Michael L. Gasperi

Life prediction modeling of bus capacitors in AC variable frequency drives

Aluminum electrolytic capacitors are the leading choice for ac variable-frequency drive (VFD) bus filters. Predicting the expected life of these components in this application is complicated by four factors. First, the electrical impedance of aluminum electrolytic capacitors is nonlinear with both frequency and temperature. Second, motor drives produce a spectrally rich ripple current waveform that makes energy loss difficult to compute. Third, the heat transfer characteristic of capacitor banks is dependent on design geometry. Fourth, the expected life of aluminum electrolytic capacitors is extremely sensitive to operating temperature. This paper describes a method for predicting bus capacitor life that addresses these problems by using a multiple component model for capacitor impedance, a motor drive simulation to create ripple current waveforms, a heat transfer model based on bank geometry, and a capacitor life model derived from the device physics of failure. An example is given showing the effect of ac line impedance on the relative expected life.

Initial rotor angle detection of a nonsalient pole permanent magnet synchronous machine

This paper presents a technique that calculates the absolute angular position of a permanent magnet (PM) rotor within a pole pair at standstill. The algorithm works with nonsalient pole motors. By choosing an appropriate voltage pulse width and applying it to each phase winding, the stator currents partially saturate the stator iron, enabling the algorithm to discern between a north pole and a south pole, and subsequently, the absolute position. The scheme is computationally simple and does not rely on the knowledge of any of the motor parameters.

A method for predicting the expected life of bus capacitors

Predicting the expected life of motor drive bus capacitors is complicated by several factors: the equivalent series resistance of aluminum electrolytic capacitors is frequency and temperature sensitive; the ripple current is a composite waveform; and the capacitor expected life is very sensitive to operating temperature. This paper describes a method that addresses all these factors by developing separate models for capacitor equivalent series resistance, motor drive circuitry, heat transfer conditions and capacitor life.

A permanent magnet rotor containing an electrical winding to improve detection of motor angular position

Sensorless detection of rotor angular position is desirable for decreasing the cost and increasing the reliability of permanent magnet AC motor drive systems. This paper describes tests performed on a rotor that has surface mounted magnets and a conducting winding that links only the d-axis of the rotor. The conducting winding allows rotor angular position to be detected by stator-winding inductance variations.

Practical issues in the design of active rectifiers for AC drives with reduced DC-link capacitance

For AC drives with an active front-end boost rectifier, the DC-link voltage can be regulated with fast dynamic control and made immune to line and load variations. Hence, it is possible to reduce the DC-link capacitance and use film capacitors instead of electrolytics, thus improving the life expectancy of the drive. Typically, active rectifier control includes load power feed-forward to enhance the response of the DC voltage regulating loop to load transients. In this paper, it is shown that under certain load transients encountered in practice, the rise of the DC bus voltage is inevitable and cannot be mitigated by control. It is also shown that with power feedforward control, the limited sampling rate of the pulse-width-modulated DC link current in a practical system affects the quality of the input line currents. Detailed design and stability analysis and experimental results under several practically encountered situations are presented.

Modeling and analysis of DC link bus capacitor and inductor heating effect on AC drives. I

The overall performance and efficiency of a voltage source inverter (VSI) can be improved by selecting a combination of DC link capacitance and DC link inductance based upon a weighted optimization criterion. Developments of semiconductor technology have enabled present semiconductor devices to operate with improved power losses, to a great extent. However, there is still a considerable amount of power loss in the DC link capacitor bank and DC link inductor. These loss components are normally ignored to a certain level in terms of design/efficiency optimization, in the design phase of the drive. Therefore, there are additional opportunities to optimize the design by selecting the capacitor/inductance combination for minimum power losses or for any other optimum such as size and weight or cost. This paper describes the modeling of the PWM AC drive, AC motor and special modeling of the DC bus capacitor and DC link inductor, using the SABER simulator. The paper compares the SABER simulation results with the experimental data to verify the accuracy of the model, for given values of DC link inductance and bus capacitance as the first step towards modeling the overall system for accurate prediction of the drive performance.

Heat transfer model for capacitor banks

The life expectancy of aluminum electrolytic capacitors used for DC bus filtering in motor drives depends upon their operating temperature. This paper discusses convection and radiation heat transfer from capacitor banks and presents a heat transfer model for banks of capacitors that can be used to estimate temperature rise. The model is verified with experimental data and is further extended by using computational fluid dynamics and radiation theory. Bank design options are explored for their impact on bank cooling efficiency.