Monitoring of electric motors and industrial frequency converters

Electric motor monitoring is one of the predictive maintenance technologies that has undergone the most significant evolution in recent decades, surpassing innovation with RMS (Root Mean Square), which is the instrumentation used to obtain an effective value of alternating current electrical measurements, as well as voltage measurements.

It is important to note that voltage and current signature analysis is now the basis for modern instrumentation, as it has greater visibility in applications such as industrial frequency converters without the need for proximity to an engine.

This technology provides the ability to monitor electric motors online while they are operating, as well as control and monitoring of frequency converters, through a transparent sensor that produces shaft torsion signatures, thus enabling control of the motor cabinet and advanced RCM (Reliability Centred Maintenance) to diagnose voltage and system conditions.

Previously, electric motor maintenance was carried out by professionals in the field using a megohmmeter in one hand and a digital multimeter in the other. However, with insulation tests performed at lower voltages twice, the nominal voltage showed that this practice does not meet professional standards.

As it turns out, online electric motor monitoring consists of accurately assessing a motor's load from the electrical system, motor terminals, voltage sensors, and proper analysis of the signals obtained. This approach to load monitoring not only shows the precise steady-state speed and operating torque, but also provides visibility of the instantaneous torque on the shaft.

Now, how is this information useful to us? These data are invaluable when determining which problems may be caused by mechanical or electrical faults. On the other hand, we have the variable frequency drives which are widely considered viable alternatives to direct current units for all sectors of industry.

It has been demonstrated that the energy savings in pumping systems through the use of variable speed drives and frequency is significant, which is why these units are often more attractive, because they not only reduce maintenance requirements, but also improve control and have the potential to increase system efficiency.

However, the challenge they present is to achieve monitoring of electric motors, due to a shortage of available technology that maintenance professionals can utilise. The variable frequencies and voltage levels present with the use of industrial frequency converters created a challenge for both standard field instrumentation and the demanding assessments of field personnel.

This article conveys what can be expected from testing and using modern instruments to monitor electric motors online, recognise the most frequent problems and the impacts they can have on this equipment. The idea is to show you how a wealth of information can be obtained using modern instrumentation to troubleshoot systems.

Frequency converters and electric motors can be accurately monitored, regardless of their implementation as frequency, speed, or torque control devices (whether in open-loop or closed-loop vector control applications). Steady-state, transient, and dynamic frequency converter functions can be adequately monitored with capable portable instrumentation.

Demonstration of performance achieved by monitoring electric motors in a controlled environment

To investigate the capabilities of a modern online electric motor performance monitor, it was necessary to operate industrial-sized motors in a controlled environment. A motor load capacity test was performed on the laboratory test benches, which consisted of creating an emulated motor load application based on voltage level, voltage distortion, and voltage imbalance ramps independently.

Therefore, sufficient dynamic control of the frequency converter operation was required to evaluate the capabilities of these online units and the monitoring of online electric motors. To verify this, a 300 hp four-quadrant vector drive dynamometer was used under various operating conditions designed for this purpose.

The online monitoring instrument was connected to the vector drive conductors with current and voltage clamps, regulating both the load and the positive and negative direction of rotation, with variable frequencies and amplitudes, including changes in torque deviation.

These tests and diagnostics have rendered the dynamometer almost obsolete for successful predictive maintenance programmes, demonstrating instead that estimation techniques are becoming a useful and intuitive tool for the evaluation and online monitoring of electric motors.

Obtaining the effective values of shaft voltage through online monitoring of electric motors is of utmost importance for industries if we want to reduce maintenance times and the costs associated with this activity, since the voltage of a sine wave varies over time and is therefore not equal to the voltage reached at its peaks.

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