Sensors are an important tool to monitor the condition of electrical and rotating machines. They are available in different types and sizes. ‘Which sensor is ultimately the most suitable depends on the critical properties within the production unit and the failure modes of the machine,' says dr. Bram Corne, founder of Orbits. He supports companies and gives advice on signal and data processing of electrical or mechanical systems and processes with a general focus on condition monitoring of electrical machines. Corne did extensive doctoral research at Ghent University on condition monitoring of electrical rotating machines based on electrical current measurements.

“Electric machines use about 65 to 70 percent of the electricity generated worldwide. Industry uses the majority of these machines. They, therefore, play a crucial role in the production process of many companies. In recent years, much research has been carried out into monitoring such as monitoring the condition of these critical assets. Because in the event of unexpected failures, the costs can quickly increase considerably.”

Several techniques have been investigated, such as measuring temperature using temperature sensors. “If a part in a machine does not function as it should, this often leads to overheating. This is not always accurate at the component level, but can sometimes be sufficient to schedule a maintenance intervention.”

Another, more advanced technique is measuring vibrations. “If the measured vibration patterns of a machine deviate from the zero measurement (which is considered normal, ed.), this can indicate, for example, a bearing problem, an imbalance or misalignment.” Another method used is current analysis. “Based on measured current, both potential mechanical and electrical problems are revealed.”

Detecting both mechanical and electrical problems

Corne focuses on the differences between monitoring the condition based on vibration and current sensors. “In current analysis, the electrical machine is used as a sensor. It is possible to determine the nature of the problem when anomalies are detected because many causes of failure leave a specific fingerprint of failure in the current spectrum.”

Current measurement can determine both mechanical and electrical problems. “This is the big difference with vibration sensors. In vibration analysis it is often impossible to detect electrical problems or the problem is detected at too late a stage,” says Corne. “Whenever a motor experiences the start of an electrical failure, the deviation should result in such a strong force to actually induce mechanical movement of the machine. Only when the failure causes significant movement of the stator housing can the problem be detected vibration analysis. When monitoring the current, this problem can quickly be spotted before collateral damage is developed”. Current measurement can therefore often fail at an earlier stage.

Detecting electrical problems using current sensors have been going on for quite some time. In recent years, the detection of mechanical problems using these sensors has grown considerably. “In the past, it was very challenging to accurately determine the severity of mechanical problems. Mechanical problems are spotted in the current due to unique variations in the air gap between the rotor and stator (e.g., due to a bearing failure). This change can be detected in the current, but the electrical phenomenon must be linked to the severity of the mechanical problem. A company that installs a condition monitoring system wants to know exactly where it stands at a particular moment in time. (can’t end the sentence with when, missing text?)

Suppose that the detection system detects a bearing problem in the electric motor. The first question a company asks itself is: How much time do I have before a fault occurs? A year or a few days? If he knows the answer to this question, he can plan the maintenance, strategically. It is therefore very important to link the severity of the mechanical damage with the severity is reflected in the stator current. In recent years, an increase in modelling power and accumulated knowledge have made much better connections and estimates. Current sensors are consequently suitable for detecting both mechanical and electrical faults at an early stage.”

Installation of sensors

Another big difference between vibration and current sensors is the location of the sensors. “An advantage of current over a vibration analysis is that it is not necessary to carry out the measurement on site at the motor. Current measurement is possible from inside the motor control cabinet or at a central location. This makes installation easier, the environment is safe, clean and accessible, and it is easy to install. This enables this technique to have an advantage on motors installed in harsh environments (blast furnaces, cryogenic applications, submersible pumps, wind turbines, etc.)”. Laying an internet cable or amplifying a Wi-Fi signal is also relatively easy to conduct in a central location.

The flexibility of vibration sensors

On the other hand, vibration sensors are flexible. Corne: “It is possible to place a vibration sensor on almost every component to measure their condition, no matter how close or far away this component is from the engine.

When measuring with current sensors, the focus is only on the motor. It is only possible to identify the electrical and mechanical problems of the motor and the systems directly connected to the motor. Components that are very far away from the motor are more difficult to monitor with current sensors.”

Current sensors combined with machine learning models provide the solution here. Through employing machine learning, anomalies or irregularities that would not be visible to a trained analyst can be detected. It shows when a pattern deviates from what it usually is, no matter how small the effect. “Thanks to the built up historical database of a machine, we can use a kind of fingerprint of the machine as a frame for reference. For example, as soon as the machine consumes a little more than before in the same load condition, the monitoring system generates an alarm. This small change in engine operation can, therefore, indicate a mechanical problem that occurs far inside the drivetrain. Through additional tests or inspections, you can then very specifically locate the causal error. The more knowledge you build up in this way, the more input there is to make the next current measurements more accurate. In this way, the system becomes smarter and smarter”.

Choice depending on asset and failure mechanisms

Both vibration and current sensors have advantages and disadvantages. The most suitable choice depends on the situation in which the drive is located. “Companies often have sufficient knowledge about the history of the most critical machines. They know from the past where and how the failure mechanisms occur that have a negative influence on the activity. If a component that often fails is far away from the engine, then perhaps a vibration sensor on the component is the best option. If a failure mechanism is often in or around the motor itself, or if the electrical components play a critical role, current sensors are the best option for condition monitoring.”

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