Figure 4.89: Gate signals for the three-phase inverter in twelve-step starting.
An alternative to the above mentioned method, the twelve-step starting ,
improves the starting performance. In the twelve-step pattern, the inverter
gate is switched over every 30 ◦ , and hence the power electronics devices in
the converter circuit conduct for 150 ◦ , instead of 120 ◦ in the six-step start-
ing. The corresponding gate signals are shown in Figure 4.89 and the phase
voltages with step amplitudes V DC /3, V DC /2and2V DC /3 are illustrated in
To realize the twelve-step pattern, an additional switching state is inserted
between every two successive states of the six-step sequence, making it twelve
states in the starting sequence. The switching sequences of both the six-step
and twelve-step are listed in Table 4.4.
Table 4.4: Switching sequence for six-step and twelve-step starting method
(AH, BL) (AH, CL) (BH, CL)
(BH, AL) (CH, AL) (CH, BL)
(AH, BL) (AH, BL, CL) (AH, CL) (AH, BH, CL)
(CH, AL) (AL, BL, CH) (CH, BL) (AH, BL, CH)