On September 27 at 11:30 a.m. in the Parque Barigui room, engineer Fabio Toshio Nakatani will give a presentation on: Hybrid analysis (man+technology) in predictive maintenance of rotating electrical machines. Discussing Industry 4.0 in the area of maintenance in relation to industrial automation and the integration of different technologies. Artificial intelligence, robotics, neural networks and big data with the aim of improving processes and increasing productivity and reliability. But where does this technological evolution leave the role of man (specialist)? How will integrated work between technology and specialists increase the degree of reliability of rotating electrical machines?

Register at https://cbmga.abramanoficial.org.br/ and visit the Nishi stand during your participation in the country’s main Maintenance and Asset Management event.

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What is corrective maintenance?

Still very present in companies, corrective maintenance is only used when some equipment breaks down or fails. This method does not use maintenance planning, which leads to the need for parts inventory and availability of professionals, since the failures are not foreseen and cause the idleness of machines and labor.

For example, a machine seems to be in perfect condition and in constant production, but at some point a part presented a failure and the production had to be stopped. Since there was no planning, the maintenance professional, before correcting it, needs to understand the problem, check if the part can be replaced and if there is in stock. This situation will cause the company to make the machine unavailable, which will generate costs per machine stopped, part that needs to be replaced, and labor costs.

When performed without planning, corrective maintenance generates huge inconveniences, compromising the fulfillment of deadlines and especially operational profitability.

What is preventive maintenance?

When we talk about maintenance planning, one of the best paths to follow is certainly the preventive maintenance. It is the maintenance that corrects problems and failures even before they happen, thus reducing costs, machine unavailability, and increasing maintenance efficiency. Its main characteristic is the possibility of scheduling maintenance, avoiding problems with parts inventory, idle machines, and lack of production.

The tip we leave in this maintenance is to focus on the expected result, to evaluate the condition and operational reliability of the equipment, and not to focus only on the execution of the tasks pre-established by the planning. In an electrical machine, the electrical and mechanical tests and visual inspection are conditions to analyze its results and from there make a critical analysis and conclude potential problems and the degree of reliability that it is, that is, for this the presence of an electrical machine specialist is essential.

In the market we also commonly hear the term “rejuvenation” for preventive maintenance, which can lead to a misinterpretation. There is no way to rejuvenate what is already aging. We can prevent accelerated aging.

What is predictive maintenance?

It is safe to say that predictive maintenance is a step ahead of preventive maintenance. It is the most accurate follow-up and planning maintenance, requires greater qualification of the professional and aims to prevent any failure of parts before they even present a defect. It is characterized by monitoring equipment parameters, such as partial discharges, vibration, temperature, etc.

Predictive maintenance uses the monitoring of the parameters of the parts so that their replacement can be done before the end of their useful life. By inspecting the machine, the maintenance professional – based on the monitoring parameters – identifies the need to change components and schedules the best moment for maintenance, which is done without intervention in production, facilitating maintenance and avoiding higher costs with machine idleness.

What company wants to stop its production because of a failure or defect? Very similar to preventive maintenance, predictive maintenance has as a positive point the possibility of programming its actions. Planning and follow-up of the parts are important factors, which working together, reduce costs with losses and increase the effectiveness in the production of the machines.

NISHI offers solutions for all types of maintenance. Contact us and request an evaluation of reliability and operational warranty of your electric machine.

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In machines that work at high temperatures (close to the temperature limit of the insulating material), the insulation between the plates deteriorates over time, causing the appearance of eddy currents or “eddy currents”, which circulate between the plates of the core that are shorted. The temperature at this location can often be 30 to 40ºC higher than the average core temperature.

The test commonly used to detect possible “hot spots” is the “Loop Test”. We wind coils around the magnetic core where the number of turns is calculated to achieve a flux similar to the nominal flux. We leave the core fed until its temperature stabilizes, and by thermovision we evaluate the temperature differences.

If one or more points are identified where the temperature variation is above average (10ºC or more), these points must be investigated and/or repaired. The repair is done by completely disassembling the core, stripping the insulation material, applying the new insulation, and assembling it.

Learn more about the services provided by Nishi and contact one of our experts for information about maintenance and standards applied to electrical tests of rotating machines.

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A common first question is what determines motor life? Is it determined by the lifetime of the insulation material? By the wear period of the bearings? By the deterioration of the steel? Others?

When we “explode” a motor and stratify its components we see in its composition basically steel, copper and insulating materials. When comparing the materials, we conclude that the insulating material is the most fragile in relation to the others. Is it correct to say that the life span of the motor is determined by the life span of the insulating material? What about the bearings that wear out over time, is it correct to say that the end of their life is the end of the motor’s life?

With good maintenance practices, ensuring the integrity of the insulation system can be a determining factor for long motor life. In medium and high voltage motors, predictive and preventive maintenance will prevent the acceleration of the aging of the insulation system. Statistics show that the stator winding alone accounts for almost 40% of failures.

Having knowledge of the failure mechanisms of a motor’s insulation system and intervening in them will ensure long-lasting uptime. We list below the main causes of failure or accelerated aging:

• high motor temperature;
• electrical stress;
• high contaminant environment.

It is common for all three phenomena to act simultaneously.

Learn more about the services provided by Nishi and contact one of our experts for information about failure modes of insulation systems.

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Watch the video below with engineer Fábio Toshio performing an insulation resistance test. He comments on the main points of attention for proper execution and interpretation of applied standards and test results:

• Applicable standard IEEE Std 43:2013 – Recommended Practice for Testing Insulation Resistance of Rotating Machinery;
• Stress levels;
• Acceptance criteria;
• Polarization Index;
• Cautions that interfere in the measurement.

Learn more about our products and services for predictive and preventive maintenance of motors and generators. Contact our team and request an evaluation of your maintenance scope.

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Moisture and dirt on stator or rotor windings decrease the insulation resistance by reducing surface resistivity. Operating a machine under these conditions can accelerate the aging of the insulator and even lead to equipment burnout.

A cleaning technique used in the market, besides the traditional one with electric solvent, is the application of CO2 in the insulation system. This cleaning consists of blasting CO2 in the solid state, by means of appropriate equipment, where kinetic energy, thermal shock, and thermo kinetic energy (sublimation effect) must be controlled to remove the dirt without affecting the insulating material. The knowledge of the insulating material is fundamental to determine the parameters of the application method and to avoid damage to the material to be cleaned.

The principle is based on throwing a jet of ice at high pressure and speed that, when it collides with the surface to be cleaned, causes two phenomena (a) cooling of the set dirt + substrate (base or surface to be cleaned), which by the temperature gradient, instantly causes cracks that become weak points in the mass of contaminants on the equipment to be cleaned; (b) in parallel, the ice molecules collide with the surface and their sublimation occurs (they pass from the solid state directly to the gaseous), with the molecules expanding their volume by approximately 800 times, instantly. In a colloquial way, we can say that millions of microbursts occur that cause the “dirty mass” to be pulled off the surface from the inside out, by detachment. Since the dirt or contaminant surface is not cohesive (it is “cracked” by instant cooling), the layer of material to be cleaned is removed with impressive efficiency.

This procedure should not be confused as a simple machine “washing” technique, but rather as a process of recovering the insulating system from dirt and moisture. The technician who operates the system is not a simple “washer”, but an electric machine specialist who knows the winding and the insulating system and evaluates its degradation stage, thus avoiding undesirable results. We also caution that the types of soiling are crucial to the effectiveness of the result and also to the choice of intervention method – traditional solvent or dry ice.

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As most cases of leakage are solved only by correcting the seals and adjusting the oil flow (adjusting the pressure and flow rate), we may be led to conclude that the solutions boil down to these actions. It turns out that this is not always true. There are more complex cases where the cause may be associated with the pressure difference between the bearing and its environment. Like wind in the atmosphere, which is the movement of air from a high pressure zone to a low pressure zone, lubricating oil also tends to make this movement if we have pressure differences between the bearing and its environment. In these cases, the solution demands a detailed analysis of the pressure levels in this region and how to act for its equalization.

External and internal visual inspection of the equipment, as well as monitoring the temperature of the bearings are recommended practices to identify oil leaks in the lubrication system. If the bearing lubrication is done through an oil ring, oil leakage will lead to the exhaustion of the reservoir, and significant damage to the bearing may occur due to loss of lubricating oil.

If the oil leaks, it can contaminate the winding and damage the insulating system, mainly because it carries other conductive dirt along with it. Periodic inspections are recommended by measuring the clearances of the components in the bearing sealing system, visual inspection of the assembly, and using the lubricating oil pressure and flow rate as specified by the manufacturer.

Contact our team and clarify your doubts about check items and evaluation in periodic preventive maintenance of your equipment.

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The principle of an electrical machine is that the conduction of electric current is the basic condition for its operation. In order to conduct this current continuously through the various coils, there are electrical connections that connect one coil to another, both in the rotor and in the stator. These connections are made by soldering or brazing.

In brazing, the connection established between two metallic elements is made by means of an addition metal, which is in the liquid state and whose fusion temperature is higher than 450ºC and lower than the fusion temperature of the base materials, and the base metals do not participate in the constitution of the joint.

Welding, on the other hand, is a weak brazing, in which the melting temperature of the filler metal is below 450ºC. Normally, the filler metal penetrates by capillarity between the surfaces of the parts to be joined. Uniform heating of the surfaces should be promoted, because with the heating, the base material molecules get farther apart and the filler metal in liquid state penetrates between these molecules.

The goal to be achieved in the connections is that in this bridge, which connects the coils, there is no bad contact generating a high ohmic resistance value and consequent heat losses and Joule effect. The mechanical part is also important, because in addition to the Joule effect (electrical losses) there are centrifugal forces and thermal movements that generate stress in these connections, reasons that cause wear and malfunctions. Predictive and preventive maintenance is essential to identify the integrity of these connections and thus avoid unscheduled failures in them.

As an illustration, in an 8 160 hp, 13 800 V, 1 800 rpm motor, the client had a history of failures in the damping cage and requested a detailed inspection of the equipment for preventive evaluation and intervention, considering the need for high reliability of the equipment for operation. Through tests on the rotor circuit we concluded that the connections were intact, which enabled the return in operation, restoring peace of mind to the customer.

winding rotor connections

Inspection of electrical connections of a high voltage wound rotor	Wound rotor test to evaluate electrical connections

In another case, a 66 760 kVA, 13 800 V, 257.14 rpm hydro generator, a partial winding was performed on the stator winding with bar change. Since this generator had already failed due to problems in the brazing of the series connections of the bars, as a preventive measure, a general evaluation of all the connections was made before releasing the machine for operation.

Brazing process on electrical connection	Test to evaluate problems in the brazing of serial connections of the bars

Request a quote for a preventive evaluation of your equipment and count on our team of specialists in electrical machines!

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When we analyze the nominal voltage between turns of a machine, even for large equipment, the level is low and its operation is around 100/150 volts/turns. At this level, the electrical deterioration is low and the failures are due to thermal and mechanical actions on the insulating material. Because it is in direct contact with copper, the highest temperature point, it is the insulator that heats up first. Another wear factor is the movements by dilatation due to thermal cycles in operation. Electrically we can have deterioration coming from the impurities in the environment in which the machine is operating.

We have several switching sources inserted in the circuit that can generate voltage surge waves with fast rise time wavefronts. These waves, when they reach the winding, distribute the voltage (the circuit is seen as a capacitive divider) in an uneven way and concentrate high values in the first windings of the machine that can cause early deterioration by the appearance of electrical discharges due to the high voltage level. Remember that this insulation is made for a low voltage level, and is basically composed of the conductor’s own insulation (enamel and/or fiberglass) or in some projects an insulating layer is applied, but of only one turn.

Hardly ever does the machine go out of operation due to a short turn insulation. What normally happens is that this short deteriorates the main insulation, causing a short to ground that causes the protection to operate.

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