PRACTICAL REAL-TIME PWM MODULATORS - AN ASSESSMENT

Many techniques have been proposed for generating PWM waveforms for controlling AC motor drives. This paper demonstrates the practicality of generating real-time, software-based PWM waveforms using low-cost microprocessors. It uses a space vector methodology and extends the concept of a computationally efficient trapezoidal modulation strategy to more desirable sinusoidal schemes. Within the algorithm, the apportioning of null (zero) voltage vector times (during which all three output phases are equal) is identified as a degree of freedom which can be used to optimise the resultant PWM waveform. The optimisation criterion chosen here was that of minimum motor harmonic losses. The traditional space vector modulation technique, which divides the null voltage vector times equally between the two possible null states, is shown to surpass both the triangulation and the bus-clamping generation strategies when evaluated in terms of both this criterion and other practical factors.