# Fan Law

The fan law, or affinity law, states that the power used by an electric motor to rotate a pump or fan is proportional to the cube of its rotational speed. This 3 minute video explains the fan law and its effect on data center power use.

### Data Center Example

Lets say we have a 15 horsepower motor that is powering a fan for a CRAH unit.  That motor will draw ~10 kW of power at full speed.  If we added a VFD to that fan, we can control its speed.  Figure 1 shows the power draw for that unit as its speed is reduced.

You can see at 80% of fan speed, the power is ~5 kW.  As I described in the video, this is because the power decreases very quickly as the fan speed decreases.  A good rule of thumb is that you can save 50% energy by reducing fan speeds by 20%. This is why it is so attractive to add variable speed capabilities to existing fans.  See my post on EC Plug Fans to learn more ways to save on fan energy in a data center.

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