Pulse Width Modulation Waveform Research Articles

Novel algorithm for generating near optimal PWM waveforms for AC traction drives

A harmonic elimination optimised pulse-width modulation (PWM) scheme is an attractive alternative to both natural and regular sampled PWM schemes for voltage source inverter drives used in railway traction. However, its application has been limited by the fact that the exact switching angles cannot be calculated online by a microprocessor based waveform generator. The paper describes an algorithm for calculating the near optimal switching angles, which will permit a fast and efficient realisation in a microprocessor based controller. With this type of PWM scheme, it is possible to show analytically that components at the signalling frequencies are not generated in the DC supply input current, for existing power frequency track circuits. The switching angles calculated with the algorithm agree closely with the exact solutions over most of the range of the fundamental output.

Optimum PWM Waveform Synthesis---A Filtering Approach

A fundamentally different approach is proposed for the synthesis of optimum pulsewidth modulated (PWM) waveforms for high-power inverter applications. Conventional optimum PWM waveform synthesis techniques which seek to control harmonic levels in the inverter output directly are seen to be equivalent to a filtering operation. Digital filter structures capable of processing PWM waveforms are examined and waveform synthesis strategies are proposed and verified experimentally. Finally, the design of a high-performance PWM waveform generator is detailed.

The Effect of Word Length on the Harmonic Content of Microprocessor-Based PWM Waveform Generators

In a microprocessor-based pulsewidth modulated (PWM) waveform generator, the quality of the waveform depends on the accuracy with which the position of the switching edges of the waveform can be determined. This in turn depends on the length of the binary word or digital counter used to control the edge positions. The quantization effects experienced in microprocessor-based PWM waveform generators are analyzed for various word lengths. It is demonstrated that good waveform quality can be achieved by an 8-b microprocessor for a wide range of applications, including those in which the carrier frequency is in the ultrasonic range. The microprocessor employs a fast method of voltage control based on addition. Furthermore, an increase in the line-to-line output voltage is obtained without output distortion by adding a measure of third harmonic to the phase waveform.

Spectral Errors in the Application of Pulsewidth Modulated Waveforms

Possible spectral errors resulting from the application of theoretically optimized pulsewidth modulated (PWM) waveforms to physical power converters are considered. These errors, produced by commutation rise and fall times, timing errors, and amplitude ripple, are synthesized and analyzed in terms of switching, ramp, and ripple functions. Probabilistic and heuristic results show the relative sensitivity of the error producing terms independent of the PWM technique.

Losses and Parasitic Torques in Electric Motors Subjected to PWM Waveforms

Some aspects of the theoretical performance of an electric motor, supplied by an unfiltered pulsewidth modulated (PWM) inverter are studied. The stator copper losses and the winding insulation lifetime are emphasized. An induction motor system is assumed and the creation of low-frequency parasitic torques are described. Their relative importance are estimated. It is shown that PWM supplies may have a negative impact on the motor performance and lifetime and can make a modification of the motor design more desirable than in the case where a ``simpler'' inverter system is used.