019 Measurement system for diagnosis of rotor cage defects in induction motors

Authors: Maciej Sułowicz, Arkadiusz Duda,Dariusz Cholewa, Jarosław Tulicki, Mirosław Czechowski, Jakub Zielonka

Development of a measuring instrument for diagnostic evaluation of electric motors for the implementation of the task titled “Research and Development Center for Electric Machines” at the Department of Electrical Machinery of EMIT S.A.

Diagnostics of electrical machinery is a key issue to ensure continuity in the operation of industrial units. Proper operation of machines results in increased service life and reliability. The concept of the project was born during the analysis of the market for dedicated diagnostic instruments. The main task of the instrument is diagnostics of induction motors – rotor cage damage. According to statistics, these damages are among the most common that occur in induction machines. The concept was to build the entire instrument in one robust industrial box. Robust and waterproof enough for industrial conditions, but also relatively lightweight so that the whole thing could be handled and transported by a single person. The result was a mobile diagnostic device, including a computer, measurement card, batteries, Rogowski Coils and current probes. A proprietary application was developed, which was eventually named CAGELab. The application fully executed in the MATLAB environment. However, the customer does not need to know the MATLAB environment or have a license for it. The developed application was divided into two functional modules: Database Module and Measurement Module. The first module is responsible for storing data about the machines under test and the diagnostic measurements made for them, and for generating a PDF report containing graphs, rating parameters, descriptions and diagnostic evaluation based on legal standards. The Measurement Module is responsible for performing signal acquisition and then analyzing measurements taken at steady state or during machine start-up. Stator current recordings for the aforementioned types of rotor cage condition tests can be performed in the switchgear (by recording the current in the secondary circuits of current transformers using current probes) or at the motor (using Rogowski Coils hooked up to the power wires at the point where the wires are not shielded). To assess the condition of the rotor cage during steady state, the so-called RFI (Rotor Fault Index) is used, while to diagnose cage damage during start-up, the DRFI (Dynamic Rotor Fault Index) is used. These indices can be referred to the relevant standards and based on them, the condition of the rotor cage can be initially determined.

The prepared diagnostic instrument complies with the following Standards:

1. PN-EN 61010-1:2011 Safety requirements for electrical measuring instruments, automation and laboratory equipment — Part 1: General requirements
2. PN-EN 50191: 2011 Installation and operation of electrical test benches
3. PN-EN 61000-6-4:2019 Electromagnetic compatibility (EMC) — Part 6-4: General standards — Emission standard for industrial environments
4. PN-EN 61000-6-2:2019 Electromagnetic compatibility (EMC) — Part 6-2: General standards — Immunity in industrial environments

The developed diagnostic tool has the functionality that is demanded by the current industrial market.