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AC Motor Duty Meter For Excavating Machines: Feasibility Study

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Published: October 13Project Number: C21040

Get ReportAuthor: Galina Mirzaeva | The University of Newcastle

The Australian mining industry has started to adopt AC-driven electrical excavators, draglines and electric rope shovels. This transition is expected to continue and possibly accelerate, necessitating development of condition monitoring tools for AC machines in digging applications.

 

A number of AC motor condition monitoring tools are available off-the-shelf. They are based on off-line or on-line measurement of external parameters, typically vibration or stator current, in order to detect signature harmonics indicating faults.

 

While this approach has proven its ability to detect faults, sensitivity of externally measured parameters is not always sufficient to capture a fault at an early stages of development. Nor do the existing methods provide an accurate information on how long a motor may operate until it fails.

 

Additionally, most of the existing methods require significant intervals of steady state operation to be able to detect faults, but in dynamic applications, such as digging, motors almost never operate in steady state. Finally, signature harmonics indicating faults are not well pronounced under inverter control due to current regulator action and injection of switching noise.

 

These recognised difficulties in existing fault diagnostic tools have inspired the current project.  The Duty Meter approach is radically different from existing techniques in that, rather than looking for signs of faults that have already developed, it predicts how a fault is developing in a healthy motor based on the motor duty.

 

Along with signals measured externally to the motor, such as stator current and leakage flux, we use main flux density signals measured inside the motor air gap. This approach was prompted by its utilisation in an earlier investigation of DC motor fault mechanisms. Our continuing partnership with the motor OEM has made it possible to include the installation of the miniature flux density sensors inside the motors during their manufacturing.

 

This report summarises findings of the first stage of the project, namely, of a feasibility study of the AC Motor Duty Meter for excavating machines to prove the feasibility and advantages of the Duty Meter approach, using a laboratory size prototype AC motor.

 

The project describes the motor and other hardware used in the study; provides a brief overview of the existing fault diagnostic methods; and describes the tests completed within the fault study. It further explains how each of the AC motor faults was introduced to the test motor and then detected; and what advantages the proposed methodology has over the existing methods with respect to each fault.

 

Four main types of AC motor faults have been studied: winding insulation faults; rotor bar faults; eccentricity faults; and bearing faults. A large number of illustrations are included throughout the report to assist in understanding of its content. The main findings of the study and the next steps in the development of AC Motor Duty Meter development are summarised in the Conclusions.

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