Why do energy-saving motors with power frequency operation have shaft current?
November 06, 2023
Why do energy-saving motors with power frequency operation 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 generating shaft voltage during the operation of energy-saving motors, one is alternating magnetic flux, and the other is static charge accumulation.
The shaft voltage generated by the former is continuous and periodic. Under normal circumstances, the rotor of the 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 a 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 an energy-saving motor is unbalanced in the circumferential direction, asymmetric alternating electromotive force will be generated, resulting in shaft voltage. This voltage is generated along the axis. The axial voltage generated by static 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 operation of large and medium-sized AC motors, once the rotor shaft voltage forms a loop, shaft current will be generated, which is a typical low voltage and high current mode. Oil lubrication is used between the shaft and the bearing bush, and the energy-saving motor shaft bears pressure on the oil film. Due to the low amplitude of shaft voltage, oil film insulation is generally not broken down.
During the high-speed operation of the rotor, if the lubricating oil quality does not meet the requirements or if there is a shortage of oil, the oil film will break and be broken, causing metal contact between the shaft and bearing shells. At the moment of contact, the shaft voltage will form a closed circuit, forming a low-voltage breakdown. The shaft current generated at this time is quite large, reaching hundreds or even thousands of amperes in an instant, which is enough to burn out the journal and bearing shells.
The gradual accumulation of static charges generated by friction during operation on the shaft causes the electric potential of the shaft to continue to increase due to being charged. When the operating 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 charge and generate excessive voltage. If the voltage exceeds the insulation strength of the bearing oil film, the charge will discharge in an extremely short time, forming an axial current.
The shaft current will flow through a circuit composed of the shaft, bearing inner race, bearing outer race, and bearing chamber, with the notable phenomenon being 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 race. The shaft current not only disrupts the stability of the oil film and the conditions for its formation, but also generates many corrosion spots on the surface of the shaft and bearing inner race due to discharge, which disrupts 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 on the contact surface between the journal and bearing pads, causing damage to the journal and bearing pads, causing vibration and noise in energy-saving motors, and preventing their normal operation.
