In order to measure stable harmonics, the following two conditions must be met:

First, the signal is a stable periodic signal.

Second, harmonic analysis must be carried out according to the integral multiple of the period of the stable periodic signal.

The switching frequency of traction motor inverter is relatively low, and its switching frequency is often not an integral multiple of fundamental frequency. In this way, the inverter output is not strictly periodic signal (of course, there is no strict periodic signal in the objective world, which means that the error is large). For example, each signal cycle contains 8.2 switching pulses. In this way, five cycles account for 41 pulses in total. However, the pulses in each cycle are not integers, so the waveforms of these five cycles must be different. Since the waveforms are different, they are not the real periodic signals. When the switching frequency is increased, the integer pulses may not be guaranteed. However, the synchronization is not an integral multiple, and the switching frequency is high, so the influence is relatively small. For example, each cycle contains 24.2 pulses, although it is not an integer, but the impact of these 0.2 pulses on 24 pulses is certainly less than that of 0.2 pulses on the previous 8 pulses. Harmonic analysis equipment often takes the period of fundamental wave content of inverter output voltage as the analysis period. Because the signal of each cycle is different, the harmonic is different naturally.