Diagnostic systems for electric motors: What are they and why are they necessary

Diagnostic systems for electric motors are the complex of electrical, static and dynamic measurements, used for monitoring the health of an electric motor, for detecting any faults and for the preventive management of malfunctions. Let us see what static and dynamic measurements are and what they are for, what parameters they analyze and the impact of preventive management of malfunctions on the performance of the electric motor.

Static electrical measurements

Static electrical measurements verify the characteristic parameters of an electric motor through cold tests of the machinery, i.e. tests performed on the component that is not in operation. These measures accurately assess the state of operation of the engine and its components, according to different purposes:

• Quality Assurance (QA), quality control following the engine overhaul;
• Trending monitoring, monitoring and evaluation of the state of health of the engine over time;
• Diagnostic troubleshooting, analysis of malfunctions in specific engine components – in-depth analysis of problems detected on the engine.

The measuring devices are generally battery powered in order to autonomously generate the desired voltage / amperage in the electric machine:

• the measurement system applies a standard voltage from the outside;
• the transducers of the measuring equipment detect, directly or indirectly, the characteristic values ​​of interest, such as ohmic resistance, inductance, electrical capacity, each with its own resolution;
• the processing and analysis software performs a normalization of the measurements and compares the measured values ​​with the threshold values, in order to evaluate the correct functioning of the electrical machine.

Through static measurements and their comparison with characteristic threshold values, it is possible to evaluate many ways of failure of electric motors, here are some examples of measurements:

• the insulation resistance of the windings;
• the polarization index and the leakage currents
• the electrical capacity;
• the ohmic resistance of the individual phases;
• the inductance of the individual phases;
• the characteristic parameters of the earthing.

By collecting this information, the operator is able to assess the occurrence of any malfunctions on the electric motor and resort, if it is convenient, to targeted scheduled maintenance.

Dynamic electrical measurements

Dynamic electrical measurements are essential for a less invasive electric motor diagnostics, which allows the component to remain in operation during the procedures for acquiring measurements useful for diagnostics and failure prevention. They can be performed on three-phase or direct current motors.

Dynamic measurements can be performed on the running motor, requiring the component to stop only if anomalous values ​​are detected that require more accurate control and which can also include static electrical measurements.

Furthermore, the dynamic measurement operations exploit the monitoring of parameters useful not only for the prevention of faults, but also for the evaluation of the efficiency of the engine.

For three-phase alternating current (AC) motors, dynamic electrical measurements can provide useful information to identify potential faults and to ensure good motor efficiency, here are some examples of standard measurements:

• waveforms of voltages and currents
• spectral analysis of the absorbed currents
• measurement of active, reactive and apparent power
• absolute values ​​of voltage, current and power
• THD, HVF parameters, etc.

Static and dynamic electrical measurements: why do both?

An integrated control system, which provides in parallel the static and dynamic measurements, guarantees significant benefits to the operation of the motor, in fact some failure modes can be accurately detected using both types of measures described. Here are some benefits regarding these measures:

• reduction in the number of stops due to engine failures thanks to early diagnosis
• reduction of engine repair / replacement costs, being able to accurately identify the repair to be carried out
• Evaluation of the quality of electro-mechanical reviews
• continuous performance control, through dynamic measurements on the running engine
• complete diagnostics on all motor fault areas: power quality, power supply circuit, insulation, stator, rotor, air gap.