The amount of power a fan requires changes based on the speed (RPM), airflow rate (CFM), and static pressure (in w.g.).
The amount of input power a fan requires to operate at a given point is called the break horsepower (BHP). A motor must meet or exceed the BHP of the fan to operate at a given point. A simple ratio determines if an inverter duty fan can provide the necessary BHP when it is not running at full speed.
A fan and motor work together in an HVAC system to keep the air moving and transferring heat into and out of the space. The design of the ductwork, filters, coils, and other miscellaneous components generate static pressure drop that resists the flow of air. The amount of power the fan requires to overcome this pressure drop and provide the desired amount of airflow (CFM) is the break horsepower (BHP). Next, we will start to look at this from the perspective of the motor trying to satisfy the BHP requirement.
Motors are primarily specified by their nominal speed (RPM) and horsepower (HP) for a given voltage. The nominal speed is the full speed of the fan running at 60 Hz, or your standard frequency. Inverter duty fans can be over sped (run at a higher RPM than nominal) without a loss of available horsepower, though over speeding the motor can reduce the overall life of the motor and VFD.
Alternatively, if a motor is running at less than the full rated RPM then the amount of horsepower it can provide is reduced proportionally, though there is no impact to motor life due to operating at lower speeds as long as there is sufficient airflow to keep the motor from overheating.
For example, a typical inverter duty or premium motor would be 1800 RPM nominal and any of several standard HP steps. Let’s say we are looking at using an 1800 RPM 10 HP motor. The fan we have selected needs 8.25 BHP at 997 RPM. So, our equation becomes:
This motor will not work for this fan under these conditions as it does not have enough horsepower. If we had looked at a motor that was 10 HP and 1170 RPM nominal, then it would derate to 8.5 HP would be just enough for the fan to operate.
Be sure that any motor selected meets or exceeds the required fan BHP at the operating RPM. If your system is variable fan speed then you can refer to the fan affinity laws to check the BHP requirements at different operating points.