What’s the ideal type of smart sensor for your industrial assets?
Sensors are an important tool to monitor the condition of electric motors 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. The company provides industrial clients with training and advice on using signal and data processing to diagnose faults in electrical and mechanical systems. Corne did extensive doctoral research at Ghent University on condition monitoring for electric rotating machines based on electrical current measurements.
“Electric machines use 65 to 70 percent of the electricity generated worldwide,” Corne notes. “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 on monitoring the condition of these critical assets, because in the event of unexpected failures, the costs can quickly skyrocket.”
Several techniques have been investigated, such as measuring temperature with sensors. “If a part in a machine isn’t functioning as it should, it often leads to overheating,” says Corne. “This isn’t always accurate at the component level, but can sometimes be sufficient to schedule a maintenance intervention.”
Another, more advanced technique is measuring vibration. “If the measured vibration patterns of a machine deviate from the baseline measurement, this can indicate, for example, a bearing problem, an imbalance or misalignment,” says Corne. Another method used is current analysis, Corne’s specialization. “Both potential mechanical and electrical problems are revealed by measuring current.”
Detecting both mechanical and electrical problems
Corne focuses on the differences between monitoring a machine’s condition based on vibration and current sensors. “In current analysis, the electrical machine is used as a sensor,” Corne explains. “It’s 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 identify both mechanical and electrical problems. “This is the big difference with vibration sensors,” Corne says. “In vibration analysis it’s often impossible to detect electrical problems, or the problem is detected at too late a stage. When a motor starts to experience an electrical failure, the deviation needs to produce such a strong force that it actually induces mechanical movement in the machine. Only when the failure causes significant movement of the stator housing can the problem be detected by vibration analysis. When you monitor the current signal, this problem can quickly be spotted before collateral damage develops.” Current measurement can therefore often detect upcoming electrical failure at an earlier stage.
The use of current sensors to detect electrical problems has 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,” says Corne. “Mechanical problems such as bearing failure, for example, are spotted in the current through unique variations in the air gap between the rotor and stator. This change can be detected in the current, but that 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.
“Suppose 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 it knows the answer to that question, it can strategically plan maintenance. That means it’s vital to link the severity of the mechanical damage with the severity reflected in the stator current. In recent years, increases in modeling power and accumulated knowledge have enabled much better connections and estimates. As a result, current sensors are now suitable for detecting both mechanical and electrical faults at an early stage.”
Another big difference between vibration and current sensors is their location. “An advantage of current over vibration analysis is that it’s not necessary to carry out the measurement on site at the motor,” Corne says. “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. That gives this technique an advantage for equipment installed in harsh environments, such as blast furnaces, cryogenic applications, submersible pumps, wind turbines, and so on.” A central location also makes it easier to laying an internet cable or amplify a Wi-Fi signal for the data transfer from sensors to the analysis platform.
The flexibility of vibration sensors
On the other hand, vibration sensors are flexible. “It’s possible to place a vibration sensor on almost any component to measure its condition, no matter how close or far away this component is from the motor,” Corne says. “Current sensors place the focus on the motor. It’s only possible to identify electrical and mechanical problems in 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.”
The solution here is to combine current sensors with machine learning models. By using artificial intelligence, anomalies or irregularities that would not be visible to a trained analyst can be detected. Machine learning will detect any deviation from the normal pattern, no matter how small the effect. “Thanks to the accumulated historical database for a machine, we can use a kind of machine fingerprint as a reference frame,” Corne explains. “For example, as soon as the machine consumes a little more power than before under the same load condition, the monitoring system generates an alarm. This small change in motor 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 equipment is located. “Companies often have sufficient knowledge about the history of their most critical machines. They know from the past where and how the failure mechanisms occur that have a negative influence on operations. If a component that often fails is far away from the motor, 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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