Minimization of the Electromagnetic Torque Ripple Caused by the Coils Inter-Turn Short Circuit Fault in Dual-Redundancy Permanent Magnet Synchronous Motors
With the development of electric vehicles and More-Electric/All-Electric aircraft, high reliability is required in motor servo systems. The redundancy technique is one of the most effective methods to improve the reliability of motor servo systems. In this paper, the structure of dual-redundancy permanent magnet synchronous motor (DRPMSM) with weak thermal coupling and no electromagnetic coupling is analyzed and the mathematical model of this motor is established. However, there is little
... ere is little research on how to suppress the torque ripple caused by short-circuited coils in the DRPMSM. The main contribution of this paper is to present the advantages of DRPMSM and to find a way to suppress the torque ripple caused by the short circuit fault in DRPMSM. In order to improve operation quality and enhance the reliability of DRPMSM after a short circuit occurs, the torque ripple caused by the coils inter-turn short circuit fault in DRPMSM is analyzed in detail. Then, a control method for suppressing the electromagnetic torque ripple of a short-circuited coil is proposed for the first time by using an improved adaptive proportional resonant (PR) controller and a proportional integral (PI) controller in parallel. PR control is a method of controlling alternating components without steady-state error, and it can be used to suppress torque ripple. DRPMSM adopts speed and current double closed-loop control strategies. An improved adaptive PR controller and a PI controller are employed in parallel for the speed loop, while traditional PI control is adopted in current loop. From the simulation and experimental results, the torque ripple is reduced from 45.4 to 5.6% when the torque ripple suppression strategy proposed in this paper is adopted, in the case that the speed is 600 r/min. The torque ripple suppression strategy based on the PR controller can quickly and effectively suppress the torque ripple caused by the short-circuited coils, which makes the motor speed more stable. 2 of 23 widely used in various fields      . Consequently, the permanent magnet synchronous motor has drawn more and more attention and recognition. With the development of the electric vehicle and the More-Electric/All-Electric aircraft technique, higher reliability of PMSM servo systems is required. The electrical and mechanical actuator is a kind of servo system, and it is an important component of both aircraft and electric vehicles. The redundancy technique is one of the most effective methods to improve the reliability of the PMSM servo system by adding extra resources  . Especially, in the case of high reliability, dual-redundancy permanent magnet synchronous motors (DRPMSMs) will play an important role [7, 8] . Generally, the structure of DRPMSMs can be classified into two categories, that is, series structure and parallel structure            . In the series structure of the dual-redundancy motor, two motors are coaxially connected     . However, in the parallel structure of the dual-redundancy motor, the stator has two sets of three-phase windings and shares one permanent magnet rotor          . The series structure of the dual-redundancy motor obviously does not have the electromagnetic and thermal coupling between the two sets of windings, but it leads to a large volume, torque dispute and other shortcomings  . Thus, in comparison, the parallel structure of the DRPMSM has more advantages. In the past few years, dual-redundancy motors with a parallel structure have been presented in many papers. In      , the parallel structures of the dual-redundancy motor consist of two sets of independent windings with 30 electrical degree shifts in space, and a mutual rotor. The mutual inductance between two sets of three-phase windings is large as they are alternately embedded in stator slots. In  , the principle of the decoupling through two different winding connections (distributed and concentrated) for the double-star winding permanent magnet motor is presented. In  , the structure of the dual-redundancy motor is comprised of one set of windings embedded in half of the circle of stator and the other set embedded in the other half of that, and two sets of independent windings have zero electrical degree shifts in space. This structure has reduced the mutual inductance between two sets of windings compared with the previous structure of dual-redundancy motors. In  , the insertion of an additional tooth with a particular geometry reduces the magnetic coupling between the two adjacent layers and consequently reduces the coupling of the two channels significantly. However, the inductance matrix of dual-redundancy motors is not a diagonal matrix. Thus there is no complete decoupling between the two degrees of redundancy. There are still mutual inductances between the two sets of windings of the parallel structure of the DRPMSMs, which means there is electromagnetic coupling. When a set of three-phase windings faces a dangerous short circuit fault, the other set of normal three-phase windings will also be affected by short-circuited coils from electromagnetism. To address this problem, the DRPMSM with weak thermal coupling and no electromagnetic coupling is proposed in this paper  . There is no electromagnetic coupling between two sets of windings in this type of DRPMSM. Under normal conditions, two sets of three-phase windings of the DRPMSM with weak thermal coupling and no electromagnetic coupling operate at the same time, in a dual-redundancy operation mode. When a fault occurs in one set of three-phase windings, the power supply of this set of windings is cut off, and the other set of three-phase windings continues to supply power. In single-redundancy operation mode, in which the other set of three-phase windings still works, the reliability of the motor is effectively improved. The main faults of motors are winding open circuit fault and winding short circuit fault [23, 24] . When the winding open circuit fault occurs, it has no adverse effect on the normal windings. When the motor is operating in single-redundancy operation mode due to a short circuit fault, the fault windings will not have an impact on the normal winding due to the electromagnetic power. But the interaction between the current of short-circuited coils and permanent magnetic field not only produces a brake electromagnetic torque corresponding to the copper loss of short-circuited coils, but also produces an alternating electromagnetic torque with a doubled supply power frequency. The alternating electromagnetic torque affects the closed-loop speed controller, which will lead to the fluctuation of speed and increase vibration noise    . There is little research on how to suppress the torque ripple caused by short-circuited coils in the DRPMSM. Energies 2017, 10, 1798 3 of 23 This paper analyzes the inter-turn short circuit fault of the DRPMSM with weak thermal coupling and no electromagnetic coupling. The novelties of the DRPMSM with weak thermal coupling and no electromagnetic coupling include two aspects. The first aspect is that the DRPMSM adopts 12 slots, and 10 poles, which can eliminate the electromagnetic coupling between phases. In other words, the mutual inductance of each phase is almost zero. The second aspect involves placing the small teeth at the center of the slot, in which the coils on two sides of the small teeth are in different phases. The insulating plates are placed on two sides of the small teeth so that the phases have weak thermal coupling. The inter-turn short circuit fault of the DRPMSM may cause the output torque of the motor to fluctuate. The torque pulsation of the DRPMSM can generate abnormal vibration and unwanted speed fluctuation. Therefore, reducing the torque pulsation is essential to improve the torque performance of the DRPMSM drive system in single-redundancy operation mode. In order to overcome the torque ripple caused by the current of short-circuited coils and improve the performance of the single-redundancy operation of the DRPMSM, this paper firstly proposes an electromagnetic torque ripple suppression strategy for the DRPMSM, based on the proportional resonant (PR) controller [28, 29] . When a coils short circuit fault occurs and the DRPMSM shifts into single-redundancy operation mode, the PR controller is paralleled with the proportional integral (PI) controller in the speed loop to suppress the electromagnetic torque ripple output. As an inter-turn short circuit fault of the DRPMSM may occur at various speeds, a frequency adaptive PR controller is adopted in this paper. The simulation and experimental results show that the ripple of the motor output electromagnetic torque caused by short-circuited coils is effectively suppressed, and the motor speed becomes more stable. The Structure of the Novel DRPMSM The cross-section of the DRPMSM with weak thermal coupling and no electromagnetic coupling and its stator windings outspread are shown in Figures 1 and 2 respectively.