Three Phase Inverter Description :

Each leg (Red, Yellow, Blue) is delayed by 120 degrees.

A three-phase inverter with star connected load is shown below

Three Phase Inverter Basic Circuit 

Three Phase Harmonics

For three-phase inverters, there is significant advantage if MR is chosen to be:

Odd: All even harmonic will be eliminated from the pole-switching waveform.
triplens (multiple of three (e.g. 3,9,15,21, 27..):

All triplens harmonics will be eliminated from the line-to-line output voltage.

By observing the waveform, it can be seen that with odd MR, the line-to-line voltage shape looks more “sinusoidal”.

As can be noted from the spectra, the phase voltage amplitude is 0.8 (normalised). This is because the modulation index is 0.8. The line voltage amplitude is square root three of phase voltage due to the three-phase relationship

Effect of odd and triplens

 Three phase inverter with RL load 

It is desirable to have  MR as large as possible.

This will push the harmonic at higher frequencies on the spectrum. Thus filtering requirement is reduced.

Although the voltage THD improvement is not significant, but the current THD will improve greatly because the load normally has some current filtering effect.

However, higher MR  has side effects->>

Higher switching frequency -> More losses.

Pulse width may be too small to be constructed. 

“Pulse dropping” may be needed.