This article focuses on the application of frequency conversion technology in computer cooling fans. It elaborates on relevant knowledge in various aspects, including structure, working principle, circuit composition, speed regulation methods, and wiring.

I. Working Principle of Variable Frequency Motors

Cooling fans adopt variable frequency motors, which can be identified by the position of the coils. Figure 1(b) shows the variable frequency motor control circuit board. The control chip integrates DSP functions and a driver, simplifying the circuit structure. Motor speed can be adjusted by programming the control chip.

Variable frequency motors possess the characteristics of DC motors but adopt the structure of AC motors. In other words, although DC power is input externally, DC-AC voltage variable frequency converter control technology is used, and the motor itself operates entirely in accordance with the principle of AC motors. Therefore, variable frequency motors are also called "self-controlled frequency conversion synchronous motors," and the motor speed n depends on the frequency f set by the controller.

For the control circuit of a variable frequency motor with a three-phase star connection, DC power supply supplies power to the three windings of the motor in sequence through a three-phase converter circuit composed of MOSFETs. At any moment, only one pair of windings among the three pairs has current passing through, generating a magnetic field. Then, power supply to this pair of windings is stopped, and power is supplied to the adjacent pair. As a result, the magnetic field axis in the stator rotates 120° in space, and the rotor rotates 120° following the stator magnetic field under the action of magnetic force. By sequentially applying voltage to A+B-, A+C-, B+C-, B+A-, C+A-, and C+B-, a rotating magnetic field is formed in the stator, enabling continuous rotation of the motor.

The drive circuit of a variable frequency motor consists of a main circuit and a control circuit. Currently, these two parts are integrated into the same chip, so only one device is needed to realize all control functions of the variable frequency motor, simplifying the circuit structure. Common control chips include LB1964 from Sanyo (Japan), MAX6625 from MAXIM (USA), and ST72141 from STMicroelectronics. With the growing emphasis on energy conservation and noise suppression in the industrial sector, many industrial products tend to use brushless motors, placing higher requirements on motor microcontrollers. As a new generation of motor control DSP chips, TI's cost-effective TMS320C240 is very suitable for this task.

II. Circuit Composition of Variable Frequency Motors

To monitor the operating status of the fan motor, it is necessary to output a speed signal from the fan motor to the motherboard to realize real-time monitoring of fan operation. The monitoring circuit is used to display the fan speed and can implement alarms and automatic shutdown of the computer, preventing the CPU or other components from being burned due to fan stalling. Currently, variable frequency motors generally use integrated power devices to achieve this function, greatly simplifying the control circuit.

For precise control effects, it is necessary to input signals reflecting the rotor position to the integrated power devices. Therefore, variable frequency motors must have a motor position feedback mechanism. At present, Hall elements and photoelectric sensors are usually used for position and speed detection.

Hall devices are magnetic sensors based on the Hall effect, which was discovered by American scientist Edwin Hall in 1879. Currently, magnetic sensor products utilizing the Hall effect have been widely used.

When a magnetic field B perpendicular to the surface of a powered semiconductor sheet is applied, a voltage (Vh in the figure is called the Hall voltage) appears on the transverse sides of the sheet.

Hall Effect Application: Variable frequency motors use Hall devices to measure the relative position of the rotor. The obtained signals are input to the control chip to drive the motor to rotate. Meanwhile, this signal can be output through the motherboard as a speed measurement signal, serving two purposes. Since 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 number of motor phases is generally more than 2, the frequency of the commutation signal is several times the motor speed. Therefore, if the commutation pulse is used as a speed measurement signal, division operation must be performed to obtain the actual motor speed.

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