In the field of modern electrical engineering, the effective solution of control circuit faults depends on a deep understanding of the circuit topology. Taking a certain brand of smart wall-mounted fan as an example, its design adopts a combination of microcontroller unit (MCU) and driver chip. When the fan blades rotate delayedly after the device is powered on, the pulse width modulation (PWM) output waveform of the control chip should be monitored by an oscilloscope first. If the duty cycle signal is found to be abnormal, it is necessary to focus on checking whether the 22pF load capacitor in the crystal oscillator circuit has a failure problem. This type of fault often causes the clock frequency to drift, which causes the speed regulation program to run unstably. In addition, for motors that use Hall sensors for positioning, when speed fluctuations occur, it is necessary to confirm whether the gap between the sensor and the magnetic steel meets the process standard of 0.5±0.1mm. If the gap is too large, it will cause position detection errors, causing confusion in the commutation logic.
The fault repair of the power module requires a comprehensive analysis of the circuit topology and component characteristics. When the wall fan motor restarts frequently, the output voltage ripple of the rectifier bridge stack should be measured first. If the ripple factor at 100Hz exceeds 5%, the equivalent series resistance (ESR) of the filter capacitor needs to be checked. Taking a 40W wall-mounted fan as an example, the ESR of the 220μF/400V electrolytic capacitor used in it may rise from the initial 0.15Ω to 0.5Ω after the ambient temperature reaches 40℃ and runs for 2000 hours, which will significantly reduce the filtering effect. In this case, you should consider replacing it with a high-temperature resistant electrolytic capacitor and adding a 0.1μF ceramic capacitor in parallel to the circuit to effectively suppress high-frequency noise. For variable-frequency motors using switching power supplies, when the output voltage is low, it is important to check the sampling resistor of the TL431 reference source. If the temperature drift coefficient of the precision resistor exceeds 50ppm/℃, it may cause the overvoltage protection threshold to shift.
Troubleshooting of the drive system also needs to take into account the effectiveness of the power device and the protection circuit. When the motor triggers the stall protection, it is necessary to first confirm whether the gate drive voltage of the insulated gate bipolar transistor (IGBT) module is within the technical requirement range of 15±1V. Laboratory data shows that when the drive voltage is lower than 13V, the turn-on loss of the IGBT will increase by 40%, which is very likely to cause the junction temperature to exceed the safety limit of 175°C. In this case, it is necessary to check whether the turns ratio of the drive transformer is consistent with the design value, and measure whether the capacitance of the bootstrap capacitor has decayed by more than 20%. For motors using intelligent power modules (IPMs), when an overcurrent (OC) fault occurs, a thermal imager should be used to detect the temperature distribution on the surface of the IPM. If a local hot spot is found to exceed 125°C, it is necessary to check whether the thermal grease between the heat sink and the module has dried up. This fault will increase the thermal resistance by more than two times, thus affecting the stability and safety of the equipment.
