Torsional Vibrations in Open Loop Volts Hertz Variable Frequency Drive Induction Motor Driven Mechanical Systems

Variable Frequency Drives (VFD) use pulse width modulation (PWM) to modulate current/voltage input to the motor, resulting in an electromagnetic torque with a rich harmonic spectrum. The electromagnetic torque harmonics may induce severe torsional vibrations in the driven mechanical train which may culminate in fatigue failure. This paper analyzes torsional vibrations in an open-loop VFD-induction-motor-driven compressor train installed at Southwest Research Institute (SwRI) and provides methods to reduce the vibrations. VFD Torsional, an in-house VFD-motor-mechanical system solver, is used to verify two experimental observations of the SwRI system; first, the presence of severe 38 Hz torsional vibrations during compressor operation; second, the use of closed-loop control over open-loop control to reduce the vibrations. The analysis establishes that in open-loop control of induction motors the amplitude and frequency of the electromagnetic torque harmonics depend on the damping and torsional natural frequency of the motor-mechanical system. Thus, the motor manufacturer's frequency and amplitude data for the electromagnetic torque harmonics may be insufficient for the vibration analysis and design of VFD-incorporated rotating machinery. Utilizing a coupled electrical-mechanical solver, which models both the motor and the gear-train, is a better approach in designing and analyzing VFD-incorporated rotating machinery.