Analysis of the Causes of Bearing Overheating in Stainless Steel Centrifugal Fan Motors

Analysis of the Causes of Bearing Overheating in Stainless Steel Centrifugal Fan Motors

In stainless steel centrifugal fans, an imbalance in the motor’s magnetic flux can occur due to deviations in the slot dimensions of the stator and rotor, variations in the orientation characteristics of the magnetic material, or three-phase imbalance in the power supply. This imbalance is caused by the support current driven by a sinusoidal power source and poses potential risks. Shaft voltage and bearing current in stainless steel centrifugal fans are generated by the rotating shaft. Bearing voltage has a small amplitude and causes minimal damage. When driven by an inverter, the risk of motor load current increases significantly due to differences in manufacturing principles. Generally, inverters employ PWM modulation. Variable-frequency circuits use high-frequency power components (such as IGBTs) to generate an approximate sinusoidal voltage waveform for the motor. The composite vector of the fundamental components of the three-phase voltage in a stainless steel centrifugal fan is zero; however, in practice, the sum of the three-phase voltage vectors is not always zero, and the three-phase voltage is an unbalanced composite. The amplitude of the common-mode voltage equals the DC-side voltage of the inverter, and its frequency equals the inverter’s switching frequency. The common-mode voltage is generated at the same frequency as the rotor shaft through capacitive coupling between the stator and rotor. Generally, the carrier frequency on the inverter side is very high, exceeding 10 kHz. When the frequency is too high for the stator and cables, the leading and trailing edges of dv/dt become steeper, increasing waveform distortion. Through electrostatic coupling, various parts of the motor distribute capacitances of different sizes, forming a zero-sequence circuit. Under normal conditions, the ball bearings of a stainless steel centrifugal fan float on an oil film formed by grease, with the lubricating oil film acting as an insulator. If, for any reason, the oil film in the stainless steel centrifugal fan is damaged, or if excessive dv/dt bearing voltage penetrates the oil film and causes a discharge, the discharge current will result in ablation of the inner and outer rings of the bearing as well as the balls. During long-term operation, the inner and outer rings of the bearing will develop a striped pattern resembling glass, the bearing temperature will rise, the grease will dissolve, and the bearing’s performance will deteriorate further. Through electrostatic coupling, the distributed capacitance of various parts of the stainless steel centrifugal fan motor increases or decreases, forming a common-mode current discharge path for the motor stator. Most of the common-mode current flows through the stator-housing-ground-inverter housing path. For the stainless steel centrifugal fan, the path is stator-rotor-shaft-bearing-housing-ground-inverter housing. The capacitance between the inverter ground and the motor housing is higher than that between the inverter housing and the load. This creates a circuit path: stator-housing-rotating shaft-load-end bearing (motor)-coupling-bearing (load)-ground-inverter housing, resulting in shaft elongation, current flow, and discharge. This not only damages the bearings on the motor’s load side, as well as the load bearings and coupler. The two common-mode currents in the latter case flow through the motor bearings and cause damage; the second method is more harmful.