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The interference of Variable Frequency Drives (VFDs) on the vibration signature analysis of machine defects

Nader Sawalhi, Kahlil Detrick & Suri Ganeriwala
SpectraQuest Inc., 8227 Hermitage Road, Richmond, VA 23228
Published: July, 31 2012


VFDs have become the most commonly used modes to drive induction motors in a variety of applications requiring speed variations. However, vibration analysis for fault diagnosis of such systems poses some serious challenges. The signatures are complicated due to the inherent design of the VFD. This TechNote is designed to explain the operation of a typical VFD and present vibration signatures of machines operated using VFDs. VFDs control the speed of motors by varying the supplied line (mains) frequency. This is done through three main stages. Firstly, the line voltage signal (50 or 60 Hz) is rectified via a set of diodes to obtain a DC signal. This is then smoothened and stored using a set of capacitors. 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, also known as the carrier frequency, is usually set in the range from 2- 15 kHz. The vibration content as a result of using the VFD contains peaks of discrete frequencies spaced at the carrier frequency and its harmonics. Each peak contains sidebands around the carrier frequency (or one of its harmonics) spaced at both the rotor speed and the VFD frequency. Analysts examining high frequency content of their data signals needs to be aware of the carrier frequencies and the associated side bands of their VFD.

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VFD and Usage of VFD units

1. Introduction This technical note highlights the effect of the variable frequency drive unit (VFD) on the vibration signal measured at the motor and being transmitted to the mounting bearings of the machinery fault simulator (MFS). The purpose of this TechNote is to give some insight into the frequency content of vibrations measured in machines using variable frequency drive (VFD) systems. Specifically, to discuss factors that must be considered when analyzing vibration spectra at frequencies greater than 3 kHz and in particular at the motor side. 1.1. Why using a VFD A Variable Frequency Drive (VFD) converts an input AC voltage and frequency to an output of a different voltage and frequency. VFDs are commonly used to control an AC motor’s rotational speed and torque by varying the voltage or the voltage frequency it outputs to the motor. They are also used to convert single phase to three phase. SpectraQuest uses VFDs to power/control all of our induction motors.