3.5 Zero-Cross Point Detection

In this implementation, the method employed to measure the zero-cross point is by using a comparator with a variable reference consisting of the sum of all the phase voltages.

Figure 3-13. Zero-Cross Point Detection Using Comparator

The neutral voltage can be expressed as $V_{n} = \frac{B E M F A + B E M F B + B E M F C}{3}$ , where V_n is the virtual neutral voltage, BEMF A is the BEMF voltage in phase A, BEMF B is the BEMF in phase B, and BEMF C is the voltage in phase C.

Due to the asymmetry in the coil impedance, the virtual neutral signal will not be constant, but one that will vary based on the driving cycle. To avoid any phase shifts in the commutation point, the virtual neutral point must not be heavily filtered. Only high-frequency noise must be suppressed with a low RC time constant low-pass filters.

The irregularity of the virtual ground can be seen in Figure 3-14, where the first three waveforms represent the PWM signal for each of the three phases and the fourth waveform is the virtual ground.