External rotor motors are usually contained within the width of the fan housing, 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. Some of the smaller double inlet fans are IP54 protected to enhance the dust protection, and are resilient mounted at each shaft end to reduce the transmission of vibration.
Low cost speed control is possible by supply voltage variation, either by electronic (triac) method or by transformer. Detachable discharge flanges enable ‘without flange’ or special mounting arrangements to be specified.