Shaft current is a major quality killer of variable frequency motors, large motors, high voltage motors and generators, and it is extremely harmful to the motor bearing system. There are many cases of bearing system failures due to inadequate shaft current prevention measures.
The characteristics of shaft current are low voltage and high current, and the damage to the bearing system can be said to be hard to prevent. The generation of shaft current is due to shaft voltage and closed circuit. To solve the shaft current problem, there are two ways to solve it: eliminating the shaft voltage or cutting off the circuit.
Magnetic circuit imbalance, inverter power supply, electrostatic induction, static charge and external power supply interference may all generate shaft voltage. The amplitude of shaft voltage is generally small, but when it reaches a certain value, it will break through the bearing lubricating oil film and pass through a closed loop on the bearing surface. The large shaft current will cause the bearing to burn due to heat in a very short time. The bearing burned by shaft current will leave a washboard-like mark on the outer surface of the inner ring of the bearing.
To solve the problem of avoiding shaft current, necessary measures must be taken in the design and manufacturing process of the motor, such as adding necessary insulation measures to the end cover and bearing sleeve. For small-sized products, insulating bearings can be used, and leakage carbon brushes can be added during use. From the perspective of use, taking disconnection measures on components is a once-and-for-all measure, while the use of diversion may cause the problem of replacing the carbon brush device. At least, the carbon brush system should not have problems during the maintenance cycle of the motor.
01
Insulated bearings
The size and load capacity of insulated bearings are the same as those of ordinary flange bearings. The difference is that insulated bearings can prevent the passage of current very well. Insulated bearings can avoid damage caused by electrical corrosion. Therefore, they can be used in motors to ensure more reliable operation than ordinary bearings. Insulated bearings can avoid the electro-corrosion effect of induced current on bearings and prevent current from damaging grease, rolling elements, and raceways.
02
Why does inverter power supply generate shaft voltage?
When the motor is operated with inverter power supply, the power supply voltage contains high-order harmonic components, which cause electromagnetic induction between the stator winding coil ends, the wiring part, and the rotating shaft, thereby generating shaft voltage.
The stator winding of the asynchronous motor is embedded in the stator core slot. There are distributed capacitances between the turns of the stator winding and between the stator winding and the motor base. When the general frequency converter is operated at a high carrier frequency, the common mode voltage of the inverter changes sharply, which will form a leakage current from the motor casing to the ground terminal through the distributed capacitance of the motor winding. This leakage current may form two types of electromagnetic interference, radioactive and conductive. Due to the imbalance of the motor magnetic circuit, electrostatic induction and common mode voltage are the causes of shaft voltage and shaft current.
Post time: Aug-13-2024
