In cement production practice, the axial vibration value of bearing pedestal of a 280kW high pressure circulating fan suddenly raised, and the abnormal noise at the coupling increased. After taking measures including replacing the fan bearing, aligning the motor and fan, and cleaning the dust on the impeller, the fan vibration value has not recovered to the normal value. Through the mechanical and electrical aspects, the causes of equipment failures are investigated. It is determined that the trigger of A1 power unit inverter in the inverter is not in place, the three-phase output current of the inverter is unbalanced, and the coupling is not synchronized. After replacing the A1 power unit, the three-phase current output is balanced, and the fan vibration value returns to the normal level.