Virtual flux (VF)-oriented control (VFOC) and VF-based direct power control (DPC) (VF-DPC) have been developed to improve voltage-oriented control and DPC of three-phase active rectifiers. The VF space vector is utilized in transformations between stationary and rotating coordinates in VFOC and in obtaining instantaneous power in VF-DPC. The VF space vector is calculated by integrating the grid voltage space vector. This integration is usually performed using a first-order low-pass (LP) (FOLP) filter, which counteracts the saturation and dc-drift problems associated with pure integrators. However, the dynamics of FOLP filters can be enhanced to a great extent. This paper presents a new, simple, and fast integration algorithm for VF-based control methods. Simulations and experimental tests on a VF-DPC-based system showed that the proposed algorithm leads to rapid recoveries after grid voltage sags occur. Moreover, the performance of VF-DPC under nonideal grids is discussed.

Different techniques have been developed to detect rotor asymmetries in asynchronous motors. Although the reliability and the amount of information these techniques provide about a machine's state are indubitable, they still have a serious limitation: failure detection when the motor is driving a variable load torque. In this case, the motor phase currents are modulated by torque oscillations, and the information they contain about the integrity of bars and end rings is altered. This paper reports a new diagnostic method, based on the measurement of the magnetic flux linked by one stator tooth, which allows perfect simple discrimination between the actual presence of rotor asymmetries and the spurious effects caused by the oscillations in the load torque off the driven machine even when it is part of an adjustable speed drive and the motor is operating under a variable supply frequency.

The detection of rotor asymmetries in squirrel-cage induction motors has been an important research topic during the last two decades that has given rise to the development of various diagnostic techniques. Despite these techniques being reliable and providing much information about a machine's state, they still lack the capability to detect a failure when the motor is driving a machine, producing oscillations in the load torque. If this happens, then the motor phase currents are modulated by torque oscillations and the information that they contain about the integrity of the bars and endrings is no longer correct. Currently, the authors are not aware of a simple and reliable method that exists for the industrial diagnosis of rotor asymmetries in working cage motors under arbitrary load conditions. This paper describes a new diagnostic method and a new portable electronic instrument that are based on the measurement of the magnetic flux linked by one stator tooth, which allows for an optimal simple discrimination between the actual presence of rotor asymmetries and the spurious effects that are caused by the oscillations in the load torque of the driven machine.

One of the best known control methods for three-phase active rectifiers is the so-called direct power control (DPC). The control algorithm of the DPC is primarily based on the regulation of instantaneous active and reactive power. Because the DPC method can operate without any grid voltage sensors, instantaneous power and grid phase voltages must be estimated. The value of the essential coupling inductor between the rectifier and its three-phase supply play an important role in these estimations. This value is theoretically known; however, the grid inductance, which is sometimes difficult to determine precisely, or the coupling inductor tolerance, among other factors, may lead to errors in the inductance value. These errors have a negative and considerable effect on the instantaneous power and grid voltage estimation. Therefore, they provoke an improper operation of the DPC algorithm and the rectifier itself. To avoid these drawbacks, two simple, fast and robust inductance online estimation algorithms (analytic and iterative) are proposed in this paper. The proper operation of both methods is shown by simulation and experimental results.

Insulation faults in electrical rotating machines are a matter of great industrial interest since the economic losses associated with these events are usually equal or even higher than the cost of the machine itself. For this reason, the development of monitoring and diagnostic techniques to detect this failure during its earliest stages has been a subject of great interest. On the other hand, the accuracy of the quality controls applied to the manufacturing of insulation systems is an essential issue to reduce fault rate and increase machine life. The aim of this paper is to present a comprehensive review of the scientific literature in this field, including not only research papers but also existing standards for off-line tests. A critical review of their diagnosis capability, especially on low voltage machines, is essential to clarify the reliability of these techniques as well as the convenience of its generalized industrial use.

Coupling inductors are the most commonly utilized filtering option in grid-connected direct power control (DPC)-based active front-end rectifiers integrated into three-phase adjustable-speed drives (ASDs). Inductor faults, e.g., common interturn short circuits, can drive the whole system to failure. In this paper, a novel strategy is proposed for the early detection of incipient faults in the coupling inductors of the aforementioned ASDs. The proposed method enables a fast identification of the defective phase. The strategy also provides an accurate estimation of the inductance value of the coupling inductors that improves the performance of the DPC method. Simulations and experimental tests demonstrate the validity of this detection and estimation strategy.