About this calculator This calculator determines the VFD operating frequency needed to achieve a target fan RPM based on the motor nameplate RPM and frequency. It uses the linear relationship between motor speed and frequency. Use it when setting up a VFD for a fan or when adjusting fan speed to meet changed airflow requirements without replacing sheaves.
Use when You need to determine VFD frequency setting based on fan and motor RPM.
Formula Operating Hz = Motor Frequency × (Fan RPM ÷ Motor RPM)
Variables
Fan RPM Desired fan speed in RPMMotor RPM Motor nameplate speed in RPMMotor Frequency Motor nameplate frequency in Hz (default 60)
Fan RPM
Desired fan rotational speed
Motor RPM
Motor nameplate speed at full frequency
Motor Frequency (Hz)
Line frequency -- typically 60 Hz (US) or 50 Hz
Operating Frequency (Hz) —

What are the fan affinity laws?

The fan affinity laws describe how fan performance changes with speed, and they are the basis for calculating VFD (variable frequency drive) setpoints. The first law states that airflow varies directly with speed: CFM₂ = CFM₁ × (RPM₂ ÷ RPM₁). The second law states that pressure varies with the square of speed. The third law states that power varies with the cube of speed, meaning a small reduction in speed yields a large energy savings. For VFD-controlled fans, speed is proportional to frequency: a motor running at 48 Hz instead of 60 Hz operates at 80% speed and approximately 51% power. The adjusted frequency for a new airflow target is: New Hz = Base Hz × (New CFM ÷ Base CFM). These laws apply to fans operating within the same system, and they assume no change in system resistance. During system rebalancing, the fan affinity laws provide a reliable way to predict the new frequency setting without trial and error.

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