Take VFD losses into consideration.
Hydraulic Institute
11/29/2018
How can I measure the power consumed by a pump in the field to compare to the OEM’s curve? Monitoring a pump system’s performance through power measurements is a great way to evaluate the condition of equipment over time. Pump manufacturers typically publish the power required by the pump as a function of flow rate at a given rotational speed, which does not incorporate losses from other components. The input power to the pump is determined by measuring the torque and rotational speed at a given flow rate, or by measuring the electrical input power to a motor that has been characterized so that the motor input power can be correlated to motor output power (pump input power). Image 1 shows a typical performance curve.

- Measure the pump shaft’s torque and rotational speed.
- Measure the motor electrical input power and incorporate the losses of the motor. If a variable speed drive or other equipment is used in the system, the losses of these components need to be considered.


- Is the process water-like?
- If the process fluid is not similar to the water test, the liquid density must be corrected. If the viscosity differs from the manufacturer’s test, the head, flow, efficiency and pump input power will be affected. ANSI/HI 9.6.7 Rotodynamic Pumps – Guidelines for Effects of Liquid Viscosity on Performance should be consulted.
- Is the in-situ test speed the same as the manufacturer curve?
