This article mainly talks about the application of frequency conversion technology in the computer cooling fan, and explains its structure, principle, circuit composition, speed regulation method, wiring and other related knowledge in detail from various aspects.
I. Working Principle of Frequency Conversion Motor
Figure 1 (a) is a photograph of a dismantled fan motor. The fan is a variable frequency motor, which can be identified from the position of the coil. Fig. 1 (b) is the control circuit board of the variable frequency motor. The control chip integrates the functions of the DSP and the driver, which simplifies the circuit structure. By programming the control chip, the speed of the motor can be changed.
Frequency conversion motor has the characteristics of DC motor, but adopts the structure of AC motor. That is to say, although the external access is DC, but the use of DC-AC variable voltage converter control technology, the motor itself is fully in accordance with the principle of AC motor to work. Therefore, the variable frequency motor is also called "self-controlled variable frequency synchronous motor". The speed n of the motor depends on the frequency f set by the controller.
Three-phase star-connected variable-frequency motor control circuit, DC power supply through the three-phase converter circuit composed of MOS transistors to the three components of the motor when power supply. At each moment, only one pair of windings in the three pairs passes by current to produce a magnetic field, and then stops supplying power to the two windings, while supplying power to another pair of adjacent windings. Thus, the magnetic field axis in the stator rotates 120 degrees in space, and the rotor rotates 120 degrees with the stator magnetic field under the action of magnetic force. When the voltage is applied to A+B-, A+C-, B+C-, B+A-, C+A-, C+B-in turn, a rotating magnetic field is formed in the stator, and the motor rotates continuously.
The drive circuit of the variable frequency motor consists of two parts, the main circuit and the control circuit. Now the two parts are integrated into the same chip, so that all the control functions of the variable frequency motor can be realized by using only one device, which simplifies the circuit structure. The commonly used control chips are LB1964 of Sanyo Corporation of Japan, MAX6625 of MAXIM Company of America, and ST72141 of Semiconductor Company of Italy. And so on. With the increasing attention of industry to energy saving and noise suppression, many industrial products tend to use brushless motors, which put forward higher requirements for motor microcontrollers. As a new generation of motor control DSP chip, TI's high cost-effective TMS320C240 is very suitable for this task.
II. Circuit Composition of Frequency Conversion 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 motherboard to monitor the operation of the fan. The monitoring circuit is used to display the speed of the fan, and to realize alarm and automatic shutdown of the computer, so as to prevent the burning of CPU or other devices due to the shutdown of the fan. Nowadays, integrated power devices are widely used in variable frequency motors to realize this function, which greatly simplifies the control circuit.
In order to achieve precise control effect, the signal reflecting the rotor position must be input to the integrated power device, so the variable frequency motor must have the motor position feedback mechanism. At present, Hall element or photoelectric sensor are usually used to detect position and speed.
Hall device 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.
On an electrified semiconductor sheet, plus a magnetic field B perpendicular to the surface of the sheet, a voltage appears on both sides of the sheet (the Vh in the figure is called Hall voltage).
Hall effect
The frequency conversion motor uses Hall device to measure the relative position of the rotor, and the obtained 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 motherboard as a speed measurement signal, which can be said to kill two birds with one stone. Because the commutation pulse is a square wave signal, it can be transmitted to the motherboard for display and control after simple processing on the motherboard. Since the motor's phase number is usually more than 2 and the frequency of commutation signal is several times of the motor's speed, if the commutation pulse is used as the speed measurement signal, the real motor speed can only be obtained by dividing operation. Enen