A normal-looking current may show that the motor is running healthily, but it does not prove that the airflow is sufficient. The problem is usually on the fan side: belt slip, a dirty impeller, a clogged filter, a damper in the wrong position, a leak in the ductwork or an impeller turning the wrong way. Moreover, on some fan types the current also drops when airflow falls; the value taken as "normal" may in fact be lower than it used to be.

DRG-100L1-4 2.2 kW 1500 rpm cast iron frame DRG three-phase motor – reduced fan airflow with normal current

Interpreting the current correctly

On forward-curved centrifugal fans, the motor load decreases as airflow falls. So on a fan with a clogged filter, the current may be below the nameplate value and be perceived as "normal". The real comparison should be made with a reference value measured when the system was healthy. If there is no reference, recording the current with the system in clean condition for future use is a good habit.

Order of checks

  1. Direction of rotation: A centrifugal fan running backwards still delivers air, but noticeably less. Compare with the arrow on the casing.
  2. Belt: A loose or worn belt slips; the motor runs at full speed while the fan turns slowly. Measuring the fan shaft speed with a tachometer shows this immediately.
  3. Impeller: Dust, grease or fibres building up on the blades reduce its ability to move air.
  4. Filters and grilles: Blockages on the suction and discharge sides.
  5. Dampers and flaps: Dampers left half-closed or with broken linkages.
  6. Ductwork: Separated flanges, punctured flexible connections, branches added later.

Possible causes on the motor side

On the motor side, a possible cause is a speed lower than expected; for example, a motor with a different number of poles has been fitted. If the speed on the motor nameplate does not match the speed intended for the fan, check this first. Electrical measurements should be carried out by qualified electrical personnel.

For detailed information by fan type, you can read the centrifugal fan motors page; if a motor replacement is really needed, you can review the three-phase motor models in the shop.