Estimation accuracy of a vector-controlled frequency converter used in the determination of the pump system operating state

Pump, compressor and fan systems often have a notable energy savings potential, which can be identified by monitoring their operation for instance by a frequency converter and model-based estimation methods. In such cases, sensorless determination of the system operating state relies on the accurate estimation of the motor rotational speed and shaft torque, which is commonly available in vector- and direct-torque-controlled frequency converters. However, frequency converter manufacturers seldom publish the expected estimation accuracies for the rotational speed and shaft torque. In this paper, the accuracy of these motor estimates is studied by laboratory measurements for a vector-controlled frequency converter both in the steady and dynamical states. In addition, the effect of the flux optimization feature on the estimation accuracy is studied. Finally, the impact of erroneous motor estimates on the flow rate estimation is demonstrated in the paper.