How to Prevent Bearing Damage Caused by Bearing Currents in Compressors

This article applies to general cage induction motors for transmission systems as well as motors deployed in variable frequency speed control systems. 

For cage-type variable-frequency induction motors powered by variable frequency drives (VFDs), their structural design parameters (frame size and dimensions can refer to general cage induction motors). 

However, attention should be paid to adverse operating effects brought by wide frequency operation of variable-frequency motors. One major adverse effect is bearing damage induced by shaft currents during motor operation.

I. Types and Formation Mechanisms of Bearing Currents
1. Low-frequency Bearing Currents

Low-frequency shaft currents are generated by asymmetrical motor magnetic circuits, which commonly occur in motors with power above 400 kW. 

An asymmetrical magnetic circuit creates circulating alternating flux inside the magnetic yoke. This flux induces alternating voltage within the conductive loop formed by the motor shaft, bearings, 

end covers and stator frame. When the induced voltage breaks down the insulating properties of bearing lubricant, current flows through the closed loop including the front and rear bearings of the motor.

2. High-frequency Bearing Currents
Formation mechanism of high-frequency bearing currents:

Three-phase sinusoidal power supply at industrial frequency is balanced and symmetrical with zero neutral-point voltage. Nevertheless, the output voltage of variable frequency drives is generated via Pulse Width Modulation (PWM). 

The inverter converts DC voltage into three-phase sinusoidal AC voltage.

Although the fundamental frequency component is balanced and symmetrical, non-absolute synchronization of switching actions of diodes inside the inverter unit generates asymmetrical high-order harmonics, 

leading to increased zero-sequence voltage components, i.e., non-zero neutral-point voltage. This zero-sequence voltage is defined as common-mode voltage in relevant standards. 

It can be measured at the neutral point of motor windings. Its frequency equals the switching frequency of inverter diodes, and its amplitude is proportional to DC bus voltage. 

Bearing currents generated by common-mode voltage are defined as high-frequency bearing currents.

Categories of High-frequency Bearing Currents

Under the action of common-mode voltage, high-frequency flux circulating along the stator yoke induces high-frequency voltage. When the induced voltage is sufficiently high to break down the insulation of bearing lubricant, 

circulating current flows within the closed loop formed by bearings, shaft and stator frame.

Common-mode voltage creates a voltage drop exceeding 100 V between the motor frame and variable frequency drive frame. Current leaks into the stator frame and flows back to the VFD through metal couplings and driven mechanical equipment, 

forming shaft grounding current.

Under high-frequency common-mode voltage, impedance of various stray capacitances inside the motor decreases, providing low-impedance paths for current flow. 

High-frequency bearing currents are generated upon internal capacitive discharge inside the motor. This current returns to the power supply via grounding conductors and capacitors of the variable frequency drive.

II. Hazards of Bearing Currents

Current flowing through bearings changes rapidly, and its rate of change depends on bearing manufacturing conditions. When bearing balls are fully immersed in non-conductive lubricant, 

the bearing capacitance remains electrostatically charged. Once electrostatic charging voltage exceeds the dielectric strength of bearing lubricant, the lubricant oil film will be broken down. In addition, 

induced voltage generated by asymmetrical motor magnetic circuits can also damage the insulation performance of bearing lubricant and trigger severe bearing currents.

When the current density of bearing current exceeds 1.5 A/mm², high energy released by partial discharge of shaft current generates extreme local temperature, melting tiny areas on bearing inner rings, 

outer rings or rolling elements and forming pits. This leads to noise and vibration. If undetected and untreated in a timely manner, bearing failure will occur and bring severe losses to production.

III. Preventive Measures Against Damage from Bearing Currents
Adopt Electrically Insulated Bearings
Electrically insulated bearings fundamentally eliminate electrical erosion by integrating insulation performance into bearing structures, improving equipment reliability and extending machine uptime.
Electrically insulated bearings normally feature an aluminum oxide coating on the outer ring outer diameter surface and end faces.
Insulated bearings with coatings on inner ring bore surfaces and end faces deliver enhanced protection against high-frequency currents, as the coated surface area of inner rings is smaller than that of outer rings.

Manufactured by special spraying processes, insulated bearings are coated with high-performance protective films on outer surfaces. The coating maintains strong bonding force with the base material and excellent insulation performance. 

It can protect bearings from electrical erosion induced by induced currents and prevent damage to grease, rolling elements and raceways caused by electric current.