Shaft current is the hacker of energy-saving motor bearing system

Shaft current is the main quality killer of variable frequency motors, large motors, high-voltage motors, and generators, which can cause significant damage to energy-saving motor bearing systems. Bearing system failures caused by insufficient preventive measures for shaft current are common.

The characteristic of shaft current is low voltage and high current, which can cause damage to the bearing system. The generation of shaft current is caused by shaft voltage and closed circuit. The problem of shaft current can be solved by eliminating shaft voltage or cutting off the circuit.

Unbalanced magnetic circuit, inverter power supply, electrostatic induction, electrostatic charge, and external power interference can all generate shaft voltage. The amplitude of the shaft voltage is generally small, but when it reaches a certain value, it will break through the lubricating oil film of the bearing. Through the closed circuit on the surface of the bearing, larger shaft currents can cause the bearing to burn out due to heating in a short period of time. Bearings burned out due to shaft current will leave washboard like marks on the outer surface of the bearing inner ring.

In order to solve the problem of avoiding shaft current, necessary measures must be taken in the design and manufacturing process of energy-saving motors, such as adding necessary insulation measures in the end caps and bearing sleeves. For small-sized products, insulated bearings can be used, and leak proof carbon brushes can also be added during use. From a usage perspective, taking open circuit measures on components is a permanent solution, while using shunt methods may lead to the replacement of carbon brush devices. At least, the carbon brush system should not have any problems during the maintenance cycle of energy-saving motors.

01 Insulated bearings

The size and load-bearing capacity of insulated bearings are the same as those of ordinary flange bearings. Differently, insulated bearings can effectively prevent current from passing through. Insulated bearings can prevent damage caused by electrical corrosion, thus ensuring more reliable operation than ordinary bearings. Insulated flange bearings can prevent the electrical corrosion of bearings caused by induced currents, and prevent damage to lubricating grease, rolling elements, and raceways caused by currents.

Why does an inverter power supply generate shaft voltage?

When the electric motor operates using the inverter power supply, the power supply voltage contains high-order harmonic components, causing electromagnetic induction between the ends of the stator winding coils, wiring parts, and the rotating shaft, thereby generating shaft voltage.

The stator winding of an asynchronous motor is embedded in the stator core slot, and there is distributed capacitance between the turns of the stator winding and between the stator winding and the energy-saving motor base. When a universal frequency converter operates at high carrier frequencies, the common mode voltage of the inverter changes sharply, causing leakage current to form between the energy-saving motor housing and the grounding terminal through the distributed capacitance of the energy-saving motor winding. This leakage current may cause two types of electromagnetic interference, radioactive and conductive. Due to the imbalance of the magnetic circuit in energy-saving motors, static induction and common mode voltage are both reasons for the generation of shaft voltage and shaft current.