MBVI systems generate a spectrum of the residual current – ie the distortions on the current waveform not caused by the voltage supply. This signal contains information about a number of different features. Although the description about detecting bearing faults in the section above uses the example of bearing faults leading to changes in torque on the motor, there are many other factors that can affect the motor current, including eccentric rotor, varying air-gap between rotor and stator, distortion of the motor frame etc, and all of these factors reveal themselves in the residual current spectrum.
By contrast, a simple torque spectrum only shows the torque. The two spectra, if laid on top of one another, can generally be seen to be noticeably different.
Torque spectrum is prone to distortion effects if the voltage waveform is distorted – something the model-based approach removes. This would suggest MBVI systems are better suited to situations where the voltage supply is distorted, such as on inverter driven equipment.