[4A11] Interference of variable frequency (VFD) on rotating machinery fault diagnosis
S Ganeriwala
SpectraQuest, USA
VFDs are most commonly used to drive induction motors in applications requiring speed variations. However, vibration analysis for fault diagnosis of such systems poses serious challenges as the signatures are complicated due to the inherent design of the VFD. This paper will explain the basic operation of a typical VFD and present vibration signatures of machines operated using VFDs. VFDs control motor speed by varying the frequency of the supply line. This is done in three stages. Firstly, the line voltage signal is rectified to obtain a DC signal. This is then smoothed and, finally, the constant DC voltage is used to construct a pseudo AC voltage using a set of transistors, which act like switches (gates). The switching frequency, or carrier frequency, interferes with the vibration signal. The vibration signal as a result of using the VFD contains discrete frequencies spaced at the carrier frequency and its harmonics. Each peak also contains sidebands around the carrier frequency (and its harmonics) spaced at both the rotor speed and the VFD frequency. There can be side bands associated with other rotational discrete frequencies such as gears and gear mesh frequencies. Analysts examining high-frequency content of their data signals need to be aware of the carrier frequencies and the side-bands of their VFD. By understanding these issues, one can use them for diagnostics purposes.
