The induced motor voltages show harmonics with odd ordinal number ( ), because the flux in the air gap has a rectangular distribution. Harmonic voltages with ordinal numbers divisible by three, create zero phase-sequence systems, which are not accessible by the connections of three phase conductors. As shown in Fig. 5, all third harmonics are in correct phase - they cancel each other out between the three phase conductors and can therefore not be detected from the three phases of the motor in wye connection.
If the wye connection of the motor is accessible, the zero phase-sequence systems can be detected between this and a virtual wye connection, realized with three resistors. Fig. 6 shows the corresponding circuit. The voltages are tapped off by potential dividers. The reference potential of the sensing circuit may be anywhere between the positive or the negative potential of the DC link.