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Inverters: the progression from high-tech to commodity

13 November 2015

Twenty years on and there’s really little to pick between the drives brands in terms of hardware; now it’s a matter of services and software innovation, says Lenze's Geoff Spear.

This month at the SPS/IPC show in Germany, Lenze launches the new i500 inverter range

Back in 1995, frequency inverters were considered ‘high-tech’ and were just becoming part of machine designers’ armoury; dc drives were just as common. Early reliability problems with inverters had been largely overcome. Lenze were selling the new 8200 range of inverters up to 11kW, a compact bookshelf design that triggered massive sales growth, in part at the expense of mechanical variators because the electronics gave a wider speed range and were maintenance free.

Twenty years on and frequency inverters are close to becoming a commodity for many applications. For simpler machines, just about any brand on the market will do the job. The design engineer’s choice is increasingly on ease of use, which focuses on the software and other factors such as technical support, size and cost.

Reliability is no longer an issue now that modern production lines test under load. Inverters have got smaller; will they continue to do so? Probably not, as power and control cable connections have to be made and reasonable access is needed.  The boundaries between frequency inverters and servo drives are already blurred and that trend will continue. Future product developments will be known simply as inverters with variants extending from basic up to servo levels.

There is much talk these days about drive train efficiency and saving energy. Regulations are on the way stipulating IE energy classes for inverters and IES system efficiency classes for the inverter together with a motor. Implementation is likely to be slow due to the complexity of measuring and setting standard levels under a variety of loads.

Inverters have typical efficiencies around 94 percent.  Designers should remember that adding an inverter to a fixed speed drive actually increases energy use and should be done only where there is good reason. Fortunately inverters are capable of saving a lot of energy in the right application, much more than energy-efficient motors as can be seen from some examples:

- Many electric motors are lightly loaded for a significant part of their duty. Inverters can offer magnetic flux adjustment with potential savings up to 30 percent under light loads; for example, the VFC eco software that Lenze includes free of charge in many of its drives.
- In cyclic applications inverters can be used to optimise the speed and acceleration profiles with potential energy savings up to 30 percent.  As when you drive a car, smooth acceleration and deceleration gives maximum efficiency.
- Re-using braking energy in applications such as winding or where there are stops and starts can save up to 40 percent. This can be done by simply connecting a dc bus, by capacitor storage or by regeneration to the mains.

So, although inverters themselves introduce energy losses, when applied with knowledge they can save massive amounts of energy. This area is under-exploited today, but will contribute to strong market growth in the future.

A significant factor holding back the use of inverters for energy saving is a shortage of skills and drive expertise. Particularly at machine builders we see fewer engineers, and those remaining have a broader and heavier workload. Increasingly customers need help to select, apply and to optimise inverters to their machine requirements.

Lenze is well positioned to help here with sizing software called Drive Solution Designer (DSD). This is a process-based software program that can optimise the complete drive train through accurate simulation and comparison of drive options. It is available for machine builders and even academic institutions to use, although in most cases the extensive content from ten years of development means that customers prefer Lenze to run it for them.

The data input to create a model in DSD starts with the ambient conditions for the machine. To that are added dynamic requirements in terms of motion. The program has built-in formula for most types of motion required for a process. For example, conveyors can be horizontal or angled and suggestions are in place for coefficients of friction.

For positioning applications it is only necessary to enter the end position and time available – the program adds ‘S’ shaped ramps and optimises the motion profile to match the available drives and motors. DSD is not confined to single axes and an almost unlimited number can be added to model multi-axis machines. This allows energy-saving scenarios to be added, for example sharing braking energy by linking the dc bus.

Outputs of the DSD software are partly graphical, such as distance, acceleration and input power against a time axis. The outputs of inverter and servo drives are shown superimposed on an image of the required power for the application, making it possible to make a selection that is ‘right-sized’, i.e. neither too big or too small. Different selections can be easily tested and compared.

The accuracy of models created in DSD is remarkably high reflecting the many years of experience and development that has gone into it – tests on real-life machines reveal typical errors below 2 percent. This level of accuracy makes DSD a useful tool to forecast running costs, and an additional output is an Energy Certificate which predicts the running cost of the machine for a year based on the prevailing electricity cost per kWh.

Product development in the world of inverters has slowed from the heady days 20 years ago.  At the SPS/IPC show this month Lenze will be launching its new i500 range with an emphasis on flexibility and ease of use. As with many inverter product launches these days, at first glance there does not seem to be much that is new.  However there are innovations here such as easier programming by USB or WiFi and keypads that are more powerful and user-friendly.

Looking ahead, the focus is likely to be saving time through ease of use.  This means software wizards that allow the setting of key parameters to be faster, plus clearer screen displays, easy setting up of network comms, and the ability to create simple applications without any inverter knowledge.  Where the dynamics are more demanding, pre-configured software will be used to achieve peak performance. Examples are winding, cross cutting and robotic motion, which are already available in the Lenze FAST software suite. Here the programmer is not required to write code and instead spends much less time entering parameters and fine-tuning performance. The result is major savings in engineering time and fewer errors to debug.

Geoff Spear is marketing manager, Lenze and Techdrives


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