Phase Voltage and Line Current

Figure 7-2 shows the phase voltages of the windings by the 3-phase BLDC in both sinusoidal drive and trapezoidal drive. Three oscilloscope probes are connected to the voltage dividers of each motor winding. The difference between the driving phase voltages can be observed. The sinusoidal drive produces a saddle shape waveform where the switching voltage is smoothly varied. The trapezoidal drive, on the other hand, generates a trapezoidal waveform.
Figure 1. Phase Voltages

Using a current probe, the phase current of the trapezoidal and sinusoidal current drive is obtained and shown in Figure 7-3. Both drive runs at 2400 RPM. Ripples are observed in trapezoidal drive. The sinusoidal drive provides a smoother line current compared to the trapezoidal drive. Ripples are more apparent in the trapezoidal drive. The sinusoidal drive current waveform follows a sinusoidal like pattern in which ripples and audible noise are reduced.

Figure 2. Phase Current