As the filter clogs, system resistance rises and the fan's operating point shifts; how the motor responds to this change depends on the type of impeller. The motor should therefore be chosen not from the old nameplate but from the fan's blade type and the load with the filter both clean and dirty. Frequent clogging is also a system problem in its own right and should be solved first.

DRG-132S-6 3 kW 1000 rpm aluminium frame DRG three-phase motor – fan motor selection with filter clogging

Fan type is decisive

On forward-curved centrifugal fans, power consumption rises markedly with airflow. When the filter clogs and airflow drops, the motor is relieved; the moment the filter is changed, airflow and power jump up. On this type of fan, the motor should be selected for the cleanest filter condition. On backward-curved fans the power curve peaks at a certain point and the tendency to overload is lower. On axial fans, increased resistance can raise the power in some cases. For details, see the forward-curved fan motor page.

What to look at when selecting

  • Power limit: The highest shaft power read from the fan curve with the filter clean and dirty
  • Protection: Correct selection of the device protecting the motor against overcurrent; settings are determined from the motor nameplate and by qualified electrical personnel
  • Dust load: If the filter leaks, dust builds up on the motor fins and reduces cooling; the outer surface of the IP55, self-ventilated (IC411) frame needs to be kept clean
  • Start/stop frequency: On fans that are stopped often for filter cleaning, the number of starts should also be assessed

First look at the cause of the clogging

The filter class may be too fine for the application, there may be no pre-filter, the dust load may have increased over time, or the cleaning system (shaking, reverse air) may not be working. Fitting a larger motor without fixing these problems only increases energy consumption.

Differential pressure measurement

Monitoring the pressure difference across the filter is the most practical way to track both when to change it and how the motor load is changing. In dust-heavy applications you can read the notes on dust collection fan motors and choose among the three-phase models in the shop suited to fan duty.