Klaus Krischan

Calculation of load-dependent equivalent circuit parameters of squirrel cage induction motors using time-harmonic FEM

Two methods to determine the equivalent circuit parameters of squirrel cage induction motors (IM) are presented and compared. The first one is based on the time-harmonic finite element method (FEM) simulation of the measurement process (short-circuit and no-load test), yielding an equivalent circuit with constant parameters. In addition, the time-harmonic FEM is applied in the second method to calculate the load-dependent equivalent circuit parameters of squirrel cage IMs. Here, the material properties in a two-dimensional finite element model of the induction machine are linearized in each operating point and the superposition principle for the magnetic flux is applied to define the leakage inductances. Hence, the parameters are calculated for every operating point separately, thus the variations of the parameters due to skin effect in the rotor bars and due to material saturation under arbitrary load condition and rotor speed can be taken into account.

Low cost speed control for single phase induction motors - comparing different approaches with regard to efficiency

Operating single phase fed motors across a wide range of speed and load single phase fed motors across a wide range of speed and load with high efficiency is of growing interest. The paper at hand presents an investigation into the efficiency of different low cost variable speed drive concepts (namely phase control, integral (half) cycle control and integral switched cycle control) based on capacitor run single phase induction motors. Simulations and measurements concerning for example the efficiency achieved and the speed-torque characteristics of the different concepts are presented. Subsequently, ideas for improvements (selecting a different run capacitor, lowering the inrush current and the amplitude of the fundamental component by combining phase control with integral switched cycle control) are put forward.

Condition monitoring and failure prognosis of IGBT inverters based on on-line characterization

An inverter characterization technique is used to improve inverter fault diagnosis and prognosis techniques with the demonstrative bond wire lift-off fault. Knowledge of the nonlinear voltage-current characteristic of each inverter device is used to achieve these improvements. Three contributions are made in this paper to the field of inverter condition monitoring by using the characterization technique: use of the inverter device characteristics to improve a classifier-based diagnostic technique, direct use of the device characteristics for diagnosis, and improving the calculation of remaining useful life with the device characteristics. The diagnostic method improvements are verified experimentally by adding resistances to create an artificial fault. Thermal and electrical simulations are used to demonstrate the improvements made to the prognostic method.

Experimental determination of specific power losses and magnetostriction of electrical steel coils

A method for measuring the specific losses and magnetostriction of grain-oriented electrical steel coils up to six tons is presented. Since transformer core manufacturers are mainly supplied with entire steel coils, a fast incoming goods inspection is desirable. The proposed method identifies the specific losses at certain flux densities of the entire steel coils. In addition, the magnetostriction of the coiled material is measured. With the magnetostriction the main property for transformer no-load noise can be classified. The structure and setup of the developed coil measurement system (CMS) are shown. The measurement technique is explained and its requirements are identified. Initial results of specific power losses and magnetostriction of electrical steel coils as well as the reproducibility of the results are highlighted.

Inverter Device Nonlinearity Characterization Technique for Use in a Motor Drive System

This paper introduces a detailed nonlinearity characterization technique for insulated-gate bipolar transistor inverters in a drive system which is able to measure device nonlinearities. Methods are derived from measurements of the voltage on the inverter phases. These measurements are obtained while applying a set of direct currents to an electrical machine. These methods are then applied to tests using alternating current. The study of inverter nonlinearities is useful in order to conduct condition monitoring. These methods fulfill the need to characterize each device in an inverter. In addition to characterizing every device using either ac or dc current, the method is resistant to noise; for these reasons, the method is applicable in the field. Simulations and experimental work demonstrate the benefits and applicability of the methods.

Direct steady-state computation of mechanical vibrations in electrical machines

A method for steady-state calculation of mechanical or structural systems formulated by the finite element method (FEM) is presented. The procedure introduced is used to calculate the mechanical vibrations caused by the electromagnetic excitation of induction motors. Compared to other approaches, this method enables the free choice of the time discretisation algorithm. The method is presented for second order systems and demonstrated by a numerical example.

A single stage 54V to 1.8V multi-phase cascaded buck voltage regulator module

This paper proposes a new design approach to the conventional two stages solution to convert the 48-60V low voltage bus to the 1-1.8V point of load (PoL) voltage for microprocessors in large telecoms server farms. The core idea is to stack or cascade the buck voltage regulator module (VRM) input voltage in series and connect their output in parallel. This single stage multi-phase cascaded buck (MCB) VRM has a large degree of design freedom depending on the load requirements. A 150W 6-phase MCB (two 3 phase modules in parallel) achieved a maximum efficiency of 91.3% in simulation. This paper also presents the results of a demonstrator of the same circuit which achieved a measured peak efficiency of 90.1%.

Modular test system architecture for device, circuit and system level reliability testing

Reliability stress testing of power semiconductors requires significant development effort for a test apparatus to provide the required functionality. This paper presents a modular test system architecture which focuses on flexibility, reusability and adaptability to future test requirements. Different types of tests for different devices in application circuit configuration can be implemented based on the same modular test system concept. Vital parameters of the device under test (DUT) can be acquired in situ during the running stress test. This enables to collect drift data of this parameters. The control and data acquisition parts of the test system are separated from the actual test circuit. With this physical separation, the same control part can be used for different types of tests. Experimental results of a prototype test system are provided.

Calculation of load‐dependent equivalent circuit parameters of squirrel cage induction motors using time‐harmonic FEM

Two methods to determine the equivalent circuit parameters of squirrel cage induction motors (IM) are presented and compared. The first one is based on the time-harmonic finite element method (FEM) simulation of the measurement process (short-circuit and no-load test), yielding an equivalent circuit with constant parameters. In addition, the time-harmonic FEM is applied in the second method to calculate the load-dependent equivalent circuit parameters of squirrel cage IMs. Here, the material properties in a two-dimensional finite element model of the induction machine are linearized in each operating point and the superposition principle for the magnetic flux is applied to define the leakage inductances. Hence, the parameters are calculated for every operating point separately, thus the variations of the parameters due to skin effect in the rotor bars and due to material saturation under arbitrary load condition and rotor speed can be taken into account.

A self-commissioning method for permanent magnet dc-motor drives

The conventional model describing the electrical and mechanical properties of a permanent magnet dc-motor drive is extended in order to take into account the oscillations of the induced voltage that are caused by slotting effects, and - as the electrical machine investigated is fed from a PWM-inverter - in order to include the inverters voltage drop. Furthermore, a method is proposed to estimate the extended models parameters from one single experiment that is easy to be carried out. The method is based on recording the inverters dc-link voltage, the dc-motors armature current and the rotors angular position as a function of time while the dc-motor is fed from the inverter. As the inverter is controlled by a digital signal processor system, this can be done without using additional measuring devices, but just with the dsp-systems measurement facility which is required anyway for the controlled operation of the drive. No additional mechanical components have to be coupled if the motor is equipped with an incremental encoder. Besides the methods aptitude for self commissioning purposes, it is also perfectly suited for quality control as in addition to the relevant electrical parameters one can also estimate mechanical parameters (e.g. to describe the load torque). This makes it easy to control the assembly quality of the motor. Last but not least the models predictions are compared to experimental results and this comparison clearly shows the benefits of the method proposed.