Why does energy-saving motors operate at power frequency and have shaft current?

The basic conditions for the formation of current are voltage and closed circuit. The prerequisite for generating shaft current is the presence of shaft voltage and a closed circuit. Why do energy-saving motors have shaft voltage? There are two reasons for the generation of shaft voltage during the operation of energy-saving motors: alternating magnetic flux and static charge accumulation.

The shaft voltage generated by the former is continuous and periodic. Under normal circumstances, the rotor of an electric motor operates in a symmetrical sinusoidal alternating magnetic field, and the alternating electromotive force induced by the cutting magnetic field of the motor rotor also generates symmetrical alternating current. So under normal circumstances, there will be no asymmetric voltage at both ends of the rotor. But when the magnetic resistance of the stator core of the energy-saving motor is unbalanced in the circumferential direction, asymmetric alternating potential will be generated, resulting in shaft voltage. This voltage is generated along the axial direction. The shaft voltage generated by electrostatic charges is intermittent and non periodic. During the operation of energy-saving motors, the fluid on the load side will rub against the rotating body, generating static charges on the rotating body, which will gradually accumulate and generate shaft voltage.

During the operation of large and medium-sized AC motors, once the rotor shaft voltage forms a circuit, shaft current will be generated, which is a typical low-voltage high current mode. Oil lubrication is used between the shaft and the bearing shell, and the energy-saving motor shaft is pressurized on the oil film. Due to the low amplitude of the shaft voltage, the oil film insulation is generally not broken down.

During high-speed operation of the rotor, if the quality of the lubricating oil does not meet the requirements or there is a lack of oil, the oil film will break and be punctured, resulting in metal contact between the shaft and the bearing shell. At the moment of contact, the shaft voltage forms a closed circuit, resulting in low voltage breakdown. The shaft current generated at this time is quite large, reaching hundreds or even thousands of amperes in an instant, enough to burn out the shaft neck and bearing.

The gradual accumulation of static charges generated by friction on the shaft causes the potential of the shaft to continuously increase due to charging. When the running shaft contacts any component other than the rotating body, it discharges through that component. If the running shaft does not come into contact with the rotating parts outside the body, it will accumulate charges and generate excessive voltage. If the voltage exceeds the insulation strength of the bearing oil film, the charge will discharge in a very short time, forming axial current.

The shaft current will flow through the circuit composed of the shaft, bearing inner ring, bearing outer ring, and bearing chamber. The significant phenomenon is the small and deep circular corrosion points generated by arc discharge at the position of the shaft bearing and the surface of the bearing inner ring. The shaft current not only destroys the stability of the oil film and the conditions for its formation, but also generates many corrosion spots on the inner surface of the shaft and bearing due to discharge, which damages the good fit between the shaft and bearing, resulting in the bearing being unable to work. In special circumstances, strong shaft currents can generate strong arcs at the contact surface between the shaft neck and the bearing shell, causing damage to the shaft neck and bearing shell, resulting in vibration and noise of the energy-saving motor, making it unable to operate normally.