External rotor motors are usually contained within the depth of the fan impeller, thus saving the space normally occupied by a standard motor. They are located in the air stream, so temperature is limited, and are generally of protection class IP44, so the air needs to be clean and dry to avoid motor contamination.

Backward-curved motorised impellers develop pressure within the envelope of the impeller and do not therefore rely on a scroll housing for this purpose. They are of relatively low cost and can be simply and cheaply housed. Air is drawn through a radiussed inlet ring, which minimises pressure losses and air leakage, and is discharged at 90° to the inlet axis.
The impeller has a 'non-overloading' characteristic in that it draws maximum power near the centre of its pressure/volume flow curve.

Low cost speed control is possible by supply voltage variation, either by electronic (triac) method or by transformer.