Exciterless Wound-Field Medium-Power Synchronous Machines: Their History and Future

The excitation system (ES) of the wound-field synchronous machine (WFSM) has remained largely unchanged over the past 50 years. Nonetheless, there has been significant research into exciterless solutions, which, through the integration of the ES with the main machine, aim to offer improved power density, simplified manufacture, and better cooling opportunities.

DANIEL FALLOWS , STEFANO NUZZO , and MICHAEL GALEA T he excitation system (ES) of the wound-field synchronous machine (WFSM) has remained l a r g e l y u n c h a n g e d over the past 50 years. Nonet hele ss, t here has been significant research into exciterless solutions, which, through the integration of the ES with the main machine, aim to offer improved power density, simplified ma nufacture, and better cooling opportunities.
This work presents the history of exciterless solutions for WFSMs, focusing on the challenges for medium-power generators. The very latest innovations are discussed, signposting the future for the exciterless machine, along with new challenges and opportunities for engineers.

The Current State of WFSMs
The WFSM has a long history of power-generation applications, yet it has had limited success in emerging applications due to low power density, despite other advantages [1]. This is largely due to the ES required to supply power to the rotating field winding, for which the typical ac exciter represents a significant additional volume [2]. Exciterless has long been suggested as the next step-improving power density by integrating the exciter and main machine magnetic circuits. Additional benefits, including simplified manufacture, are also key drivers, as illustrated in Figure 1.
This work begins with a brief discussion of the ESs in common use today before an in-depth look at exciterless topologies, in which the excitation magnetic circuit is combined with that of the main machine. Modern ESs using wireless power transfer (WPT) topologies and how these signpost the future of WFSM excitation are discussed.

Conventional ESs
Before the invention of the silicon rectifier, excitation was achieved using a complex dc ES [ Figure 2    a controllable dc generator produces the excitation power, and the commutator provides rectification [2]. The need for both a commutator and slip rings posed a key failure point such that, with the development of robust silicon diodes, alternatives in the form of static slip ring excitation [ Figure 2(b)] and the ac exciter [Figure 2(c)], were developed. A slip ring system is supplied by an externally controlled dc source, typically from a power converter. It is prone to the same wear as the dc ES but allows for static excitation by decoupling the field current from the rotation of the machine, which results in faster transient response and start-up.
In contrast, the "brushless" ac exciter replaces the commutator/slip ring arrangement with a shaft-mounted diode rectifier. This provides a maintenancefree solution while still leveraging the prime mover power, thereby minimizing the converter requirements of the control system. These advantages make it the dominant choice for medium-pluspower machines [3], [4]. Both of these solutions are still in common use today despite a number of exciterless topologies having been proposed.

The Exciterless Synchronous Machine
The most apparent solution to exciterless is the integration of the ac exciter windings within the main machine magnetic circuit; however, this presents limitations in the form of increased magnetic loading, lack of a damper winding (fitted to improve harmonic and transient performance [5]- [7]), and greater winding complexity. Exciterless development has mainly focused on winding complexity, producing topologies that allow for the excitation and main machine currents to flow in the same windings. These simplifications, shown in Figure 3, have been "mixed and matched," producing many unique topologies.
These combined magnetic circuit exciterless methods can be categorized by their excitation scheme into three types: ■ dc field excitation methods, in which the stationary excitation field winding produces fixed magnetic poles ■ fundamental ac excitation methods, in which the stationary excitation field winding is energized with ac at the same frequency as the machine output ■ harmonic ac excitation methods, in which the excitation is derived from the harmonics of the main machine output. It should be noted that all of the methods discussed in this article are demonstrated on low-power machines, with only two greater than 3 kVA. This raises questions around the scalability of these methods to higher-power machines and their suitability for medium-power applications.

Base Method-Separate Electrical Circuits
Patents from the 1960s discuss replicating the ac exciter circuits within the main machine [see Figure 3(a) and (h)], citing advantages in terms of reduced manufacturing cost and machine size [8], [16]. In general, fitting additional rotor windings presents an issue for salient pole machines, as they do not have the freedom of a slotted round rotor. Nonetheless, a solution is presented in [17] for a singlephase machine in which the pole shoe contains a slot.
A recent take on this method is presented in [9] and [18], in which the excitation magnetic circuit is an induction, rather than synchronous, machine. The rotor winding is that of Figure 3(h), whereas the stator, displayed in Figure 3(b), contains two separate three-phase windings, one an induction machine armature connected to a converter, and the second the synchronous machine armature for the output. The converter control allows for self-excitation without the need for an external excitation power supply.

Double-Star Stator-Combined Excitation and Load Circuit
A double-star stator winding configuration was patented in the 1970s that allows both an ac excitation current and ac main output current to flow in a single winding [11]. This exciterless topology saw significant development by the patent author, with publications throughout the 1980s, including an alternative structure in which the excitation current is supplied at the neutral point [10]. These topologies are displayed in Figure 3(d) and (c), respectively, and coupled with the rotor of Figure 3(h).
The winding is shown in [19] and [20] to produce a higher output for a given excitation current when excited by a dc source. Additionally, the configuration has been discussed as a rotor winding simplification, though not implemented [20], although a singlephase rotor configuration is demonstrated in [21]. The method, with dc excitation between the neutral points as depicted in Figure 3(c), is demonstrated on a salient pole machine utilizing the rotor winding topology of the "Separate Rectified Pole Windings" section [ Figure 3(i)] described in [22]. This produces an exciterless machine with a very simple winding structure.

Separate Rectified Pole Windings
A rotor winding simplification for salient pole machines is to fit a parallel rectifier diode to each pole [ Figure 3(i)]. First shown in [15] and followed by development through the 1980s and 1990s, this method relies on the significant mutual inductance between the pole windings to ensure that the pulsating pole current produces a constant air-gap flux.
A single-phase machine is shown in [23] in which a freewheeling diode is placed in parallel with the stator field winding to reduce the current ripple and improve excitation. The same concept is depicted in [24] for a two-pole machine with both threeand single-phase configurations. The finite element analysis of the machine concept is detailed in [25]- [27].
The rotor topology is typically coupled with a dc excitation winding [ Figure 3(a)], but it was used with a harmonic excitation scheme in [28], similar to Figure 3(f) but with the reactive load connected to an additional single-phase six-pole winding. This demonstrates the versatility of this rotor winding method.

Harmonic Excitation Schemes
Harmonic excitation schemes were first shown on single -phase machines [ Figure 3(e)], where the negative sequence current produced by the load provides an excitation source, with capacitors to aid regulation [12]. Three-phase harmonic excitation schemes have been shown, typically using the dual rotor winding of Figure 3(h) to allow the excitation pole number to match the harmonic frequency.
The fifth harmonic is utilized in [13] and [29] with a reactive load [ Figure 3(f)] and dc excitation winding [ Figure 3(a)], respectively, with both methods providing regulation. A form using the second harmonic is first demonstrated in [30] using a capacitive load; this is moved to an additional winding in [31] and analytically modeled with core loss in [32]. The use of a converter to impose harmonics [ Figure 3(g)] is demonstrated in [14] and [33] with a simplified round rotor structure [ Figure 3(j)]. A similar ES for salient machines is proposed in [34] using the dual rotor winding of Figure 3(h) with winding slots on the pole shoe to house the excitation armature.    [8], (b) a converter-controlled induction machine excitation [9], (c) a double-star winding with an excitation current injection between neutral points [10], (d) a doublestar winding with an excitation current injection on the phase center taps [11], (e) a single-phase harmonic excitation with capacitive loading for regulation [12], (f) a harmonic excitation with a reactive load for start-up and regulation [13], (g) a harmonic excitation current injected using a back-to-back converter [14], (h) a round rotor method with additional armature winding [8], (i) a salient pole method with pole windings individually rectified [15], and (j) a round rotor method with shorting diodes to produce a dc field [14].

Summary
designer faces many design challenges, particularly where established designs utilizing a conventional ES are the focus. The reduction of performance, due to the removal of the damper cage, and potential drops in power density, due to magnetic loading, are hard to justify to existing customers. Restrictions in winding design place further constraints and require a significant development effort to implement across an existing portfolio. The methods presented are summarized in Table 1, and, while the construction complexity is minimized, this is at the expense of design flexibility and performance. To minimize business risk and leverage current designs, an exciterless method is required that does not impact the main machine design.

WPT Excitation
WPT has been a focus of recent research for many application areas, and WFSM excitation is no exception [35]. Both inductive and capacitive WPT have been demonstrated, most often applied to traction machines, where reliability and power density are significant requirements, and conventional ESs are unsuitable. These basic topologies are shown in Figure 4.
A detailed review of both inductive and capacitive systems for smallgap applications, i.e., those similar to a WFSM ES, is presented in [36]. This work shows that systems requiring power greater than 1 kW and those with a primary/secondary gap greater than 1 mm favor inductive power transfer.
Not relying on a combined magnetic circuit, a WPT ES allows freedom in the main machine design, in contrast to the methods discussed in the "Exciterless Synchronous Machine" section, and, importantly, features such as the damper cage can remain. The implementations shown are more compact than the ac exciter and maintain the brushless benefits. However, these methods present new limitations that must be considered by the designer. Both WPT topologies require relatively high operating frequencies, which influences both the materials used and types of switching devices in the control system. These are key considerations for traditional applications where low cost must be maintained.

Inductive WPT Excitation
An inductive WPT ES is, in essence, a rotating transformer, with reactive load compensation when operated at the resonant frequency. Noncompensated ESs have also been demonstrated, such as a 50-Hz slip ring replacement system in [37], a compact 20-kHz design for traction machines in [38], and a similar 20-kHz system proposed for a 75 -MW hydrogenerator [39]. Compensated inductive WPT ESs include a 40-kW traction machine, demonstrated in a series of papers [40]- [42], followed by a 60-kW machine in [43]. At the higher power range, a 12-kW ES is proposed in [44]. These works have shown that not only is inductive WPT a viable method, but it also scales well up the power range.

Capacitive WPT Excitation
Capacitive WPT ESs were developed over a series of papers starting with [45], in which a parallel disk capacitor arrangement is used to excite a 1.5-kW machine. A similar stacked plate arrangement for an 80-kW traction machine is depicted in [46], followed by using printed circuit boards as the capacitive coupling in [47] for a 30-kW generator. Alternative coupling arrangements using fluid [48] and journal bearings [49] have also be demonstrated. While successful, capacitive methods require tight tolerances and operate at high resonant frequencies.
Summary Both WPT topologies have been shown as valid ESs for the WFSM, with traction machine applications of key interest, driven by a demand for alternatives to permanent-magnet machines. A summary of the key characteristics  of each method against the ac exciter is provided in Table 2. There remain viability challenges in traditional power-generation applications. The most notable of these is ensuring low costs from the materials used, ancillary components (such as power converters), and manufacturing methods. In addition, there remains the opportunity of further integration to produce an ES with the desired exciterless benefits.

The Future of WFSM Excitation
The WFSM exists over such a significant power range that there will never be a perfect singular solution for excitation. For classical medium-power generator applications, the ac exciter is the only viable option, shown by its current and long-term dominance in this area.

The Problem
The commercial development of the classical WFSM, as used in applications such as continuous islanded power generation, grid support, and backup generation, has stagnated, with products receiving only minor design changes for decades. Manufacturers have, instead, maintained a competitive edge through cost reduction rather than product development.
This manufacturing optimization has led to a highly integrated product range, where changes to a design can represent significant cost and business risk, including long recertification processes requiring multiple prototype machines [50]. Considering this, manufacturers are understandably reluctant to invest in development, particularly where market competition is tight. In short, a step advancement is needed to reinvigorate and kick-start development in this field at an industrial level.
Exciterless remains the "Holy Grail" to provide this step improvement in product performance, but only if it can be achieved without modification of the existing main machine design, thus ensuring an acceptable development cost and meeting additional challenges arising from strict power quality and transient performance requirements. As this article has shown, existing exciterless solutions have yet been unable to achieve this.

The Solution
WPT has been shown to be an effective ES that can be implemented without change to the machine design.
The remaining challenges are to 1) integrate the WPT system to produce an exciterless machine and 2) reduce the manufacturing cost so that it is competitive against the brushless exciter.
The classical air-cooled generator presents an opportunity for the designer to achieve an exciterless machine by integrating the WPT system within the fan and its housing. As the largest rotating component, the fan allows large coils to maximize coupling. The manufacturing cost of this arrangement, shown by the rendering in Figure 5, can be further reduced by utilizing an air-cored system rather than ferrite. The economies of scale can also be maintained by sizing the ES against the largest machine in the product range, as the fan and housing are typically shared. Finally, converter demands and costs can be reduced by targeting low resonant frequencies in the design.

Conclusion
The WFSM ES has had a long development, yet only a handful of topologies are in use today, as shown in Figure 6. While many exciterless topologies have been developed over the past 60 years, these have failed to gain widespread use against the ac exciter. Fortunately, new WPT ESs have been demonstrated, which, through the creative use of existing machine components, could provide the promised benefits of exciterless without significant compromise or costly redesign of the main machine. There are still challenges to overcome, but, for the medium-power generator market, hope is on the horizon. which he has become an expert in the field and authored a number of recent works related to this area. Stefano Nuzzo (stefano.nuzzo @unimore.it) earned his Ph.D. degree in electrical engineering in 2018 from the University of Nottingham, U.K., where he also worked as a research fellow within the PEMC Group. Since October 2020, he has been a lecturer at the Department of Engineering "Enzo Ferrari," University of Modena and Reggio Emilia, Modena, 41125, Italy. His research interests include the analysis, modeling, and optimization of electrical machines. As proven by his publication track, he has seamlessly worked and contributed to renewing the design of wound-field synchronous generators and their brushless excitation, thus becoming an expert in the field. He is a Member of IEEE.

Biographies
Michael Galea (michael.galea @nottingham.edu.cn) earned his Ph.D. degree in electrical machine design in 2013 from the University of Nottingham, Nottingham, U.K. He is a professor in electrical machines and drives with the PEMC Group, University of Nottingham, Ningbo, 315100 China. His research interests include the design and development of electrical machines and drives (classical and unconventional), reliability and lifetime degradation of electrical machines, and more electric aircraft. He has worked on synchronous generators for more than 10 years and has supervised at least six doctoral theses on these machines. He is a fellow of the Royal Aeronautical Society and a Senior Member of IEEE.