Shaft Current Issues in Variable Frequency Motors,Xinrun Microelectronics Technology (Shenzhen) Co., Ltd.

Ordinary squirrel-cage asynchronous motors designed for general transmission systems are also widely used in variable-frequency speed regulation systems. The structural design parameters of squirrel-cage variable-frequency asynchronous motors powered by frequency converters can generally refer to those of conventional squirrel-cage asynchronous motors in terms of frame size and overall dimensions. However, unlike ordinary motors operating at fixed frequency, variable-frequency motors run under variable frequency conditions, which brings various adverse factors to motor operation. One of the most prominent problems is bearing damage caused by shaft current during operation. This article elaborates on the classification, causes, hazards and preventive measures of shaft current in variable-frequency motors.

1. Classification and Generation Mechanisms of Bearing Current

Low-Frequency Shaft Current

Low-frequency shaft current is generated by the asymmetry of the motor magnetic circuit, which commonly occurs in motors with a rated capacity above 400 kW. An asymmetric magnetic circuit induces annular alternating magnetic flux in the motor yoke, generating an AC induced voltage in the conductive loop composed of the motor shaft, bearings, end covers and stator frame. Once the induced voltage breaks down the insulating property of bearing lubricant, a continuous current will flow through the front and rear bearings and form a closed loop.

2. High-Frequency Shaft Current

Generation Mechanism: The industrial three-phase sinusoidal power supply features balanced voltage and zero neutral-point potential. In contrast, the output voltage of a frequency converter is generated by PWM (Pulse Width Modulation), which converts DC voltage into three-phase AC voltage. Although the fundamental frequency component remains symmetrical, unsynchronized switching of inverter diodes produces asymmetric high-order harmonics, resulting in elevated zero-sequence voltage, namely non-zero neutral-point potential. This zero-sequence voltage is defined as common-mode voltage in industry standards. It can be measured at the winding neutral point of the load motor, with a frequency consistent with the inverter switching frequency and an amplitude proportional to the DC bus voltage. The shaft current induced by common-mode voltage is defined as high-frequency shaft current.

Types of High-Frequency Shaft Current:

1. Under the excitation of common-mode voltage, high-frequency magnetic flux circulating along the stator yoke generates high-frequency induced voltage. When the voltage is high enough to break down the insulation of bearing lubricant, circulating current flows through the closed loop formed by bearings, motor shaft and stator frame.

2. Common-mode voltage causes a voltage drop of over 100 V between the motor frame and the converter chassis. Leakage current entering the stator frame flows back to the frequency converter through metal couplings and driven mechanical equipment, forming shaft grounding current.

3. High-frequency common-mode voltage reduces the impedance of various stray capacitances inside the motor, creating low-impedance paths for current conduction. Capacitor discharge inside the motor further generates high-frequency bearing current, which returns to the power supply through the converter grounding conductor and internal capacitance.

Hazards of Shaft Current

Bearing current features a fast changing rate determined by bearing operating conditions. When bearing balls are completely immersed in non-conductive lubricant, the bearing capacitance remains electrostatically charged. Once the charging voltage exceeds the insulation limit of the lubricating oil film, the oil film will be broken down. Meanwhile, the induced voltage generated by magnetic circuit asymmetry can also destroy lubricant insulation and form large bearing current.

When the bearing current density exceeds 1.5 A/mm², the high temperature generated by partial discharge energy melts tiny areas on the inner ring, outer ring and rolling balls of bearings, forming micro-grooves. This leads to abnormal noise and mechanical vibration. Unresolved bearing electric erosion will eventually cause bearing failure and seriously affect continuous production operation.

Preventive Methods for Bearing Current Damage

Application of Insulated Bearings

Electrically insulated bearings integrate insulating structures into bearing components, fundamentally eliminating electrical erosion, improving operational reliability and extending equipment uptime.

Electrically insulated bearings are generally coated with aluminum oxide insulation layers on the outer diameter and end faces of the outer ring. Bearings with insulating coatings on the inner ring bore and end faces provide enhanced protection against high-frequency current, as the inner ring coating structure adapts better to high-frequency working conditions.

Adopting special spraying processes, high-performance insulating coatings are firmly bonded to the bearing surface with excellent dielectric properties. The coatings effectively isolate induced current, prevent electrical erosion on lubricating grease, rolling elements and raceways, and significantly extend bearing service life. Insulated bearings are widely applied in motors and generators, especially suitable for variable-frequency motor scenarios.

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