Centrifugal fans use a DC speed control motor, and their speed is controlled by adjusting the resistance to change the thyristor angle, which in turn controls the voltage across the motor terminals to adjust the speed. If the motor speed is unstable, there are generally four possible causes:
Issue with One of the Four Rectifier Diodes
Carbon Powder or Carbon Brushes Between the Motor Rotor and Rectifier
According to the principle of easy-to-difficult maintenance, first measure the adjustable resistor. If the resistance value changes regularly with adjustments, confirm that the potentiometer is working well. Then, measure the thyristor. The rectifier diode is a high-current sealed tube, with one end screwed into the circuit board and the other end soldered onto the board. If looseness is found when removing from the circuit board, shake the part during measurement to confirm whether the diode is working properly.
Problems with the Thyristor
Common issues with thyristors include short circuits, open circuits, no triggering, or poor internal contact.
Adjustable Resistor
For cabinet-type centrifugal fans, the adjustable resistor is mounted on the operating panel and is frequently used. Over time, poor contact is a common issue.
If unstable speed is observed during operation, consider these four potential causes as investigation directions. This will help quickly identify the problem and improve maintenance efficiency.