Mechanisms of shaft cracking in vertical pumps due to torque pulsation under variable frequency operation

Abstract To investigate the mechanism of shaft crack faults in a power plant’s condensate pump following a change to variable frequency operation, wireless strain testing technology was employed to measure torque pulsation during the start-stop process of the pump as a function of speed. A finite element model of the pump was developed, and the torsional natural frequencies of the shaft were calculated. When a motor operates in variable frequency mode, numerous harmonic components are introduced into the torque. At specific speeds, one of the harmonic frequencies of the torque aligns with the natural frequency of a torsional vibration mode, inducing torsional resonance. Under these conditions, the motor torque experiences significant pulsation, which can readily cause fatigue damage to the shaft. When the pump operates in variable frequency drive mode, its torsional fatigue strength must be verified.