COMPRESSORS
The inverter evolution: electronics made simple
By André Rosa, research and development (R&D) and product strategy senior director, Alexandre Cabral, R&D manager and Daniel Hense, senior R&D researcher at Nidec Global Appliance.
W André Rosa.
orking with refrigeration means always having an eye on the present and the other on the future. Compressors are becoming smaller, quieter, more
efficient, and more environmentally sustainable. However, little is said about the one device that has highly contributed to generating some of the most significant results in these topics during recent years: the inverter. This electronic device is what drives the variable speed
compressor (VCC). The kind of compressor that has been taking energy efficiency results to new levels, which in turn are pushed constantly up by the stringent energy consumption regulations for commercial refrigeration equipment in Europe and other regions around the world. After all, it has become clear that higher levels of energy efficiency are mandatory for environmental sustainability, nowadays and more so in the future. So it's past time for any player in the commercial
refrigeration segment to create more intimacy with the inverter, especially the original equipment manufacturers. This article is here to demystify the perception of complexity around this device, and to show the simplifications and gains it can bring to OEMs and the end user.
Alexandre Cabral.
What is inside an inverter? In a very simple explanation: inside the inverter’s enclosure there is an electronic board composed by an input stage called AC/DC converter (from alternate to direct current converter), and an output power stage called DC/AC “inverter”, that generates from DC current an AC current at proper level and frequency. The device’s intelligence is an embedded software installed in a 0.2 square inches microcontroller. The software implements the motor control logic and the inverter operation to drive the compressor’s BLDC (Brushless Direct Current) motor.
The use of sensorless motor control technology, which is the one used by Embraco inverters, allows the solution to be simpler.
Daniel Hense.
The inverter dismisses any sensor by reading the feedback signal from the motor’s coils (also known as back electromotive force). With that, it calculates how to generate the proper output frequency and power to drive the motor at a given speed despite the load.
24 May 2022 •
www.acr-news.com Speed is the key element
The inverter and motor compose a system that can modulate the cooling capacity by adjusting the compressor’s working speed (inverter output frequency), reducing it during low demands (low ambient temperature or no door opening) and increasing it under high demands (pull down, reload or high ambient temperature). The variable speed compressor does that without switching on and off as frequently as a traditional, fixed speed, compressor would do. A variable speed compressor can usually work in a range
from 1.600 to 4.500 rpm (rotations per minute), whereas a fixed speed only runs at the power supply frequency (3000rpm for 50Hz and 3600rpm for 60Hz). So the variable speed compressor delivers a faster pull down or temperature recovery any time the refrigerator door is opened, for example, which in food retail or food service establishments happens all the time, all day long. After reaching the desired temperature, it goes back to a speed behaviour where the coefficient of performance is maximized, which means delivering the needed cooling capacity with the least energy consumption possible. By being quicker in these transitions, the VCC not only saves energy but preserves the food quality. And we can’t fail to mention the lower noise levels delivered
by variable speed compressors thanks to the fact that the inverter controls it in order to, most of the time, operate in the lowest and more silent rotation levels possible.
Taking complexity out of the equation The key in using a variable speed compressor, dealing with an inverter, is how to make this whole system work at its optimum cooling capacity and saving energy. This goal is achieved through the adjustment of a set of parameters, which might be implemented by the electronic controller of the appliance or by the inverter itself when configured to run with an embedded control logic and a simpler controller (or even a thermostat). The embedded control logic of Embraco compressors, named Smart Drop-In (SDI), is in its second generation already, and was created to extract all the benefits from the variable speed compressor, in a variety of applications and conditions, in the most practical and simple way possible. With Smart Drop-in, the controller will be the same as the one for fixed speed (on-off) compressors, a thermostat, and
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