With the emerging power electronic technologies, numerous electrical devices use high-frequency and nonsinusoidal supplies. Such sources generate complex harmonic fields that overburden the accurate prediction of core losses using the conventional finite element method (FEM). Thereby, to predict core losses under those high-frequency and nonsinusoidal supplies accurately, this study first built a test system for core losses measurement of ferromagnetic materials (e.g., soft ferrite, laminated steels, and nanocrystalline alloy) fed by the square and pulsewidth modulated (PWM) supplies. Then, the loss prediction problems of the classical core loss model under high-frequency and nonsinusoidal supplies are revealed. Afterward, an improved FEM-based core loss model is proposed, which includes a piecewise variable parameters based hysteresis loss model, an eddy current loss model considering different cross sections of magnetic circuit and supply waveforms, and improved excess loss coefficients. Experimental validations on different materials, the nanocrystalline alloy in an 11-kVA intermediate frequency transformer, the ultrathin silicon sheets in a 10-kW high-speed permanent magnet motor, and the nanocrystalline alloy in a 3-kW wireless power transfer system, are conducted systematically. The effectiveness and generality of the improved core loss model were verified.

This article proposes a rotor temperature estimation method for in-service induction machine (IM) based on parameter identification, which combines the advantage of recursive least squares (RLS) and model reference adaptive system (MRAS). The RLS with forgetting factor is firstly adopted to identify the parameters of motor inductances. Then, the online identification of rotor resistance can be realized via the MRAS based on the instantaneous reactive power (IRP) and the proportional integral (PI) regulation adaptive law designed by the Popov hyper-stable theory. Thereby, the rotor temperature is computed by the resistance-temperature relationship of metals. To achieve a fast convergence of the parameter identification, rotor slot harmonics are extracted from the stator current and used to determine the rotor speed in real time. Furthermore, to obtain a more accurate initial value of the stator and rotor leakage inductances and resistances, the first 5-15 cycles of the IM starting process are used to mimic the locked-rotor test condition. A merit of the proposed method is that it requires significantly less time to estimate the rotor resistance than the traditional methods. With a novel test bench, experimental validation is performed on a 22-kW IM. In the test, the rotor temperature is measured in three different ways: 1) wireless sensors inserted in the rotating rotor core and rotor end rings, 2) infrared sensor for rotor end ring temperature, and 3) PT100 installed in stator end winding. With this test bench, the real rotor temperature was revealed, and the effectiveness of the presented method is also verified.

Renewable energy units are required to have reliable fault ride-through capability to prevent large-scale disconnection of renewable energy plants due to grid faults. Since synchronous generators have better voltage and current resistance capabilities, one of the advantages of renewable energy connected to the grid through the motor-generator Pair (MGP) system is the isolation of grid faults, which is totally different from existing fault ride-through methods. First, the structure and mathematical model of the MGP system are introduced in this paper. Then, a novel control method for the MGP system based on DC link voltage feedback is given, which is very important for fault ride-through in the control power balance on both sides of the DC bus. The isolation mechanism, damping and reactive power support functions of the MGP system on grid faults are analyzed by means of the rotor motion equation and phasor diagram of synchronous machine. A 5-kW MGP simulation and experimental platforms are built to verify the isolation and protection effects of the MGP system on a renewable energy generation system during the transient processes of grid voltage sag, overvoltage and multiple voltage sags. The results demonstrate that the MGP system can isolate grid faults on the generator side, while the oscillation amplitude on the motor side is much smaller, and the generator can provide strong reactive power support to the grid during three grid faults.

A synchronous condenser (SC) can provide the reactive power for the Ultra High Voltage Direct Current (UHVDC) converter station to keep voltage stability and prevent commutation failure. The single-phase short circuit at a transmission line is prevalent and it can result in not only the voltage drop but also the increase of the rotor loss and temperature of the SC. In order to study the ability of the SC to withstand a single-phase short-circuit fault, the rotor loss and temperature field of a 300-MVar SC are studied by coupling the electromagnetic and temperature field models of the SC with the models of the electric circuit and power grid. The loss distributions in the rotor core and slot wedges are calculated under a single-phase short-circuit fault at the different positions of the transmission line. The rotor temperature distributions of the SC under over- and under-excitations are calculated and verified by the experiments. The influence of short-circuit duration on the rotor temperature of the SC is calculated under the single-phase short circuit. The study can provide a theoretic basis for the improvement of the operating ability of the SC under the single-phase short-circuit faults.

Due to the coaction of supply harmonics and the harmonics resulting from the motor structures, the mechanism and distribution characteristics of iron losses become more complicated in inverter-fed induction motors. Therefore, accurate prediction and fine analysis of iron loss are very important at the design stage of high-efficiency inverter-fed induction motors. In order to predict the iron losses accurately, this paper proposes a piecewise iron loss model whose parameters vary with the magnitude and frequency of flux density, and two additional flux density terms are introduced to the classical iron loss model considering the nonlinearity of magnetic material and harmonic fields. With this model, the iron losses are calculated for an inverter-fed 5.5 kW induction motor. The results reveal the distribution characteristics of hysteresis and eddy current losses in stator and rotor cores, and the characteristics of additional iron loss caused by harmonic fields. By the comparison of predicted and measured no-load iron loss under different supply voltages and switching frequencies, the proposed model and the analysis results are validated.

Accurate prediction of iron losses in electrical machines requires detailed information of flux density waveforms in the iron. In the case of an induction machine operating at partial load, many supply cycles must be modeled to obtain accurate flux density waveforms on the rotor. This may be particularly troublesome when nonsinusoidal supplies are used and many small time steps are required. This paper presents an approach to predict rotor flux density harmonics with data from a finite-element model of a single supply cycle. Using this approach, the influence of particular harmonics on total iron loss can be extracted. Predicted rotor losses are compared with those obtained using direct calculation of losses from time-stepped data and with test measurement.