1、 Working principle of variable frequency motor
The fan uses a variable frequency motor, which can be identified from the position of the coil. Figure 1 (b) is the control circuit board of the variable frequency motor. The control chip will integrate the DSP function and the driver, simplifying the circuit structure. By programming the control chip, the motor speed can be changed.
Variable frequency motor has the characteristics of DC motor, but it adopts the structure of AC motor. That is to say, although the external access is DC, the DC-AC variable voltage converter 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 "automatic variable frequency synchronous motor". The speed n of the motor depends on the frequency f set by the controller.
The three-phase star connected variable frequency motor control circuit, DC power supply through the MOS three-phase converter circuit to the three windings of the motor time-sharing power supply. At each moment, only one of the three pairs of windings has current passing through it, generating a magnetic field. Then, stop supplying power to this pair of windings, and supply power to the adjacent pair of windings. In this way, the magnetic axis of the stator rotates 120 ° in space, and the rotor rotates 120 ° with the stator magnetic field under the effect of magnetic force. When the voltage is applied to a + B -, a + C -, B + C -, B + a -, C + a -, C + B -, the rotating magnetic field is formed in the stator, and the motor rotates continuously.
The drive circuit of the variable frequency motor consists of the main circuit and the 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 using one device, and the circuit structure is simplified. The commonly used control chips are lb1964 of Sanyo company of Japan, max6625 of Maxim company of America, and ST72141 of Italian semiconductor company Wait. With the increasing attention of industry to energy saving and noise suppression, many industrial products tend to use the brushless motor, which puts forward higher requirements for the motor microcontroller. As a new generation of motor control DSP chip, TMS320C240 with high cost performance of TI company is very suitable for this task.
2、 Circuit composition of variable frequency motor
In order to monitor the operation of the fan motor, it is necessary to output the speed signal from the fan motor to the main board to monitor the operation of the fan. The monitoring circuit is used to display the fan speed and realize the alarm and the automatic shutdown of the computer, so as to prevent the CPU or other devices from burning due to the fan shutdown. Now the frequency conversion motor generally uses the set success rate device to realize this function, which greatly simplifies the control circuit.
In order to achieve the precise control effect, it is necessary to input the signal reflecting the rotor position to the set success rate device, so the variable frequency motor must have the motor position feedback mechanism. At present, Hall element and photoelectric sensor are usually used to detect the position and speed.
Hall device is a kind of magnetic sensor based on Hall effect, which was discovered by American scientist Edwin hall in 1879. At present, the magnetic sensor using Hall effect has been widely used.
A voltage (VH in the figure is called Hall voltage) will appear on the transverse sides of a semiconductor chip when the magnetic field B perpendicular to the surface of the chip is applied to the chip.
Hall effect:
The relative position of the rotor is measured by Hall device, and the signal is input into the control chip to drive the motor to rotate. At the same time, the signal can also be output through the main board as a speed measurement signal, which can be described as a double shot. As the commutation pulse is a square wave signal, it can be transmitted to the main board for display and control after simple processing on the main board. Because the number of phases of the motor is generally more than two and the frequency of the commutation signal is several times of the speed of the motor, if the commutation pulse is used as the speed measurement signal, the real speed of the motor can only be obtained through division operation.