The power required to drive a fan relates to the fan shaft speed as follows:
Initial_Horsepower/New_Horsepower = (Initial_Fan_RPM/New_Fan_RPM)^3
A small speed change can result in a substantial increase in power consumption. Therefore, this relationship should be considered when replacing a V-belt drive with a synchronous belt drive for energy savings.
To ensure that the fan speed does not increase, the design speed ratio should be based on a measured fan shaft RPM of the existing V-belt drive. This measurement can be made with a contact or a strobe tachometer.