Single inlet centrifugal fans with forward-curved impeller blades and thermal motor protection are directly coupled to the external rotor motor. Thanks to their compact and robust design, they are used whenever a large volume of air has to be moved into a confined space. They are equipped with dynamic balancing on two planes which enables stable and vibration-free operation. Voltage control can be used to modulate the rotation speed and the noise level of the double inlet fans. The single inlet centrifugal fans are also available in the TRE impeller version with a motor.