This article discusses the maximum torque per ampere (MTPA) control of synchronous motors, which have become an indispensable part of highly efficient motor drives. It explains the nature of torque produced by synchronous motors, ways to find its maximum and algorithms to operate at this point, despite changes of loads and motor parameter variations. The authors propose a classification of the MTPA methods, based on the features of each algorithm or group of similar methods. They demonstrate the conventional control scheme and discuss the modifications necessary for the implementation of each method. This article reviews existing MTPA control algorithms, discusses their pros and cons, and suggests possible areas of usage for each group of methods. The authors of the article share their vast experience in the industry and research aspects, which were obtained by developing industrial, commercial, traction, and military drives, and report on their views on the perspective of each method taken into consideration.

Flux-Weakening (F-W) based on feedback voltage regulation is commonly adopted in Interior Permanent Magnet Synchronous Motor (IPMSM) drives. Voltage space vector magnitude is controlled in closed-loop, modifying the current reference, following a value related to the inverter limitations. Stable and fast voltage control allows to operate with lower voltage margin, leading to higher torque vs. speed capability. Theoretical analysis and gain adaptation of the flux-weakening regulation loop was reported in [1][2], partially overcoming the difficulties due to the strong non-linearity of the plant. An approximated closed-form design method was proposed in [3]. This allowed application of the algorithm to drives where auto-tuning is needed, ensuring stability and dynamical performances. The method was further refined in [4]. A fundamental enhancement is introduced in this paper, namely the use of a speed regulator with explicit torque reference output. An advantage of this technique is that the speed loop becomes linear in the whole operating range and accurate control of machine torque is possible. Thanks to a novel gain adaptation for the voltage regulator, the flux-weakening behavior is decoupled from the speed control response at speed steady state (at constant or slowly-changing torque), improving performance of the overall drive control. Adoption of this novel method allows smooth operation with invariant dynamical behavior of both the speed and F-W control loops. Extensive simulations and experimental tests are reported to prove the validity of the proposal. Sensorless operation of the drive system has also been considered in the tests to further validate the proposed solution.

Flux-Weakening (F-W) methods based on feedback voltage regulation are widely adopted in Interior Permanent Magnet Synchronous Motor (IPMSM) drives. Generally, the setpoint must be lower than maximum available voltage, to prevent saturation of current regulators during transients. However, the margin should be small, to exploit torque capability of the machine and avoid additional current magnitude during F-W operation. The trade-off between F-W control bandwidth and the necessity of voltage margin has not been analyzed in-depth in previous literature. In fact, F-W regulator design aiming at a certain bandwidth has been addressed by means of gain adaptation, but the choice of the target bandwidth remained as an open issue. In this paper, the crucial transient conditions which can cause the saturation of current regulators, specifically the reduction of input voltage, the fast acceleration of the drive and the transient torque demand, are considered. The input voltage reduction affects the available voltage, while fast acceleration and increase of torque demand push towards higher voltage, requiring disturbance rejection action by the F-W controller. All these conditions are analyzed in terms of worst-case requirement and the proposed analytical modeling is verified by means of simulations and experiments. The findings can be applied in the design of automatic tuning procedures and design for high-reliability or safety-critical applications.

An auto-tuning and self-adaptation procedure for High Frequency Injection (HFI) based position and speed estimation algorithms in Interior Permanent Magnet Synchronous Motor (IPMSM) drives is proposed in this paper. Analytical developments show that, using conventional approaches, the dynamics of the high-frequency tracking loop varies with differential inductances, which in turn depend on the machine operating point. On-line estimation and adaptation of the small signal gain of the loop is proposed here, allowing accurate auto-tuning of the sensorless control scheme which does not rely on a priori knowledge of the machine parameters. On-line adaptation of Phase-Locked Loop (PLL) gains and of the injected voltage magnitude is also possible, leading to important advantages from the performance, loss and acoustic point of view. The theoretical basis of the method has been introduced first and the main concept demonstrated by means of simulations. Implementation has been carried out using the hardware of a commercial industrial drive and two Interior Permanent Magnet Synchronous Motors, namely a prototype and an off-the-shelf machine. Experimental tests demonstrate the feasibility and effectiveness of the proposal.

In the field of power electronics-based electrical power conversion, the Dual Active Bridge (DAB) topology has become very popular in recent years due to its characteristics (e.g., bidirectional operation and galvanic isolation), which are particularly suitable to applications such as interface to renewable energy sources, battery storage systems and in smart grids. Although this converter type has been extensively investigated, its analysis and control still pose many challenges, due to the multiple control variables that affect the complex behavior of the converter. This paper presents a theoretical model of the single-phase DAB converter. The proposed model is very general, i.e., it can consider any modulation technique and operating condition. In particular, the converter is seen as composed by four legs, each capable of generating voltage on the inductor, and by the two output legs, which can steer the resulting inductor current to the load. Three variables are considered as the control inputs, i.e., the phase-shifts with respect to one leg. This approach results in a very simple yet accurate closed-form algorithm for obtaining the inductor current waveform. Moreover, a novel analytical model is proposed for calculating the average output current, based on the phase-shift values, independently of the output voltage. It is also shown that average output current can be varied cycle-by-cycle, with no further dynamics. In fact, average output current is not affected by the initial value of inductor current or by DC offset (which may arise during transients). The proposed models can be exploited at several stages of development of a DAB: during the design stage, for fast iteration, when selecting its operating points and when designing the control. In fact, based on the analytical results, a novel control loop is proposed, which adopts a “fictitious” (i.e., open-loop) inner current regulation loop, which can be applied to any modulation scheme (e.g., Single Phase-Shift, Triple Phase-Shift, etc.). The main advantage of this control scheme is that the simple dynamics of the output voltage versus the average output current can be decoupled from the complicated relationship between the phase-shifts and the output current. Moreover, a Finite Control Set (FCS) method is proposed, which selects the optimal operating points for each operating condition and control request, ensuring full Zero-Voltage Switching (ZVS) in all cases. The analytical results obtained and control methods proposed are verified through simulations and extensive experimental tests.