In modern AC motor systems, a current can flow through the bearings, and this current often operates in an intermittent manner. When the circuit is periodically interrupted, it can lead to the formation of an electric arc. These arcs can cause significant damage to the bearing components over time.
There are two primary causes for current passing through the bearings of an AC motor. First, voltage can be induced due to an imbalance in the internal electromagnetic field. Second, high-frequency currents may travel through stray capacitance paths within the motor.
In an ideal AC induction motor, the magnetic field is perfectly balanced. When the three-phase windings carry equal currents with a 120-degree phase difference, no voltage is induced on the motor shaft. However, when the inverter outputs a PWM voltage, it can create an asymmetrical magnetic field inside the motor. This imbalance leads to a voltage being induced on the shaft, typically ranging from 10V to 30V, depending on the driving voltage. The higher the driving voltage, the greater the voltage on the shaft.
When this voltage exceeds the dielectric strength of the lubricating oil in the bearing, a conductive path is formed. As the shaft rotates, the oil film may temporarily block the current, creating a switch-like on-off behavior. This process can generate an arc, which erodes the shaft, rolling elements, and raceway surfaces, forming small dimples. If there's no external vibration, these dimples may not significantly affect performance. However, under vibrational conditions, grooves can form, which can severely impact the motor’s operation.
Furthermore, experimental studies have shown that the shaft voltage is also influenced by the fundamental frequency of the inverter output. Lower fundamental frequencies tend to result in higher shaft voltages, leading to more severe bearing damage.
At the start of motor operation, when the lubricating oil is cold, the current flowing through the bearing is usually between 5mA and 200mA. Such low currents generally do not cause damage. However, as the motor runs for a longer period, the temperature of the lubricating oil increases, and the peak current can rise to between 5A and 10A. This increased current can lead to arcing and the formation of small pits on the bearing surfaces, gradually degrading the motor’s performance over time.
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