J.F. Lindsay

Programmed PWM techniques to eliminate harmonics - A critical evaluation

The authors establish the superiority of programmed PWM (pulsewidth modulation) techniques over the conventional carrier-modulated PWMs and provide a critical evaluation of the programmed PWM schemes applied to single and three-phase inverters, thereby providing the framework and guidelines for the selection of the appropriate technique for each application area. Evaluation criteria include harmonic loss factor and total harmonic distortion factors defined at the input and output of the inverter terminals. A simple low-cost solution for obtaining the required PWM switching points is proposed. Selected results are verified experimentally.<<ETX>>

Analysis and design of a series voltage unbalance compensator based on a three-phase VSI operating with unbalanced switching functions

Voltage unbalance in AC supply systems is typically corrected by means of a shunt connected thyristor-controlled static VAr compensator. This approach has the disadvantage of slow response, harmonic injection into the AC system, and the requirement for large passive components. The proposed system consists of a three-phase PWM voltage source inverter, connected in series with the line through a three-phase transformer. The unbalance compensation is achieved by canceling the negative sequence component of the line-to-line voltages of the source. It is also shown that by having the inverter operate with unbalanced switching functions, it is possible to balance the load voltage and to control the amplitude of the positive sequence component in order to perform load voltage regulation. A complete mathematical description of the method is presented, demonstrating that the compensation can be achieved with low kVA inverters and low harmonic injection. Implementation procedures, design equations, and a design example are also included in order to illustrate the proposed method. Experimental results of a 1.5 kVA laboratory prototype system confirm the feasibility of the technique. >

A current source PWM inverter with instantaneous current control capability

A new approach for the control of a current source pulse width modulated (PWM) inverter is proposed to achieve instantaneous current control capability. The proposed scheme provides instantaneous current response in the AC motor by altering the modulation index of the current source PWM patterns. In order to achieve this task, optimum programmed PWM patterns with selective elimination of lower order harmonics in the motor line currents are used. Output filter capacitors are provided to bypass the current harmonics in the inverter output and allow for the instantaneous changes in the line currents. This method of control provides sinusoidal output voltages and sinusoidal currents in the AC motor. The proposed scheme guarantees fast response characteristics combined with high-performance steady-state behavior. A design procedure for the current source PWM and selection of programmed PWM current patterns for a variable-speed situation is illustrated. Selected results are verified experimentally. >

Analysis and design of a series voltage compensator for three-phase unbalanced sources

Voltage unbalance typically present in three-phase AC supply systems adversely affects power system components, static converters, drive systems, electric machines, etc., connected to the system. A method to eliminate this unbalance by means of a voltage compensator connected in series with the supply through transformers is described. The technique is based on extracting the negative sequence voltage component of the supply and canceling it in order to obtain balanced voltages. The positive sequence component is then adjusted to achieve voltage regulation. It is shown that the compensation can be achieved with low kilovolt-ampere inverters and that harmonic injection is reduced to a minimum. The authors include implementation principles, design equations, and a design example. Simulated and experimental results confirm the theoretical concept and feasibility of the proposed system. >

Sensitivity of pole arc/pole pitch ratio on switched reluctance motor performance

The sensitivity of the pole-arc/pole-pitch ratio of the stator and rotor on the performance of a switched reluctance motor is investigated. An analytical method based on magnetic flux path and a two-dimensional finite-element analysis are used. The sensitivity study is performed by comparing the average torque developed for different stator as well as rotor pole-arc/pole-pitch ratios and choosing the ratio combination that produces the greatest value of average torque.<<ETX>>

Comparative Study of the Losses in Voltage and Current Source Inverter Fed Induction Motors

The losses in an induction motor fed from six-step voltage and current source inverters are calculated and a comparison of efficiencies when supplied from these sources is given. The equivalent circuit that includes the effect of space harmonics and corrected for the skin effect in rotor bars is used for the calculation of main and stray copper losses. Stray iron losses due to magnetomotive force (MMF) and permeance harmonics, end leakage and skew leakage are also considered. Losses in the motor when supplied from these sources are also calculated using the modified equivalent circuit having stray and core loss resistors and their efficiencies are compared.

Analysis and design of a series-connected PWM voltage regulator for single-phase AC sources

Regulation of load voltage in single-phase applications is becoming an important issue for critical loads. This paper proposes a method to regulate a single-phase AC source by using a series connected auxiliary voltage source. A voltage source inverter, connected in series with the AC supply through a transformer, is used to compensate the input voltage variations to achieve a regulated load voltage. Input power factor improvement can also be obtained, when operating with reactive load, by phase-shifting the compensating voltage with respect to the input voltage. The operation of the compensator in all four quadrants allows the compensation of any voltage drop, and not only the reactive voltage drop within the same path. The total kVA rating for the system is a fraction of the load total power, and is determined by the maximum allowable line voltage variations. The use of a series connection associated with a high-performance modulation technique results in a high-quality output with a small size filter. This paper includes a description of the proposed method, implementation principles, design equations and a design example. Steady-state performance features are investigated. Simulated and experimental results confirm the concept and feasibility of the proposed system.

A new PWM speed control system for high-performance AC motor drives

An approach to speed control for AC motor drives that uses programmed PWM (pulse width modulation) switching patterns over the complete range of output speed is presented. The scheme provides smooth operation during the required switching-pattern changes and guarantees high-quality output voltage and current in the AC motor load, making it most suitable for high-performance, high-efficiency applications. A detailed description of the scheme and its realization is provided. Results of an experimental investigation on a variable-speed induction motor and a permanent-magnet synchronous motor drive system illustrate the advantages of the scheme. >

Performance analysis of parallel-processing UPS systems

Parallel-processing inverter-based uninterruptible power supply (UPS) systems have advantages over the conventional rectifier-inverter systems. Control is more complex and design tools are not readily available. The modeling, control and performance of a typical system are analyzed, and important parameters are identified. This UPS consists of an inverter-battery subsystem in shunt between the AC mains and the load. Two control loops are employed to fully exploit the advantages of the proposed UPS system: (1) a fast load voltage regulation loop; and (2) a slower battery charging loop. Mathematical models are developed to design the required regulators and compensation networks for both loops. The predicted performance characteristics are verified by simulation. Experimental results on a 5 kVA unit confirm the theoretical analyses.<<ETX>>

A new current control scheme for AC motor drives

A current control scheme based on programmed PWM switching patterns is presented for AC motor drives. In this scheme, reference and feedback current correspondence is maintained with simultaneous optimization of a performance objective. The proposed scheme overcomes the disadvantages of the existing current regulators using the direct control approach and exhibits several advantages in terms of reduced switching frequency, minimum current/torque ripple, higher efficiency, and improved performance in the AC motor drive. Analytical and experimental investigations on a current-regulated induction motor drive confirm the predicted advantages of the scheme. >