SOURCE-ALIGNED CALCULATION WORKSPACE
Pump Affinity Laws & VFD Calculator
Estimate how centrifugal-pump flow, head and power change when rotational speed changes, then verify the actual operating point against pump and system curves.
C6 · VALIDATED
01Base pump point
02Target speed
The cached workbook example uses a 0.8 speed ratio; confirm the source reference speed before using a converted duty point.
ENGINEERING REFERENCE
Pump affinity laws and VFD calculator
Compare a pump operating point at two speeds using the documented affinity relationships for flow, head and power. The speed ratio and target values make the effect of a VFD change easy to review alongside the original duty point.
Affinity-law results are an engineering estimate for the same pump and operating regime. Check the manufacturer performance curve, efficiency change, minimum flow and control limits before applying a speed change in service.
Pump Affinity Law Equations
For the same centrifugal-pump geometry under the assumptions represented by the affinity laws, flow varies approximately with speed, head with the square of speed, and power with the cube of speed.
Q₂ / Q₁ = N₂ / N₁; H₂ / H₁ = (N₂ / N₁)²; P₂ / P₁ = (N₂ / N₁)³.
How RPM Changes Flow, Head and Power
A relatively small speed change can have a much larger effect on head and especially power. The final operating point still depends on where the changed pump curve intersects the actual system curve.
Limits of Affinity-Law Predictions
Affinity-law results are not a substitute for manufacturer pump curves, efficiency curves, NPSHr data or the installed system curve.
Frequently Asked Questions
Does pump head change linearly with RPM?
No. Under the standard speed relationship, head varies approximately with the square of speed.
Why does pump power change quickly with speed?
Under the standard affinity relationship, power varies approximately with the cube of speed.
Do I still need the manufacturer pump curve?
Yes. Use it for final performance, power, efficiency and NPSH review.