As the "Preface to the Pavilion of Prince Teng" puts it, the moment when Wang Zi'an ascended to a lofty height and composed his fu was one in which all four beauties—auspicious time, splendid scenery, delightful heart, and joyful occasion—were perfectly combined. It was a rare and exquisite moment, marked by the simultaneous presence of a brilliant host and exceptional guests, thus giving rise to a masterpiece that transcends time and stands unmatched throughout history.

　　Following in the footsteps of our distinguished predecessors, STEP industrial control designers have been relentlessly pursuing the path to perfection. After more than a decade of persistent technological leaps and continuous product iterations, coupled with the growing accumulation of diverse on-site industry demands, we’ve proudly introduced to the refrigeration industry guest Miaoke this four-in-one AS720D model. So, what exactly are these “four beauties”?

　 　First-class beauty | Extremely high power density

　　The AS720D inherits the AS720’s exceptionally high power density and has been developed on the basis of the original product. It features a non-standard, custom-designed cabinet that supports integrated onboard installation for central air-conditioning compressors. A 630-kW unit fits into the size of just a chest of drawers. By contrast, engineering-grade variable-frequency drives with the same power output typically have much larger volumes, as illustrated in the comparison below.

　 　Second Best | Low-Harmonic, Bidirectional Front-End Rectifier with Energy Feedback Capability

　　The AS720D integrates a low-carbon, energy-efficient front-end rectifier with low harmonic distortion and regenerative capability. Within such a compact enclosure, STEP has spared no expense in incorporating an expensive four-quadrant active front end. This front end not only features six IGBT modules—just like the inverter unit—as its rectifying components, but also includes a sophisticated and comprehensive LCL filter, a dedicated AFE control unit, and a soft-control system. So when you open the cabinet door, you’ll notice two handheld controllers inside—a clear indication of its dual-head design. This innovative design is friendly to grid waveform quality, enabling users to achieve exceptionally high energy savings while reducing grid-connected harmonic content to below 4%. To put it another way, while conventional inverters rely on brute force when driving ultra-high-speed operation of magnetic levitation screw compressors, the AS720D acts more like a Tai Chi master skillfully manipulating a Tai Chi ball—accumulating energy endlessly yet effortlessly and gracefully, striking a perfect balance between high-efficiency, high-frequency output, and energy conservation, low carbon emissions, and noise reduction. Compared to ordinary inverters, the AS720D boasts vastly superior technological sophistication and significantly higher manufacturing costs.

　　 Third best | The AS720D uses refrigerant cooling.

　　Thanks to the use of four-quadrant technology, the switching frequency is high and the heat flux density is nearly twice that of conventional two-quadrant inverters. Whether such a design can deliver high energy efficiency and enhanced power density depends critically on the cooling system. Traditional engineering-grade inverters rely on air-cooling, which requires large air ducts, resulting in significant operational noise and making them prone to dust and oil buildup. This necessitates frequent maintenance and leads to low thermal efficiency. Later, water-cooled inverters were developed, offering much higher heat-exchange efficiency than air cooling. However, water-cooling systems must be equipped with chillers to rapidly cool the circulating water, consuming extra electricity and generating considerable noise. Over time, scale may accumulate in the cooling water circuits, reducing thermal efficiency and making cleaning a hassle. Since the AS720D drives a refrigeration compressor, using refrigerant-based cooling is exceptionally convenient and highly efficient. It eliminates the need for an additional cooling system compared to water cooling and avoids all the aforementioned issues. As a result, it achieves ultra-low noise levels and significantly reduces the overall size—transforming what was once a bulky 110-630 kW unit into a compact backpack that can be mounted directly onto the compressor. This makes on-site installation far more convenient for customers, providing end-users with greater space savings and design flexibility.

　　 The Fourth Beauty | The AS720D is available in a variety of products with different protection levels.

　　The AS720D offers products with multiple protection ratings—IP22, IP54, and IP55—allowing it to meet diverse customer needs. With its robust design and comprehensive protection, the device can be directly installed outdoors, serving as a reliable guardian for central air-conditioning systems. Moreover, for certain motors rated at 460V according to U.S. standards, this unit can operate efficiently using only a 380V input while providing a stable voltage boost function. This significantly broadens customers’ motor selection options and enhances compatibility with upgrades and retrofits of existing equipment.

　　In addition to the four-star product offerings, the STEP HVAC service team also provides “genuine manufacturer care, professionally delivered right to your home” on-site installation and maintenance services, further meeting customers’ needs for long-term use and reliable operation, and fully embodying the principle of respecting and serving our customers with sincerity.

　　As the saying goes, "When the host is elegant, guests come willingly." Faced with the lingering intensity of the "autumn tiger" that still rages today, let the AS720D—paired with magnetic levitation, air suspension, and high-speed direct-drive units—lock the heat away, propelling the industry one giant step closer to its goal of low-carbon emissions and reduced pollution. Together, we’ll achieve a perfect application that combines four exquisite virtues and resolves two seemingly conflicting challenges. What do you say—sounds wonderful, doesn’t it?

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No. 1560, Siyi Road, Jiading District, Shanghai Municipality