Motors
Motor Sizing for Centrifugal Pumps
Motor sizing for centrifugal pumps is more than calculating power at the rated point. A correctly selected motor must cover the actual pump power demand across the required operating range.
Basic pump power
Pinput = ρ · g · Q · H / η
Rated point is not enough
The motor must often cover power at rated point, maximum impeller, end-of-curve or specified operating range. Some pumps have rising power curves toward runout.
What affects motor size?
- flow and head,
- liquid density,
- pump efficiency,
- maximum impeller diameter,
- end-of-curve power,
- service factor,
- starting method and power supply limits,
- VFD operation,
- ambient temperature and area classification.
Common engineering checks
- rated power vs motor nameplate power,
- non-overloading requirement,
- service factor margin,
- starting current limitation,
- torque during acceleration,
- temperature class and ATEX/IECEx requirements,
- VFD compatibility.
Practical recommendation
For procurement, always ask vendor for power curve over the full operating range and clarify whether offered motor covers rated point only or full curve with maximum impeller.