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Belt driven Fans - Fan vs. Motor Speed

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Belt driven fans are used in heating, ventilation, air-conditioning and cooling (HVAC) systems.

The speed of a fan wheel versus the speed of the motor with a belt drive can be calculated as

df nf = dm nm                          (1)


df = pulley diameter fan (inch, mm)

nf = speed of fan (rpm - rounds per minute)

dm = pulley diameter motor (inch, mm)

nm = speed of motor (rpm - rounds per minute)

Example - Speed of Fan

A synchronous electrical motor rotates with speed 1000 rpm. The motor pulley diameter is 76.5 mm and the fan pulley diameter is 205.5 mm. The speed of the fan can be calculated by modifying (1) to

nf = dm nm / df

- or with values:  

nf = (76.5 mm) (1000 rpm) / (205.5 mm)

   = 372 rpm

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