​Analysis of Causes for Unstable Speed in Cabinet-Type Smoke Exhaust Fans

Overall, cabinet-type smoke exhaust fans are highly efficient and environmentally friendly ventilation equipment. However, minor issues may arise during practical application. A relatively common problem is unstable rotational speed in centrifugal fan motors, which leads to further complications. For instance, when cabinet-type smoke exhaust fans utilize DC speed-regulated motors, these motors occasionally exhibit unstable rotational speeds during operation. The motor's operating speed is primarily controlled by adjusting the thyristor conduction angle via a variable resistor. The causes of this phenomenon should be considered from the following four aspects: First, fluctuating motor speeds in cabinet-type smoke exhaust fans may stem from issues like carbon powder buildup between the rotor and commutator or defects in the carbon brushes themselves. Therefore, during maintenance, we generally follow the principle of addressing simpler issues first. Thus, the variable resistor should be measured initially. If the resistance value changes predictably with adjustments, the potentiometer is functioning correctly. Subsequent measurements of the thyristor also indicate normal operation. Further testing reveals abnormalities in the diode. Second, when the cabinet-type exhaust fan's speed becomes unstable, some issues stem from the thyristor. Common failures include breakdown short circuits, open circuits, failure to trigger, or internal contact issues. Thirdly, poor contact in the adjustable resistor can cause fluctuating speeds. This typically occurs in cabinet-type exhaust fans with extended usage, where contact degradation may arise. One potential cause is an abnormal rectifier diode. In summary, centrifugal fans are complex equipment requiring targeted measures for different anomalies. New Concepts, Insights, and Methods in Pneumatic Design for Shell-less Centrifugal Fans With advancements in modern aerodynamic design methods for shell-less centrifugal fans, we encounter new challenges during high-performance fan development—particularly through collaboration with frontline production management technicians. Simultaneously, resolving these issues often sparks innovative ideas and approaches, continuously driving the evolution of contemporary design methodologies. Next, we briefly explore the selection of aerodynamic design methods for airflow, aiming to deepen our understanding. The aerodynamic design of shell-less centrifugal fan equipment is primarily based on the design flow rate requirements provided by the user. Moreover, the default operating condition is set to achieve high efficiency under such conditions. However, analysis of actual applications reveals that the operating point for many shell-less centrifugal fan units does not align with the aerodynamic design flow rate. The deviation in direction and magnitude is related to the specific speed. Typically, fans with specific speeds below 27—commonly termed low-specific-speed fans—often exhibit flow rates exceeding the original design conditions. Moreover, the deviation becomes more pronounced as the specific speed of the shell-less centrifugal fan decreases. Fans with medium specific speeds generally operate close to their original design conditions. When the specific speed exceeds 55, the operating flow rate falls below the original design flow rate. For such scenarios, we can initially adopt a flow-based aerodynamic design approach when selecting the startup design method. That is, when designing shell-less centrifugal fans, flow design and total pressure aerodynamic design should be conducted reasonably according to different user requirements. While optimizing design performance, the non-design performance of shell-less centrifugal fan equipment should also be considered.