Lag-free terminal voltage sensing in low-pass filtered PWM converters

Several high-performance control algorithms for ac machine drives and other applications require accurate knowledge of the instantaneous converter output voltages. The use of a simple low-pass filter to remove the PWM switching frequency harmonics from the AC voltage measurements introduces undesirable amplitude and phase lag errors into the voltage measurements. This work presents compensation functions that make it possible to compensate these measured filter output voltages for both amplitude and phase lag errors introduced by zero-order-hold samplers and two different types of PWM algorithms. The impact of the selected filter time constant and the required accuracy of the compensating equations relative to the ADC accuracy are also addressed, leading to identification of the optimal filter break frequency for maximum accuracy.