The 180 degree mode and the 120 operation mode are two different strategies for changing the electronic power switches to obtain the desired AC output waveform. In the 180-degree operating mode, if you divide a cycle of 360 degrees into 6 intervals of 60 degrees each, you will have 3 devices in each interval.
If I mention the upper leg switches as 1,3,5 and the lower leg switches as 4, 6, 2, the switching table would be- (180 degree mode)
0-60 degrees- Switches 1,6 and 5 are on
60-120 degrees- Switches 1, 6 and 2 are on
120-180 degrees- Switches 1, 3 and 2 are on
180-240 degrees- Switches 4, 3 and 2 are on
240-300 degrees- Switches 3, 4 and 5 are on
300-360 degrees: switches 4, 5 and 6 are on
Inference: each switch will perform an interval of 180 degrees and for each 60 degree interval, three switches are on and three switches are off.
For the 120 degree operating mode-
Each switch is on for an interval of 120 degrees in each cycle and two switches are on simultaneously.
0-60 degrees- Switches 1 and 6 are on
60-120 degrees- Switches 1 and 2 are on
120-180 degrees- Switches 3 and 2 are on
180-240 degrees- Switches 3 and 4 are on
240-300 degrees- Switches 5 and 4 are on
300-360 degrees - switches 5 and 6 are on
Inference: each switch will carry out an interval of 120 degrees and for each 60 degree interval, two switches are on and four switches are off.