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Energy efficiency: the next frontier for drives

13 November 2015

How energy efficient can the drive itself become? With premium drive efficiencies already approaching 98 percent, Matt Handley asks: what more can be done to improve upon this? 

SiC based power module (courtesy of Mitsubishi Electric)

We all know full well how much energy can be saved by utilising the technology of a variable frequency drive (VFD) fitted to an electric motor, and the savings that can be achieved are well documented. These energy savings can be vast, so what can we do next to improve system efficiency even further?  The mechanics of gearboxes and couplings aside, the onus falls upon us as drives manufacturers to make our products more efficient in operation.

In much the same way as motors - whose efficiency is now legislated by EN60034-30 Part 1, defining efficiency classes IE1 through IE4 (the current limit for the norm) - VFDs will soon be subject to meeting minimum efficiency levels. The new standard - EN50598 - is made up of three parts. It is part 2 of this standard that will make us, as drives manufacturers, sit up and listen. Part 2, which was released in December 2014, is entitled ‘Quantitative ecodesign approach through life cycle assessment including product category rules and the content of environmental declarations’

EN50598-2 sets out efficiency values that are specified for power drive systems (PDS), motor starters and complete drive modules (CDM) that are used for electrically driven machines in the power range of 0.12kW to 1,000kW. Furthermore, the VFD is brought into scope if it also meets the criteria of having a voltage range of 100V – 1,000V.  This means the vast majority of drives placed onto the European market will fall under this new scheme.
In much the same way as motors, efficiency classes will be defined by an IE level: IE0 through IE2. Most VFDs from premium manufacturers should have no problem in meeting the highest IE2 level because the efficiency levels of their products already lie somewhere in the region of 96-98 percent. At least this means that we haven’t all got to go out and redevelop our products. So what else can be done now to improve the efficiency of the VFD?

As a manufacturer of power electronics - and particularly power semiconductors - my company is at the forefront of improving the performance and function of insulated gate bipolar transistor (IGBT) modules and intelligent power modules (IPMs).

In recent years, creating high-temperature, high-withstand voltage semiconductors having high-speed switching performance by replacing Si (silicon) wafers with SiC (silicon carbide) wafers for semiconductor materials, we have developed and commercialised power metal-oxide semiconductor field-effect transistors (MOSFETs), power modules and IPMs using power Schottky barrier diode (SBD) chips.

These new semiconductors have enabled dramatic reductions in power conversion losses as well as the size and weight of power modules and their application systems, leading to the further evolution of power electronics equipment.

SiC is a promising material; in the future it will allow manufacturers to produce VFDs that not only suffer lower power losses, but also provide higher power density. There is a great deal of interest in SiC based power electronics because of its potential to reduce the size, weight and total system cost of a variety of devices – not just VFDs. The technology is also applicable to electric home appliances, railcars, industrial equipment and automobiles, where its energy saving potential is likely to have a big impact.

By leveraging the low power loss property of SiC power devices, Mitsubishi has succeeded in creating a 400V 11kW SiC inverter prototype that reduces power loss by 70 percent, compared to same-capacity Si inverters, and which delivers an output of 10W/cm3. So, how does SiC based power electronics bring about these benefits?

Power loss reduction - SiC has approximately ten times the critical breakdown strength of silicon. Furthermore, the drift layer that is the main cause of electrical resistance is one-tenth of the thickness.  This allows a large reduction in electrical resistance and in turn, reduces power loss. This SiC characteristic enables dramatic reductions in conductivity loss and switching loss in power devices.

High temperature operation - When the temperature increases, electrons can escape to the conduction band from the so called valence band and as a result, the leakage current increases.  At times this results in abnormal behaviour.  However, SiC has three times the band gap width of silicon, preventing this flow of leakage current and thus enabling operation at higher temperatures.

High speed switching operation - With SiC, owing to the high dielectric breakdown, power loss is reduced and high voltage is easier to achieve; it is possible to use SDBs, which cannot be used with Si.  SBDs realise high-speed switching because they don’t have accumulation carriers.

Heat dissipation - SiC has three times the conductivity of silicon, which improves heat dissipation.

So far, this technology has been limited to lower power applications. The use of SiC in room air conditioner power semiconductors, for example, was a world's first for Mitsubishi Electric. Among other factors, this has decreased power loss from inverter operation by approximately 15 percent, and helped reduce seasonal power consumption by approximately 6 percent, compared with the performance of Mitsubishi's existing room air conditioners.

In January 2011, Mitsubishi Electric prototyped a 1,200V/75A rated power module by using a SiC transistor (SiC-MOSFET) and SiC diode. Using only SiC (full SiC-type) power semiconductor chips, this power module, when applied to a single-phase 200V/5kW power conditioner for photovoltaic power generation, achieved a power conversion efficiency of 98.0 percent - the highest in Japan, in experimental trials.

Using only SiC power semiconductor elements, in February 2011, Mitsubishi Electric developed the world's first full SiC- intelligent power module (IPM) to integrate both drive and protection circuitry. In addition to reducing power loss by 70 percent compared with existing Si-IPM-equipped devices, the module size was reduced by 50 percent.

At the time of writing, the quest is on to find ways of mass producing this technology.  What can be said, however, is that the future looks very good. Most recently, Mitsubishi conducted trials on a rail traction inverter in Japan and was able to achieve an overall 40 percent energy saving when compared with the performance of a conventional gate turn-off thyristor traction inverter.

Matt Handley is product manager, drives systems, Mitsubishi Electric

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