long maintenance life and high speed regulation accuracy. Therefore, brushless motors are rapidly replacing traditional brush motors, and household appliances with frequency conversion technology (such as frequency conversion air conditioners and frequency conversion refrigerators). ) Use a brushless motor. At present, almost all cooling fans adopt brushless motors.

Third, the working principle of variable frequency motor

Fig. 8(a) is a photograph of an exploded fan motor. The fan uses a variable frequency motor, which can be identified from the position of the coil. Fig. 8(b) shows the control circuit board of the variable frequency motor. The control chip integrates DSP function and driver, which simplifies the circuit structure. By programming the control chip, the motor speed can be changed.

Figure 8 DC

Motor structure

Frequency conversion motor has the characteristics of DC motor, but adopts the structure of AC motor. That is to say, although the external connection is DC, DC- AC frequency conversion control technology is adopted, and the motor body works completely according to the principle of AC motor. Therefore, the variable frequency motor is also called "self-controlled variable frequency synchronous motor", and the speed n of the motor depends on the frequency f set by the controller.

Fig. 9 is a control circuit of a three-phase star-connected variable frequency motor. DC power is supplied to the three windings of the motor in a time-sharing way through a three-phase converter circuit composed of MOS tubes. At each moment, only one of the three pairs of windings has current flowing, thus generating a magnetic field. Then stop supplying power to this pair of windings and supply power to an adjacent pair of windings, so that the magnetic field axis in the stator rotates in space 120, and the rotor rotates with the stator magnetic field under the action of magnetic force 120. When voltages are applied to A+B-, A+C-, B+C-, B+A-, C+A- and C+B- in turn, a rotating magnetic field is formed in the stator and the motor rotates continuously.

Fig. 9 Working principle of brushless DC motor

The drive circuit of variable frequency motor consists of main circuit and control circuit. Now, these two parts have been integrated into the same chip, so that all the control functions of the variable frequency motor can be realized by only one device, which simplifies the circuit structure. Commonly used control chips are LB 1964 of Sanyo Company, MAX6625 of MAXIM Company of the United States, ST72 14 1 of stmicroelectronics Company, etc. With the increasing emphasis on energy saving and noise suppression in industry, many industrial products tend to adopt brushless motors, which puts forward higher requirements for motor microcontrollers. As a new generation of motor control DSP chip, TMS320C240 of TI Company has high cost performance and is very suitable for this task.

Fourth, the circuit composition of variable frequency motor

In order to monitor the operation of the fan motor, it is necessary to output the speed signal of the fan motor to the motherboard to monitor the operation of the fan. The monitoring circuit is used to display the rotating speed of the fan, and can give an alarm and stop automatically to prevent the CPU or other equipment from burning due to the fan stop. At present, frequency conversion motors generally use integrated power devices to realize this function, which greatly simplifies the control circuit.

In order to achieve accurate control effect, the signal reflecting the rotor position must be input into the integrated power device, so the variable frequency motor must have a motor position feedback mechanism. At present, Hall elements or photoelectric sensors are usually used to detect position and speed.

Hall element is a magnetic sensor based on Hall effect, which was discovered by American scientist Edwin Hall in 1879. At present, magnetic sensor products using Hall effect have been widely used.

Figure 10 is a schematic diagram of Hall effect. On the electrified semiconductor wafer, a magnetic field B perpendicular to the wafer surface is applied, and a voltage will appear on both lateral sides of the wafer (Vh in the figure is called Hall voltage).

Figure 10 hall effect

Frequency conversion motor uses Hall element to measure the relative position of rotor, and the obtained signal is input to control chip to drive motor to rotate. At the same time, the signal can also be output through the main board as a speed measuring signal.

You can kill two birds with one stone. Because the commutation pulse is a square wave signal, it can be transmitted to the main board for display and control after simple processing. Because the number of phases of the motor is generally more than two phases, and the frequency of the commutation signal is several times of the motor speed, if the commutation pulse is used as the speed measuring signal, the real motor speed must be obtained by division.

Figure 1 1 is a brushless motor control circuit composed of Hall-locked switch circuit CS20 18, which integrates Hall voltage generator, differential amplifier, Schmitt trigger, open collector output stage, etc. The win of a low-power motor can be directly driven.

Fig. 1 1 directly drives the motor with CS20 18 hall switch locking circuit.

Some fans use photoelectric sensors to detect the speed of the fan. The specific method is to set a light shielding plate on the rotor of the motor, so that every time the motor rotates, the light shielding plate will block the light emitted by the light emitting diode on the photosensitive tube once, and the voltage on the collector of the photosensitive tube will change once, so that a pulse signal reflecting the motor speed can be obtained, as shown in figure 12.

Figure 12 photoelectric sensor principle

As can be seen from the above introduction, the velocity measurement signals obtained by Hall sensor and photoelectric sensor are different. When the speed is measured by photoelectric sensor, the frequency of the speed signal is the same as that of the motor, but when the commutation signal output by Hall element is used as the speed signal, the difference between them is several times: if it is a two-phase motor, the frequency of the commutation signal is twice that of a three-phase motor. The speed shown in the BIOS here is not the real fan speed, so please pay attention when using it!

V. Speed adjustment method

There are two speed regulation methods for DC motor: voltage regulation and frequency regulation. The fan of ordinary brush motor can change the speed by regulating voltage, and the fan of variable frequency motor can only adjust the speed by frequency modulation.

For a brush motor, changing the power supply voltage means changing the current in the rotor winding, thus changing the magnetic field strength and torque. The speed of motor increases with the increase of torque and decreases with the decrease of torque. When the load resistance of this motor increases, the speed of the motor will decrease. If we want to keep the speed constant when the load changes, we must adopt closed-loop control and realize it through speed negative feedback, so the control circuit is more complicated.

Figure 13 is a practical control scheme of brushed motor, which is realized with MIC50 1 special chip as the core and some peripheral components as the supplement. Comparing the brushless motor control circuit shown in Figure 10, we can find the obvious difference between them.

Figure 13 Voltage Regulating and Speed Regulating Circuit

In the brush motor circuit, the main circuit of the motor is connected in series with the power transistor VT, which acts as a variable resistor. The signal output by pin 7 of the chip is connected to its base through the base resistor Rb, which controls the conduction degree of VT, and the motor speed changes with the variation of the conduction degree of VT.

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