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Case Study

Inverters

Not all inverters are the same…

A discussion on why inverter selection can have a greater bearing on electrical powertrain performance than is immediately obvious.

In the search for ever increasing levels of efficiency, traditional internal combustion engines are experiencing high levels of innovation focused on downsizing and forced induction. Whilst this does not appear to have a parallel with the ever increasing levels of powertrain electrification, a focus on system performance, ie conceiving the battery, inverter & electric machine together, can have an equally dramatic effect on weight, size, performance, and of course, cost.

A traditional route to conceiving an electrified powertrain will tend to specify the system components in the order of electric machine, battery, and finally the inverter. In this sequence, the most complex unit, responsible for optimising bi-directional energy flow between battery and electric machine as well as integration with safety critical vehicle operating systems, is given the lowest priority, possibly because of the belief that an inverter is an inverter… The author would argue that inverter selection should be given greater consideration at the time of specifying the battery and motor, as the inverter will have an equally significant impact on whether or not the system’s performance and cost targets are achieved.

Zytek is a company experienced in the parallel development of electric machines, power electronics and batteries, specifically engineered for both mainstream and motorsport sectors of the automotive market. As a consequence Zytek are uniquely placed to understand the key parameters that are required to be considered in the development of a successful HV electrical system, with the inverter as the key enabler to unlock system potential.

Prior to the commencement of either an inverter selection, or the development of a new bespoke product designed specifically for an application, basic primary parameters need to be understood: input voltage range (ie battery DC voltage), output voltage range (matched to the requirements of the electric machine), output power & current, drive cycle (to ensure thermal performance can be maintained), working temperature range and coolant details (thermal capacity & flow rates). Following this, secondary information will be required: maximum electrical frequency (used to select the optimum PWM frequency to control switching losses) and further battery characteristics (to optimise DC link capacitance to reduce DC ripple current losses). Even at this high level, it has become immediately apparent that the standard information presented on a data sheet or sales flyer is not adequate to convey the necessary information to compare inverter products.

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But having selected or designed an inverter capable of operating using all of the above data (and much more!), only using very advanced, state of the art, machine control techniques can the optimum performance be realised. Given a nameplate inverter electrical rating, it is a common misconception that all similarly rated inverters will drive electrical machines to produce the same power/torque output. Zytek use a proprietary AC controller, using space vector modulation with advanced discontinuous and over-commutation techniques, all implemented on a system by system basis. At no point is any table lookup of torque vs machine current used, as this technique cannot be used for top end, high performance systems capable of extracting the maximum torque/amp (and hence minimised system loss). Live, closed-loop models of both inverter and machine loss are run in the inverter, enabling operating efficiencies of >97% to be realised, along with high levels of delivered torque accuracy (typically <1% torque error when applied to salient machines, where torque is not proportional to any measurable physical variable).

“It is a common misconception that all similarly rated inverters will drive electrical machines to produce the same power/torque output.”

Finally, from a safety aspect, many vehicle level implementations now require inverters to meet ISO26262 ASIL level D, necessitating dualisation of current sensing within the inverter using different sensing techniques, and the ability to continue to function even in the case of a complete loss of 12V KL15 & KL30 ignition signals.

Having been daunted by all of the above, rest assured that inverters are available capable of achieving all of the above and more, they just need to be thought of as less of a pure current switching device, but more of as the primary system enabler, but most importantly, as the primary choice when conceiving you vehicle electrification requirements.

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