17.1.2 Min-Max Method

There are many techniques to implement SVM out of which Min-Max method requires least computation and is used in the IP block. The third harmonic voltage is generated from min. and max. values of instantaneous 3-phase reference voltages. The third harmonic voltage (V_3H) is calculated as:

The computed third harmonic voltage is added to each phase voltage to get space vector modulated reference.

The addition of third harmonic reduces the peak of the reference signal by 15% and hence allows more fundamental without reaching over-modulation. Even though third harmonic is added in phase voltages, it does not appear in line voltages because it is common mode voltage. As a result, there is no third harmonic current but only fundamental current. For a given modulation index, the peak modulation voltage is farther from the peak carrier voltage. Thus, the number of short pulses is minimized. To utilize the DC bus fully, the voltages must be scaled up by 15% in one of the blocks that follows the SVM in the FOC loop.

If the output should be a unity peak value and the reduction in peak introduced by the block is 0.866, the factor k must be 1.155 (k * 0.866 = 1, k=1/0.866).This implies that there is a 15.5 percent boost introduced in the phase voltage, which results in better utilization of the DC bus.