What do the fan laws tell me about changing fan speed?
The fan laws are proportionalities that hold for the same fan, same air density, same operating point on the system curve: flow scales with speed (N), pressure with N², and power with N³. They are the fastest sanity check in any fan review — and they explain why small speed increases are expensive.
The three laws with a worked example
Take a fan running 3,000 rpm and raise it 20% to 3,600 rpm. Flow rises 20%. Pressure rises by 1.2² = 1.44, i.e. 44%. Power rises by 1.2³ = 1.728, i.e. nearly 73%. The same math runs in reverse: cutting speed 20% drops flow 20% but power about 49% — which is why variable-speed control saves energy in part-load applications.
| Change | Flow (Q) | Pressure (P) | Power (W) |
|---|---|---|---|
| N × 1.2 | +20% | +44% | +73% |
| N × 0.8 | −20% | −36% | −49% |
| N × 0.5 | −50% | −75% | −87.5% |
Where the laws apply — and where they break
The laws assume the system resistance curve stays the same and density is constant. They apply to the same fan geometry (not a swapped impeller) and to points that move along the same system curve. They do not apply across different fans, across density changes (altitude, temperature), or when the operating point crosses into stall.
- Use them to predict a valid speed change on the same installed fan.
- Do not use them to compare two different fan models — read each P-Q curve instead.
- At altitude or high temperature, correct for density first, then apply the laws.
Practical consequences for design reviews
If a thermal review says the design is 5°C over target, the naive fix — run the fan 20% faster — multiplies input power by ~1.7 and typically pushes current and connector ratings. Check the supply rail, the MOSFET/driver rating and the tach range before committing. The cheaper fix is often reducing system resistance: bigger inlet holes, shorter filter path, or a less restrictive heat sink.
Send MEGA Tech your airflow and static pressure target, voltage, size limit, control interface, quantity and environment. We reply with a selection proposal and a sample plan — selection by operating condition, samples validated on your unit.
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