High-resistance connections in electrical machines cause unbalances in the stator resistances, reduce the efficiency, and increase the fire hazard. In this paper, the problem of detection of high-resistance connections is investigated for multiphase induction machines with an odd number of phases. The main contribution of this paper is a control scheme that can determine the stator resistance unbalance of all phases, under transient and steady-state operating conditions. The theoretical analysis and the feasibility of the control scheme are confirmed by experimental tests.

It is known that classical fast-Fourier-transform-based steady-state spectrum analysis, such as motor current signature analysis, may fail to detect outer cage damage in double-squirrel-cage induction motors. This is because the magnitude of the rotor fault frequency components (RFFCs) in the current spectrum of faulty motors is small, due to the low-magnitude current circulation in the outer cage under a steady-state operation. The probability of misdetection is higher in time-varying load applications, such as conveyor belts, pulverizers, etc., for which double-cage motors are frequently employed. In case of load variation, the small RFFCs are spread in a bandwidth proportional to the speed variation, which makes them even more difficult to detect. A diagnosis method based on discrete wavelet transform and optimized for sensitive detection under transient operating conditions is proposed in this paper. An experimental study on a custom-built fabricated Cu double-cage-rotor induction motor shows that the proposed method can provide improved detection of outer cage faults particularly used in time-varying load applications.

This paper investigates a new approach for the detection of rotor electrical faults in three-phase wound-rotor induction machines. The method is assessed in a wind energy conversion system, where the rotor side of the wound-rotor induction machine is supplied by a static converter for the control of active and reactive power flows from the generator to the electrical grid. Here, a diagnostic space vector is presented that sensibly improves the rotor fault detection in wound-rotor induction machine by combining the spectral signatures of both rotor current and voltage space vectors. The dc component of this diagnostic space vector, that is equal to zero in healthy conditions and different from zero under rotor asymmetry, is adopted as rotor fault index. The main advantages of the proposed technique are the implementation simplicity and the capability to operate even during speed transients. The technique is also validated by means of simulation and laboratory scale experimental tests.

High-resistance connections in electric drives can cause localized overheating and motor supply voltage unbalance, which degrade the performance, efficiency, and reliability of the system. An enhanced field-oriented control scheme for induction machines that is capable of detecting resistive unbalance due to high-resistance connections, and regulating the negative-sequence current is proposed as the main contribution of this paper. Resistive unbalance is detected and located while maintaining the symmetric drive behavior both under transient and steady-state operating conditions. The negative-sequence regulator adopted in addition to the traditional current regulator for rotor field-oriented control is used to compensate for the voltage unbalance caused by the inherent asymmetries in the cable and stator winding and by the poor contacts. A model that shows the relationship between the resistive unbalance and negative-sequence current components is derived from the analysis of the proposed scheme. The theoretical analysis and the validity of the detection technique are confirmed with an experimental study on a 4-kW induction motor drive.