A variable frequency drive on a fan: where the savings actually come from

A variable frequency drive does not save energy by itself. It simply allows motor speed to be varied. Savings arise when the system genuinely does not need full airflow and the speed reduction is consistent with its function.

Why a 20% speed reduction can produce a larger reduction in power

For a given fan under similar operating conditions, with constant air density and broadly similar efficiency, the affinity laws apply: airflow varies in proportion to speed, pressure to the square of speed, and mechanical shaft power to the cube of speed.

L₂/L₁ = n₂/n₁; Δp₂/Δp₁ = (n₂/n₁)²; P₂/P₁ = (n₂/n₁)³

At 80% speed, this gives approximately 80% airflow, 64% pressure and 51,2% shaft power. Electrical input also depends on the motor and drive.

An example in kilowatt-hours

A hypothetical fan consumes 2,5 kW at full output. For illustration, assume constant overall efficiency and apply the cubic relationship to electrical power.

P₂ = 2,5 × 0,8³ = 1,28 kW

The difference is 1,22 kW. Over 2500 hours in this mode, the saving is 3050 kWh. Adding 500 hours at full speed gives a reduction in annual consumption from 7500 to 4450 kWh, or approximately 40,7%. This is the result of a specific model for one fan, not a promise of savings for the entire ventilation system or building.

When the cubic relationship is misleading

It works well for a ductwork system where losses mainly vary with the square of airflow and there is no significant constant pressure component. If the controls maintain an unnecessarily high constant pressure, dampers change position or efficiency falls substantially, the actual relationship will differ.

It is then more useful to calculate from operating parameters: electrical power depends on volumetric airflow, pressure and overall efficiency.

P_el = qᵥ × Δp / η_total

What to check before reducing speed

Determine the permitted airflow in each mode, the supply/extract air balance, critical branches and equipment requirements. Check motor limits, fan stability and heater operation.

After changing the operating mode, check that remote rooms still receive the required air and that noise has not shifted to the control dampers. The power on the motor nameplate is not the same as actual consumption at a particular operating point.

Variable speed control delivers savings when lower consumption matches a genuinely lower demand, and system operation confirms that match.