Motor Adjusted Frequency

A variable frequency drive (VFD) changes motor speed by changing the frequency of the power it supplies. An induction motor's speed is proportional to that frequency, so a motor rated 1,760 RPM at 60 Hz runs at about 1,467 RPM at 50 Hz and 880 RPM at 30 Hz. For a direct-drive fan the fan turns at motor speed, so the frequency for a target fan speed is: Hz = Motor Hz × (Fan RPM ÷ Motor RPM). For a belt-driven fan the sheaves set a fixed ratio between motor and fan speed, so use the fan's speed at full line frequency in place of the motor RPM. Because airflow is proportional to fan speed, the same ratio also gives the frequency for a new airflow target: New Hz = Base Hz × (New CFM ÷ Base CFM). Pressure and power then follow the fan affinity laws, which is why slowing a fan with a VFD saves so much energy. Most drives also have minimum and maximum frequency limits, and running a standard motor well below about 20 Hz can overheat it without an inverter-duty rating.