Views: 5 Author: Site Editor Publish Time: 2021-11-04 Origin: Site
External cause analysis
The change of the operating environment is the main external cause of the motor heating, such as: three-phase voltage imbalance, power supply voltage fluctuation, overload and overload operation, poor operating environment, etc.
1. Three-phase voltage unbalance
When a three-phase voltage imbalance occurs in the motor power supply, a reverse sequence current and a reverse sequence magnetic field will be formed in the internal complex electromagnetic interaction environment, thereby forming a larger reverse sequence torque in the motor winding. When the torque is superimposed, the internal electromagnetic torque of the motor will be reduced, which will cause the rotor speed of the motor to decrease. In the case of the same load capacity, in order to rebalance the motor system, it is necessary to increase the current of the motor. Increasing the heat generated by the internal windings of the motor will cause the motor to overheat and malfunction in severe cases. According to some engineering application statistics, when the three-phase voltage unbalance of the motor reaches about 5%, the current between the motor phases will exceed 25% of the rated current value, and the motor current value increases sharply with the increase of the voltage unbalance. Therefore, in order to ensure the safe and stable operation of the motor, it is necessary to strictly monitor the unbalance of the power supply voltage in daily management, and try to ensure that the motor runs under the condition of three-phase balance.
2. Power supply voltage fluctuation
In order to ensure that the motor has high technical and economic indicators, the manufacturer usually chooses its magnetization curve to be near saturation during the design of the motor. After the motor power supply voltage increases, the air gap magnetic flux inside the motor will increase, and the corresponding motor flux loop will be oversaturated. At this time, the excitation current will increase sharply, generating huge heat in the windings, causing The motor has malfunctioned. Since the electromagnetic torque of the motor is proportional to the square of the voltage, when the power supply voltage drops, the motor speed will be lowered when the motor load torque remains unchanged, causing the slip rate to increase, resulting in a huge The current of the rotor (stator) heats up violently in the windings, causing the motor to generate heat failure. That is to say, the heating of the motor caused by the increase of the power supply voltage is mainly reflected in the iron core of the motor; and the heating of the motor caused by the decrease of the power supply voltage is mainly reflected in the winding of the rotor (stator).
3. Overload and overload operation
Overload overload operation is a common external cause of motor heating failure. In the entire motor system, if the mechanical system driven by the motor is jammed, the transmission mechanism is too tight, the lubrication system is damaged, and the load suddenly increases, etc., the motor can run in the low-efficiency area, and overload and overload will occur. Unreasonable motor system design and selection may also cause the motor to appear "small horse-drawn cart". In this working condition, in order to ensure the balance between input and output, the motor system will reduce the motor speed, which will cause the internal current to increase, generate violent heat, and cause the motor to overheat.
Internal mechanical structure failure, stator winding failure, rotor winding failure and insulation failure are the main internal causes of motor overheating failure.
1. Mechanical failure
When the motor bearing is not installed in place, there is eccentric jamming, the bearing lubrication system has no lubricating oil, too much oil, poor oil quality, improper oil sealing system, etc., and the motor end cover and the motor frame are out of concentricity, etc. All of them can cause problems such as increased friction between the bearings, distortion of the rotating shaft, and bore sweeping of the rotating shaft, resulting in severe heating of the motor and heating failure.
2. Stator (rotor) winding failure
When the three-phase windings of the motor are connected wrongly, short-circuits between turns or layers of the stator (rotor) windings, and single-phase grounding of the three-phase windings, they can all cause the motor to overheat. When there is a virtual connection or virtual welding between the lead contacts of the three-phase winding of the motor, it will cause the contact resistance of the joint to be too large, and the violent heat at the winding joint will cause the motor to generate heat failure.
3. Insulation failure
As the running time of the motor increases, the insulation of the motor core laminate layer will continue to age or be damaged under the combined influence of thermal and dynamic effects, which will increase the internal eddy current of the motor, increase the iron loss surface of the motor, and generate intense heat at the core. After the motor winding insulation is aging and damaged, the winding insulation effect will be greatly reduced, the leakage current will increase sharply, and more heat will be generated at the windings, which will cause serious heating failures in the motor.