Revolutionizing EV Charging: New Multi-Phase Drive-Based Onboard System Unveiled
The electric vehicle (EV) industry stands at a pivotal juncture. While advancements in battery chemistry and vehicle design continue to push boundaries, one persistent challenge remains: the charging experience. Range anxiety, long charging times, and the sheer bulk and cost of onboard charging hardware continue to hinder widespread consumer adoption. However, a groundbreaking new approach, detailed in a comprehensive review published in the Proceedings of the CSEE, promises to fundamentally reshape how EVs are charged. Researchers Yu Feng, Yin Qihao, Tong Minghao, and Zhang Qianfan have synthesized years of global research into a powerful new concept: the multi-phase electric-drive-reconstructed onboard charger (EDROC). This technology is not merely an incremental improvement; it represents a paradigm shift toward a truly integrated, lightweight, and versatile powertrain, potentially eliminating the need for a separate, bulky charging unit altogether.
For decades, the standard EV architecture has been remarkably consistent. A high-voltage battery pack powers an electric motor for propulsion. To recharge this pack from the grid, a separate device, the onboard charger (OBC), is required. This OBC is a complex piece of power electronics, typically containing its own inductors, capacitors, and switching devices, all dedicated solely to the task of converting AC grid power to the DC power the battery needs. This dual-system approach, while functional, is inherently inefficient in terms of space, weight, and cost. It adds significant complexity and is a primary contributor to the “weight and volume penalty” associated with EVs. The EDROC concept, as articulated by the team from Nantong University, Nanjing University of Science and Technology, and Harbin Institute of Technology, directly challenges this status quo. The core idea is elegantly simple: why have two separate systems when one can do both jobs? The EDROC system leverages the existing, high-power components of the electric drive system—the motor and its inverter—for the charging function as well.
The fundamental principle of an EDROC system is reconfiguration. During normal driving, the system operates as a conventional electric drive. The battery supplies power to the inverter, which converts it into a controlled AC waveform to spin the motor. When it’s time to charge, a switch is flipped—literally and figuratively. The roles are reversed. The grid becomes the power source. The motor’s windings, typically used to generate magnetic fields for rotation, are repurposed as the large filter inductors needed for power conversion. The same high-power inverter, which was used to drive the motor, is now reused as a high-power rectifier to convert the incoming AC power from the grid into DC power for the battery. This elegant integration of function means that the massive inductors and the high-power switching devices, which are already present for driving, are now also used for charging. The result is a dramatic reduction in the number of unique components, leading to a lighter, more compact, and less expensive charging solution. It’s a classic case of doing more with less, a philosophy that resonates deeply with the automotive industry’s relentless pursuit of efficiency.
The initial concept of an EDROC is not new, with roots tracing back to work in the 1980s. However, the original systems, often based on standard three-phase motors, faced significant limitations. One of the most critical challenges was preventing the motor from rotating during the charging process. Injecting AC current into the motor windings to charge the battery could inadvertently create a rotating magnetic field, causing the motor—and by extension, the entire vehicle—to spin. This is not only a safety hazard but also a complete operational failure. Early solutions were often complex and compromised performance. This is where the leap to multi-phase systems becomes not just an option, but a necessity. The research team’s review highlights that multi-phase motors—those with five, six, nine, or even more phases—are the key to unlocking the full potential of the EDROC concept. These motors possess inherent advantages over their three-phase counterparts, including lower torque ripple, higher fault tolerance, and, most importantly for EDROC, greater control flexibility. This extra “headroom” in control is what allows engineers to inject charging currents into the motor windings in such a precise way that no net torque is produced, ensuring the vehicle remains perfectly stationary while it charges.
The authors categorize the current landscape of multi-phase EDROC systems into two primary architectures: static magnetic field and pulsating magnetic field systems. The static magnetic field approach is the more straightforward of the two. It relies on a specific type of motor, often a multi-three-phase machine (like a six-phase or nine-phase motor where the windings are grouped into two or three separate three-phase sets). In this setup, the AC charging current is injected as a “zero-sequence” current, meaning it flows in and out through the neutral points of these separate winding groups. This current path is carefully designed so that the magnetic flux it generates stays confined within the motor’s iron core and never crosses the air gap to interact with the rotor. Since the rotor is not subjected to any changing magnetic field, no torque is produced. This method is highly effective and elegant, requiring no physical reconfiguration of the motor windings—only the ability to access the neutral points. It’s a solution that is both robust and simple to control, making it a strong candidate for commercialization.
The pulsating magnetic field approach is more complex but offers greater versatility. This method is typically used with motors that have an “open winding” configuration, where the ends of each phase winding are individually accessible. During charging, hardware switches, such as contactors, are used to physically reconfigure the connections between the motor windings and the inverter. The windings are grouped and connected to the grid in a way that the AC currents flowing through them create a magnetic field that rapidly oscillates back and forth along a fixed axis—hence “pulsating.” While this field is dynamic, it does not rotate. The key insight, explained through the lens of Vector Space Decomposition (VSD) theory, is that the currents are controlled so that their components in the fundamental “alpha-beta” plane—the plane responsible for torque production—trace a straight line rather than a circle. A straight-line trajectory in this plane produces a pulsating force, not a rotational one. This sophisticated control strategy allows a wider range of motor types to be used as EDROCs, but it requires more complex power electronics and control algorithms to manage the switching and ensure perfect current balance.
One of the most significant contributions of this review is its deep dive into the critical issue of control strategy. Simply reusing the hardware is not enough; the system must be precisely managed to ensure efficiency, stability, and safety. The researchers identify three mainstream control approaches. The first, direct winding current control, involves monitoring and controlling the current in each individual motor phase. While intuitive, this method is challenging because the currents in the different phases are electrically coupled, making independent control difficult and controller tuning complex. The second approach, modular control, treats a multi-phase system (like a six-phase machine) as two separate three-phase rectifiers operating in parallel. This allows engineers to use well-established control techniques for managing the “zero-sequence circulating current” that can flow between the two parallel systems, a common source of inefficiency and imbalance. The third and most sophisticated method is multi-plane current control, which is rooted in VSD theory. This approach doesn’t just control the currents; it actively manages how the current energy is distributed across the different “planes” of the motor’s magnetic field. By strategically allocating current to harmonic planes (like the x3-y3 or x5-y5 planes), the controller can completely eliminate any current component in the torque-producing alpha-beta plane, guaranteeing zero rotation. This method offers the highest degree of control and paves the way for true software-based integration, where a single controller can seamlessly switch between driving and charging modes.
Beyond normal operation, the review also addresses a crucial real-world concern: fault tolerance. An EV’s powertrain must be reliable. The authors highlight that multi-phase motors are inherently more fault-tolerant than three-phase ones. If one phase fails, the others can often compensate, allowing the vehicle to continue operating, albeit at reduced power. The same principle applies to the EDROC system. The review details pioneering research into how an EDROC can continue to charge the battery even if one or more motor phases develop an open-circuit fault. For static field systems, this involves a sophisticated reallocation of current to the harmonic planes to maintain a zero net torque. For pulsating field systems, the problem is more complex, as the fault can disrupt the delicate balance needed to create a pure pulsating field. The research shows that with at least four healthy phases, a six-phase EDROC can still achieve stable, safe charging, a testament to the system’s robustness. This level of fault tolerance is a major selling point for automotive manufacturers, where safety and reliability are paramount.
The most forward-looking aspect of the paper is its exploration of the multi-energy port EDROC system, specifically designed for solar-powered electric vehicles (SPEVs). As the world seeks truly sustainable transportation, simply replacing gasoline with grid electricity is not a complete solution, especially if that grid relies on fossil fuels. Integrating solar power directly onto the vehicle offers a path to greater energy independence and reduced emissions. However, solar panels on a car are limited by surface area and efficiency, typically generating only enough power for a few miles of range per day. The challenge has been how to integrate this low-power, variable DC source efficiently into the vehicle’s existing architecture without adding more weight and complexity. The solution proposed in the review is a six-phase EDROC system that can accept power not only from the AC grid but also from a DC source, such as a rooftop solar panel or a DC fast-charging station. This is achieved by connecting the DC source between the two neutral points of the six-phase motor. In this configuration, the motor windings and inverter are reconfigured to act as a high-efficiency DC-DC converter, stepping up the relatively low voltage of the solar panel to the high voltage of the main battery pack.
This architecture enables a revolutionary new operating mode: “in-motion charging.” While the car is being driven, the solar panels can simultaneously charge the battery. The review details how this is achieved by injecting the solar-generated current as a “zero-sequence” current into the motor. This current flows through the windings but, because of its specific path, does not contribute to the torque that propels the car. It exists in a separate control dimension, allowing the propulsion and charging functions to operate independently and simultaneously. The paper presents experimental results from a 2kW prototype that validate this concept. The tests show that the system can efficiently track the solar panel’s maximum power point (MPPT), extracting the maximum possible energy even as lighting conditions change, and successfully charge the battery while the motor runs at a constant speed. This seamless integration of solar charging into the drive system is a game-changer, transforming the car from a passive energy consumer into an active, mobile energy harvester.
The implications of this technology are profound. By eliminating the dedicated OBC, automakers can free up valuable space in the engine bay or under the floor, potentially allowing for larger batteries or more passenger room. The significant reduction in component count and weight directly translates to lower manufacturing costs and improved vehicle efficiency, extending range. The ability to support multiple charging methods—single-phase AC, three-phase AC, DC fast charging, and solar charging—from a single, integrated platform offers unparalleled flexibility for consumers. A driver could plug into a standard household outlet, use a public three-phase charger, fast-charge at a station, or simply park in the sun, all with the same underlying hardware. This versatility is key to making EVs a practical choice for a wider range of users.
The research by Yu Feng, Yin Qihao, Tong Minghao, and Zhang Qianfan, published in the Proceedings of the CSEE, provides a comprehensive roadmap for the future of EV charging. It moves beyond a simple review of existing topologies to offer a clear vision of a more integrated, efficient, and sustainable automotive powertrain. The technical challenges are substantial, involving complex motor design, advanced power electronics, and sophisticated control algorithms. However, the potential rewards—lighter, cheaper, more capable, and truly renewable EVs—are too great to ignore. As the automotive industry continues its electrification journey, the multi-phase EDROC system stands out as one of the most promising and transformative technologies on the horizon. It is not just a new way to charge a battery; it is a reimagining of the entire electric vehicle from the ground up.
Yu Feng, Yin Qihao, Tong Minghao, Zhang Qianfan, Proceedings of the CSEE, DOI: 10.13334/j.0258-8013.pcsee.230269