Thomas A. Nondahl

Design and implementation of an inverter output LC filter used for dv/dt reduction

This paper presents the design and analysis of a LC output filter for use with IGBT-based motor drive inverters. The LC filter is used to limit the rate of rise of the inverter output voltage and reduce common mode noise to the motor. In typical applications where dv/dt is limited to 100-500 V//spl mu/s, the resonant frequency of the filter is above the switching frequency. A diode bridge must therefore be used to clamp the resonant voltage. Resistors are also used to help dissipate the energy stored in the resonant circuit. Crucial to the design of the filter is the handling of the additional losses in the filter. The paper describes the design considerations for such a filter for use in a 460 V induction motor drive. The losses in the circuit are estimated to show the limitations of the filter. Experimental results illustrate the control of dv/dt at the inverter terminals, and the reduced peak voltage at the motor end of a long cable.

A carrier-based PWM scheme for neutral-point voltage balancing in three-level inverters

A significant problem with neutral-point-clamped three-level inverters is the fluctuation in the neutral-point voltage. In this paper, a capacitor voltage balancing technique for carrier-based three-level pulsewidth modulation (PWM) is developed, with improved voltage control capability at high modulation index. The method incorporates a novel scheme that requires measurements of only the motor currents and capacitor voltages to implement voltage balancing control, and does not need to determine the direction of power flow between the inverter and the motor.

Circuit model for shaft voltage prediction in induction motors fed by PWM-based AC drives

The development of high-frequency pulsewidth-modulation-based adjustable-speed drives (ASDs) has increased the efficiency, performance, and controllability in induction motor applications. However, the high switching frequencies and the faster switching times of insulated gate bipolar transistors (the device of choice in these ASDs) introduce disadvantages like overvoltages at the motor terminals when long cables are used between the drive and the motor. Another industry-wide concern is the generation of rotor shaft voltage and the resulting bearing current. The grease film in a bearing acts as a capacitor that charges due to the transitions in the common-mode voltage imposed at the motor terminals by the drive. The breakdown of the film causes a spike of current to flow that can damage the bearing and reduce life. A significant amount of effort has been directed at understanding the shaft voltage phenomenon and the associated bearing current. This paper attempts to develop circuit models to predict the level of the shaft voltage. The circuit models can then be used to predict the shaft voltage levels at different installations, using simulation software like PSpice. Circuit models for two specific motors are developed. The predicted shaft voltage is very close to the actual voltage levels seen on the shaft when the motors are operated by ASDs.

Rotor Position Estimation for Synchronous Machines Based on Equivalent EMF

This paper provides and analyzes a simple, robust, and universal model for rotor position estimation for synchronous machines. The rotor position is estimated using an equivalent electromotive force (EMF) model of a synchronous machine or, alternately, using a sliding-mode observer (SMO) based on the equivalent EMF model. The SMO provides fast convergence and low sensitivity to parameter variations. Experimental results with a 3.7-kW interior-permanent-magnet machine have validated the effectiveness of the proposed equivalent EMF scheme.

Position-sensorless control of surface-mount permanent-magnet AC (PMAC) motors at low speeds

A position-sensorless field-oriented control scheme for a surface-mount permanent-magnet AC (PMAC) motor is presented. A digital signal processor is used to implement the sensorless scheme. The PMAC stator is wound like that of a conventional three-phase induction motor. The coils of the motor are all brought out and it is possible to connect the motor in different configurations. Taps are also provided which are used for voltage measurements. By measuring the tap voltages, absolute position of the PMAC motor is estimated. The estimated position information is independent of the stator resistance, thus, this scheme is even applicable at low speeds. Results are presented to show the effectiveness of the new sensorless scheme.

Analytical investigation of the switching frequency harmonic characteristic for common mode reduction modulator

Common mode voltage (CMV) generated by two-level voltage source rectifier/inverter adjustable speed drive (ASD) system is troublesome and requires extra hardware fixes, which increase production costs and can lead to increased installation costs. The use of the common mode reduction modulator (CMRM) to control the common mode voltage has received some attention over the past few years. It is attractive for its software selectivity, no additional components, etc. From a product design viewpoint, there is a need to analytically analyze and quantify the switching frequency harmonic characteristics for the CMRM. A complete design-oriented analytical analysis of the switching frequency harmonic characteristic of CMRM for a two-level voltage source rectifier/inverter ASD system is presented in this paper. An analytical harmonic flux formula for CMRM is derived, followed by a per carrier cycle and per fundamental cycle calculation of the harmonic flux RMS value. The analytical solution has been verified against both the simulated and the experimental tested load current harmonic components.

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.

An on-line position error compensation method for sensorless IPM motor drives using high frequency injection

This paper addresses the position error issue existing in sensorless IPM motor drives using the high frequency injection (HFI) method. A simple-to-design on-line compensation method is proposed based upon filter group delay theory, which substantially minimizes the error between the estimated position and the actual position caused by filters in a conventional HFI system. The method determines the position error compensation based upon a demodulation delay and a velocity or rotational frequency of the electrical machine. Experimental results with a 10-hp interior permanent magnet motor are presented showing improved sensorless performance using the proposed method.

Position sensorless control of surface mount permanent magnet AC (PMAC) motors at low speeds

A position sensorless field oriented control scheme for surface mount permanent magnet AC (PMAC) motor is presented. A digital signal processor (DSP) is used to implement the sensorless scheme. The PMAC stator is wound like that of a conventional three-phase induction motor. The coils of the motor are all brought out and it is possible to connect the motor in different configurations. Taps are also provided which are used for voltage measurements. By measuring the tap voltages, absolute position of the permanent magnet AC motor, driven by a current regulated PWM inverter, with a fixed frequency ramp comparison controller, is estimated. The estimated position information is independent of the stator resistance, thus this scheme is even applicable at low speeds. Results are presented to show the effectiveness of the new sensorless scheme.