Accuracy study of frequency converter estimates used in the sensorless diagnostics of induction-motor-driven systems

Modern frequency converters allow energy efficient operation of induction-motor-driven systems such as pumps and fans. In addition, they can be used as a sensorless monitoring unit for the pump and fan systems, as shown by previous studies. Sensorless diagnostics of induction-motor-driven systems relies on the accurate estimation of the rotational speed, shaft torque and shaft power of the motor, which is why all frequency converters are not suitable for this use. In this paper, accuracy of the rotational speed, shaft torque and shaft power estimates is studied by laboratory tests for two different frequency converters. Using the test results, estimation accuracy of a vector- and a direct-torque-controlled frequency converter are compared with each other. The effect of erroneous motor estimates on the pump flow rate estimation is also demonstrated in the paper.