On steady-state performance estimation of three-phase induction motors

In industry, more than 85 % of the used motors are induction motors, because of two main reasons: rugged in construction and low cost when comparing it to other motors. Recently, increasing higher efficiency interests and protective maintenance trends lead the on-line performance monitoring of induction motors. If it is intended to monitor performances of all motors, this can be only done by using cheaper sensors and by processing the captured information to provide a performance monitoring calculation in a central unit. For this reason, there are multiple types of research on the performance monitoring devices, which are using LTE/4G IoT technology. It is possible to estimate the motor performance by means of captured terminal quantities and motor parameters. The aim of this thesis is to define input and output steady-state performance values of an induction motor via information from simple terminal voltage and current sensors. Since motor parameters are required to estimate performance, by on-line and off-line techniques can determine the motor parameters. By conducting some fundamental tests (no-load, and blocked-rotor), the motor equivalent circuit parameters can be defined. The target of this study is to predict the performance of an induction motor by using only terminal quantities. The experimental work and estimation calculations are applied for a 4-pole, 2.2 kW three-phase squirrel cage induction motor. First, the fundamental tests such as no-load and blocked-rotor tests are conducted and the performance of the motor is monitored experimentally. Then corrected no-load test is conducted to find the more accurate value of core loss resistor and frictional losses. So motor equivalent circuit parameters and motor heating are defined. By using VisSim software, motor input impedance calculation and power distribution algorithms are formed and variation of the input equivalent impedance with the motor slip is obtained. A basic algorithm is developed to calculate the input powers, i.e. input apparent, active and reactive powers from the terminal voltage and current values. In this study, a simple method that is based on total equivalent impedance of T-equivalent circuit is proposed as a scientific contribution. The essence of the method is to find out the relation between equivalent impedance and slip. To extract the slip from the impedance expression is required a substantial of a burden of calculation. Thus, this problem is solved by using a quadratic equation of equivalent impedance that is a function of slip. A MATLAB curve fitting algorithm is used to define input impedance-slip variation with a quadratic equation. The coefficients of the quadratic equation are depending on the motor equivalent circuit parameters. These coefficients can be obtained from the manufacturer or found by means of fundamental tests. So all these calculations including the terminal powers and slip can be done via a microprocessor with a simple code. By adding frictional and stray losses, the output power and shaft torque can be defined. By doing so, the efficiency and loading can be monitored on-line. In the thesis, the tested and calculated results are in agreement. Therefore, it is shown that the performance of an induction motor can be estimated by using only terminal measurements and simple calculations. For future work, it will be searched the ways of determining motor parameters during operation and without implementing any tests. Moreover, the temperature dependency of the related parameters will analyze thoroughly.