Compensating for dead time degradation of PWM inverter waveforms

Abstract

The output voltage waveforms of a bridge inverter using some form of modulation strategy, such as pulse width modulation (PWM), are not the same as the ideal modulated waveforms because of the effect of ‘dead time’. This is the short period of time that is allowed to elapse for safety reasons between switching one device in an inverter leg off and the other device on. It has been found that the dead time causes a reduction in the fundamental component of the output voltage and introduces low order harmonics which are not intrinsically present in the ideal modulated waveforms.In variable speed drive systems the reduction in fundamental voltage causes a reduction in the torque. Furthermore, with synchronous modulation strategies there is a step change in torque when a ‘gear change’ takes place. In uninterruptible power supply (UPS) systems the low order harmonics are difficult to remove, with the result that the fundamental output voltage is contaminated.Methods of compensating for dead time are proposed and investigated and it is shown that its effects can be substantially eliminated. Tests with a compensated variable speed drive, using an inverter fed induction motor, demonstrate that the torque perturbations at a gear change are now negligible. Similarly, in a UPS application the low order harmonics are found to be removed from the filtered output voltage when dead time compensation is introduced.