Predictive Reliability Models for Variable Frequency Drives Based on Application Profiles

A Variable Frequency Drive (VFD) is a system that can control speed, torque and position of AC motors. In process automation and motion control for various industrial and commercial applications such as cooling fans, pumps, belt conveyors, rotary kilns, and elevators, VFDs are used. Reliability of a VFD is application-dependent, as the magnitude and type of failure-governing stresses vary with each application. Understanding operating conditions and quantifying reliability under those conditions is essential for ensuring proper operation of VFD to meet the service life expectation of the customer. Analysis of field failures led to identification of failure governing stress factors. Researching technical literature on operational characteristics of VFD for various applications led to proper definition of a 24-hour worst-case use load profile. Using worst-case use load profile as baseline and by accelerating the usage rate, acceleration factor (AF (usage)) is determined. In addition, by using VFD's heat sink temperatures under both worst-case use load profile and accelerated test load profile and using Arrhenius Relationship, acceleration factor due to thermal stress (AF (Thermal)) is determined. The overall Acceleration Factor (AF) is a product of AF (usage) and AF (thermal). A Reliability Demonstration Test (RDT) under accelerated conditions was performed to estimate Mean Time Between Failures (MTBF) and Life at which 10% of the population is expected to fail (L10) under worst case use load profile and also to establish life-stress relationship. This information aids in development of Predictive Reliability Models for VFDs for any application.