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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We will be discussing an example of the generator failure mechanism which can lead to a quick (months!) insulation breakdown. Do you wish to know that it is?

Please join us on Thursday 04 May 2023, 11am Brazil time (3pm UK time). We will go through details and RCA of 2 generator breakdowns, with hope that it will help you to prevent such situation in the future.

Please join the training using the link below:  
https://meet.zoho.eu/UDSxeI82x6

NOTE:
Please remember to register before the meeting, as number of participants is limited. Chrome and Firefox do not require downloading any other add-ons or apps, therefore these are preferable web browsers. Other web browsers may require downloading additional files. Please check with your IT Department.

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Voltage classification of generators/motors:

• Low Voltage: Round or Enameled Wire / Voltage Class: 127/220/380/440/760 V.
• Medium Voltage: Rectangular Wire / Coils or Bars / Voltage class: 2,3/3,4/4,16/6,6/6,9 kV.
• High Voltage: Rectangular Wire / Coils or Bars / Voltage Class: 13,8/16,5/18,0 kV or higher.

Standardized offline electrical testing:

• Insulation Resistance (IEEE-43);
• Polarization Index (IEEE-43);
• Ohmic Resistance (IEEE-118);
• HypotAC/DC (IEEE-4 / IEEE-95);
• Step VoltageDC (IEEE-95);
• Surge Test (IEEE-522);
• Off-line Partial Discharges (IEEE-1434 / IEC6034-27-1);
• Delta Tangent (IEEE-286);
• Capacitance (IEC 60034-27-30);
• Corona in darkroom) (IEEE-1799).

Turn on the subtitles in your language and watch below in Nishi’s Youtube channel the live recording and learn more about the definition and application of each electrical test according to voltage level and how it is essential for a good diagnosis of the condition of the motor/generator winding.

Take part in upcoming live opportunities to ask questions and make comments directly with an expert on rotating electrical machines.

Get in touch directly with engineer Fabio Toshio and get your questions answered: fabio.toshio@nishi.com.br

Request a commercial proposal: E-mail: nishi@nishi.com.br

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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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This subject is very important for users of large motors and generators. In machines with voltage higher than 6 kV, operating at the frequency of the electrical system (50 or 60 Hz), the coils (market standard) have a semi-conductive material (tape or paint) on the straight side in order to avoid partial discharges that deteriorate the insulating system. The fit between coil and slot is never perfect and we always have small voids in this contact. In this situation, we have electrically a circuit with 2 (two) capacitors connected in series of different dielectric constants (air and insulating).

The semi-conductive material, applied over the insulator, aims to short-circuit the air capacitance and thus eliminate possible discharges. It is unlikely that machines with voltage below 6 kV will present the problem of discharges, taking into account that the working voltage applied to the insulator will be lower than 3.4 kV (phase x ground) and would not reach the disruptive voltage of the air.

Contact our team and request an evaluation of your technical specification with the requirements of materials and manufacturing processes for coils and bars for motors and generators.

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To ensure the loss levels and yields of these machines, the generators are designed with coils formed by multiple rectangular conductors in parallel, insulated from each other (“strands”) and transposed along their length, which we call “Roebel” transposition. Each parallel conductor is designed with a small cross section because in this way the eddy currents are reduced (Fujita, Kabata – 2005).

The commonly used transpositions are 360º and 180º. Studies show that in the 360º transposition the losses by circulating current reach close to zero.

Access our blog and read more NISHISAN tips on corrective maintenance services and characteristics of electrical machines. Contact our team and ask all your questions about different coil and bar manufacturing projects for turbogenerators and hydrogenerators.

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The failure of the insulator is usually the result of a gradual deterioration, over time, which we call aging. Aging is basically the result of different stresses: thermal; thermo-mechanical; mechanical; electrical; and environmental, which act simultaneously in operation. The insulating system can change its behavior as a function of the various types of “over stresses” when compared with the behavior of normal aging.

The insulation system in large machines is one of the most important parts because of the high manufacturing and maintenance costs. Around 40% of the failures in large machines are due to the high-voltage insulating systems, according to research.

The study of the behavior of insulating material subjected to different stresses during its operation is very important to improve the design/process and the quality of the insulating system and thus avoid early aging.

One of the most common tests for approval in coil manufacturing is the VET – “Voltage Endurance Test”, according to IEEE 1043 (test method) and IEEE 1553 (acceptance criteria). This test simulates an accelerated aging through thermal and electrical stress simultaneously, in a test room, with 4 (four) prototype coils. Only after its approval the process is approved and released for coil manufacturing.

Contact our team and get to know the techniques to evaluate the insulation system and which is the best coil manufacturing project/process for 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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