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External rotor motors are particularly advantageous in the construction of fans. They are compact dimensionally and are speed controllable by voltage variation. The integration of motor and fan impeller permits ideal power matching, and balancing to be performed on the whole unit. High slip speeds are tolerated by the motor such that the fan tends to increase in speed as system pressure is increased. As the motor is generally in the fan air stream, its external rotor is continuously cooled in the manner of a rotating heat-sink. This helps to keep the motor compact dimensionally and assists the overall efficiency of the unit.
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The
external rotor motor principle 