The ability of an inverter to generate a purely sinusoidal output voltage is dependent on the ratio of the amplitude of the fundamental and DC link voltage. If the demanded amplitude is greater than the DC link voltage then the output voltage is limited to the DC link voltage. This results in distortion of the output voltage and the inverter is said to be operating in the overmodulation region. It is possible to extend the fundamental amplitude by adding a third harmonic component to the reference signal. 

In this applet you can determine how much third harmonic you need to add to get the largest fundamental amplitude before distortion occurs in the waveform. Reduce the amplitude of the third harmonic component to zero, then adjust the amplitude of the fundamental until over modulation occurs, either through the warning message or when the vector plot becomes non-circular. A measure of the maximum value of the voltage is given. Now adjust the amplitude of the third harmonic and observe the maximum amplitude that the fundamental can be increased too. Note that there is an optimal level of third harmonic that can be added. 

The third harmonic component can either be generated by a sine waveform generator (pure sinwave) locked to the required fundamental signal or by measuring the voltage between the centre point of a resistively loaded 3-phase rectifier and the neutral of the reference sources. This second method produces a triangular waveform at three times the output fundamental frequency.

• Set the amplitude of the third harmonic to zero.
• Adjust the amplitude of the fundamental until overmodulation occurs. This can be seen as a warning message on the screen or from the voltage space vector becoming non-circular.
• Note the amplitude of the fundamental at which over modulation has occurred. 
• Now, increase the amplitude of the third harmonic and find the value at which maximum amplitude of the fundamental can be achieved.