Affinity laws When the speed changes, flow rate, head and power of the pump curve shift according to fixed power laws – the basis of every design with a frequency converter.
Input (Point on the pump curve at n₁)
min⁻¹
min⁻¹
m³/h
m
kW
Result (n₂/n₁ = – %)
| Flow rate Q₂ | – m³/h |
| Head H₂ | – m |
| Power input P₂ | – kW |
–
Q₂ = Q₁ · (n₂/n₁)H₂ = H₁ · (n₂/n₁)²P₂ = P₁ · (n₂/n₁)³
| Reference | Technical book, chap. 3.4.5 · Affinity laws |
|---|---|
| Limits | Valid for constant efficiency and geometric similarity. With a large speed reduction the efficiency drops; below 50 % check the motor cooling. |