Everything revolves around efficiency

State-of-the-art controls for problem-free operation of high-efficiency IE3 motors

Energy 2.0 28.02.2015, Author Helene Steuer

The issue of energy efficiency affects the manufacturers of machines and plants in many ways. An important aspect here is presented by three-phase asynchronous motors that must meet the requirements of energy efficiency level IE3 from 2015. The properties of the new motor generation have an impact on low-voltage controls. But with the right modular system, conversion to IE3 is not a problem.

Energy efficiency is currently one of the most important issues in industrial automation. Thus EC Regulation 640/2009 stipulates that three-phase asynchronous motors in mains-fed operation must have at least efficiency class IE3 from 2015. This stipulation applies for the power output range between 7.5 kW and 375 kW for motors being newly launched on the market and intended for continuous service. From 2017, this regulation will be further tightened to apply also to smaller motors with rated power outputs between 0.75 kW and 7.5 kW.

The energy savings of the new motors are usually achieved by a lower rated current. On average, the rated current for IE3 motors is between three and six percent lower than for IE1 motors. However, the measures that were required to reduce energy consumption on the motors show effects during starting that necessitate certain adaptations during engineering and in the selection of the controls.

In practice that means: whereas the IE3 motors show a higher starting current compared with previous motor generations, the greatest change is to be seen in the inrush current. The inrush current occurs in the first and second half-wave after switching on the motor and is much higher than the actual starting current.

New challenges from higher currents

Until now, little attention has been paid to the dynamic effects during the starting procedure. Only with the new motors of efficiency class IE3 and higher are the currents that occur during starting so great that conventional controls come up against their limits. To prove this assumption, Siemens AG analyzed more than 5,000 motors from different manufacturers over a period of four years, testing some of them in the lab, and then calculated the influence of the inrush current.

The summarized results indicate the following: Although the energy savings of the new motors are usually achieved through lower rated currents, the design changes required for this at the drive end resulted in higher starting currents and increased inrush currents. The high starting and inrush currents here vary depending on the selected motor type, the requirements of the application, and the respective operating conditions. Thus, cable lengths, system rigidity, etc. also determine the actual level of inrush current.

The right controls for high-efficiency drives

The new challenges necessitate some adaptations to previously offered switching devices. On the one hand, it must be ensured that the switching capacity corresponds to the higher current load (even if this is only brief), and that the contacts do not weld. On the other hand, to avoid undesired tripping, protective devices must be designed in such a way that they do not evaluate the inrush current as a short-circuit current.

The devices of the Sirius modular system from Siemens meet these requirements because they are already designed for the physical marginal conditions of this new requirement in conjunction with energy-efficient three-phase asynchronous motors. Also significant in this context is the IEC 60947 standard for industrial low-voltage switchgear and controlgear that defines important marginal conditions for the certification of the controls. Due to the new requirements arising from the use of high-efficiency IE3 motors, discussions are already underway concerning appropriate adaptation of the conditions defined there for dimensioning and testing the controls.

It is thus important in this context that, when changing over to energy-efficient IE3 motors, manufacturers of machines and plants select industrial controls that are "IE3 ready", in other words, designed for the new properties of the IE3 motors. The change to the starting currents is greater in the low-end performance range in particular. The corresponding increase is in the range between five and 25 percent here. That alone suggests that in particular energy-efficient three-phase asynchronous motors have a perceptible influence on previous controls. The corresponding changes apply for the aforementioned inrush current, where the increase is between 20 and 48 percent.

Controls must be "IE3 ready"

Practice shows that special care must be taken particularly with the selection of devices with combined overload and short-circuit protection. After all, despite the lower rated currents of IE3 motors, the short-circuit protection must have sufficiently high factors that can differentiate between the inrush current on starting, the "normal case", and the short-circuit as a "fault". Thanks to overlapping and partly downwards expanded setting ranges, the Sirius motor starter protectors mentioned are able to perform this balancing act, in other words, to start the motor without problems and nevertheless guarantee safe short-circuit protection.

For this reason, Siemens designates its Sirius modular system including motor starters, soft starters, overload relays, etc., as "IE3 ready". The system thus meets the new requirements to suit the current era of energy saving. In addition, use of its new size S2 allows significant space savings in the control panel, as the IE3-optimized Sirius 3RT2 contactors prove: To switch a motor with 80 A rated current (37 kW) now only requires devices with a width of 55 mm, where previously only the next larger size S3 with a width of 70 mm was available. This saves valuable space in the control panel – and thus money.

Modular system for 95 percent of motor applications

For users, this means: Access to a modular, optimally matched range of components with just seven sizes up to a power rating of 250 kW/400 V with largely the same accessories. The modular system for switching and starting, protecting and monitoring also includes simplified connection to the control level, via AS-Interface and IO-Link, for example. Sizes S00, S0 and now also S2 (up to 37 kW) of the industrial controls range from Siemens have been optimized for flexible use with the stipulated energy-saving motors. With this range, the company thus covers approximately 95 percent of all motor applications worldwide.

A crucial factor is that the switching capacity for IE3 motors has been optimized: A current example of this is the new Sirius 3RV2 motor starter protector size S2 with adapted tripping characteristics, extended setting range, and increased switching capacity. The devices are characterized by setting ranges for lower rated currents, as well as an increase of the lower tolerance of the short-circuit release.

Helpful tools for simplified engineering

With its Sirius modular system, Siemens offers valuable support in designing and operating modern, energy-efficient machines and plants. In addition, manufacturers of machines and plants benefit from user-friendly tools such as the CAx Download Manager; this supplies all the data that is necessary or useful during the entire design phase. Up to twelve data types are available for mechanical (CAD) and electrical (CAE) engineering – from connection diagrams, through characteristic curves, dimension drawings and models, up to operating instructions and certificates. Siemens has calculated time savings of up to 80 percent for the use of this universal product data. With the 'Eplan Electric P8 Macro' in .edz exchange format (Eplan Data Archived Zipped) for Eplan users, the overall time required for data integration can be reduced even further.

And because comprehensive plant documentation is obligatory at the end of every project, Siemens provides a function in its 'Industry Support Portal' for directive-compliant documenting of the solutions used: The relevant chapters of the available manuals of the Siemens components used can thus be selected by dragging and dropping, and combined to form customized documentation.

Saving energy is no probleme with the right system

The legally required use of three-phase asynchronous motors of energy efficiency class IE3 supports industry in saving valuable energy and thus slowing down the consumption of resources, as well as continuously reducing the resulting CO2 emissions. However, thanks to the energy-saving features, new aspects come to light elsewhere, such as the inrush current already referred to, that require appropriate care and attention in product selection, engineering, installation, and operation.

System philosophy becomes ever more important in this context when it comes to reducing engineering outlay, ensuring safe operation, and further increasing the availability of machines and plants. All this can be implemented at low cost with the Sirius modular system from Siemens, that is also accompanied by a range of support tools. After all, stiff competition on the global market means practically everything in industry today revolves around efficiency – not only in the case of drives and processes, but also in engineering, procurement and service.