Analysis of Stator Winding Interturn Failure in Motors

Introduction

The electrical insulation failure in motors includes damage to the insulation within the coils, interturn insulation damage, and insulation failure between the coils and the ground. Insulation failure between the coils and the ground is the main cause of damage to three-phase asynchronous motors, followed by interturn insulation failure, and lastly, winding insulation failure. The electrical insulation performance of a motor is determined by the performance of the insulation material itself and the manufacturing process. The stability of the insulation material is the foundation of the reliability of the motor's insulation performance. The main insulation materials in a motor include insulating paper, insulating tubes, insulating varnish, and mica tapes that are used to enhance the insulation between the coils and the iron core, as well as the insulation materials in the electromagnetic wires and connecting wires. Damage to insulation materials is the main cause of electrical insulation failure in the motor coil windings. To reduce the likelihood of winding insulation failures, it is necessary to pay attention to the suitability of the production process and strengthen the protection measures for the insulation materials to avoid damage to the insulation layer during processing. The following is a description of the types of winding insulation interturn failures, their causes, and preventive measures.

Types and Causes of Stator Winding Interturn Failures

Interturn failure in the stator winding of a motor refers to the burning of the coils in one phase or adjacent phases in one or several slots of the stator winding. Based on the cause of the failure and its manifestation, interturn failures can be divided into three cases: grounded interturns, interphase short-circuited interturns, and internal interturns. A grounded interturn occurs when the insulation between the coil and the ground is damaged during the operation of the motor, causing the coil to short-circuit to the ground, resulting in the burning of the stator winding. An interphase short-circuited interturn occurs when the insulation between two phases is damaged, causing the coils to short-circuit, resulting in the burning of the stator winding. An internal interturn failure occurs when the insulation between the coils in the same phase is damaged, causing the coils to short-circuit and burn.

Grounded interturn failures often occur at the mouth or back of the stator slots. During the process of embedding the coils, the insulation at the mouth or back of the slots is easily damaged. During the installation of the stator, the insulation at the back of the slots is easily damaged, and during the installation of the rotor, the insulation at the mouth of the slots is easily damaged. After leaving the factory, due to the slight damage to the coil, the problem may not be fully apparent. During the operation of the motor, the insulation at the weak point is subjected to repeated impacts from the load current, causing the insulation performance to gradually decrease, and the leakage current at the weak point increases until it reaches the limit of the leakage current, causing the insulation to break down, resulting in the short-circuit and burning of the coils in the interturns, causing the motor to stop. Interphase short-circuited interturns often occur between adjacent phases at the ends of the motor stator windings. Due to the collisions during transportation or installation of the stator, the insulation between the adjacent phases and the stator itself is damaged. After running for a period of time, the temperature rises, causing the insulation at the fault point to break down, resulting in the short-circuit and burning of the interturn coils between the two phases. Internal interturn failures occur between the coils of the same phase in the same slot or adjacent slots of the stator winding. Internal interturn failures are mostly caused by damage to the insulation between the coils during the assembly process. During the operation of the motor, the imbalance block or balance column carried by the rotor becomes loose or falls off, causing damage to the internal insulation of the stator winding, resulting in interturn burnout.

Most interturn burnout failures are due to the quality of the motor. The insulation of the enameled wire has defects, and the insulation of the stator winding is damaged to varying degrees during the manufacturing and transportation process, reducing the insulation performance of the stator winding. During the use process, due to the repeated impact of the load current and the increase in humidity and temperature in the running environment, the insulation at the weak point of the interturn breaks down and short-circuits, causing the stator winding to burn out. When a short-circuit failure occurs in the interturns, it is possible that only two wires are in contact due to the wear of the insulation at the overlap, causing a short-circuit loop in the wire coil, which generates heat and further damages the insulation of the adjacent wires. As the number of short-circuited turns increases, the range of the fault expands, and when the number of short-circuited turns is sufficient, the current at the short-circuit point increases significantly, causing the stator winding to burn out. When one phase of the three-phase stator winding has an interturn short-circuit, it is equivalent to reducing the effective number of turns in that phase, causing the three-phase current of the stator to become unbalanced, reducing the torque of the motor, and causing the speed of the motor to decrease. If the motor continues to run, the range of the fault will expand, eventually leading to the burning of the stator winding.

The insulation of the interturns in the motor stator winding is either the insulation of the enameled wire itself or a very thin additional interturn insulation, such as thin films or mica pads. The contact area of the interturn insulation is the same as the contact length of the coil winding. The interturn insulation may be damaged during the winding, embedding, and connecting processes. During the operation of the motor, the insulation of the stator winding is subjected to power frequency voltage, instantaneous overvoltage, operational overvoltage, and lightning overvoltage. These voltages act simultaneously on the insulation between the coils and the ground. The rated interturn power frequency voltage is only a few tens of volts, which causes little damage to the interturn insulation. The main cause of damage to the interturn insulation is various impact overvoltages, the peak of which can be several times the rated voltage. The short-circuited turns in the motor generate induced electromotive forces under the action of the alternating magnetic field, forming a closed loop, which generates large currents, several times the rated current, causing the temperature of the short-circuited turns to be higher than that of the other turns. After long-term operation, the insulation material ages, the interturn insulation is damaged, and eventually, the interturn or ground insulation breaks down, causing the motor stator winding to burn out. Therefore, to extend the service life of the motor, it is necessary to improve the electrical insulation strength and process stability of the interturn insulation.

Preventive Measures for Grounded Interturn Short-Circuit Failures

Grounded insulation failures in motors with windings on the iron core often occur at the mouth of the iron core slot or at the ends of the windings, especially in windings with high slot fill rates. During the expanding and shaping process, the insulation at the mouth of the slot is easily damaged, causing damage to the ground insulation of the winding, resulting in grounded interturns. The repair of ground insulation failures varies greatly depending on the insulation processing technology. Vacuum pressure dipping becomes a solid state after the insulating varnish is solidified, making it difficult to repair, especially for insulation failures inside the iron core slot. When the ground insulation failure occurs at the slot mouth of the winding, the possibility of damage to the enameled wire coating is high. After repairing the ground insulation failure, the possibility of interturn insulation failure is also high. Prevention of interturn failures should consider factors such as material properties, design compatibility, winding and winding manufacturing technology, and the insulation processing of the windings, as well as the actual operating conditions of the motor. The performance compatibility of the enameled wire is particularly important for controlling interturn failures. Projects such as elasticity, coating continuity, DC resistance, and withstand voltage testing can reflect the performance compatibility of the enameled wire. During the design phase, it is necessary to consider comprehensively the manufacturing process according to different products, including the slot fill rate and end size control. The insulation processing of the windings is a key technology in the manufacture of motor windings. It is necessary to ensure that the winding dipping index and curing effect are improved to prevent the effects of vibration during motor operation, and to ensure that the motor has good heat dissipation. Additionally, it is necessary to take necessary preventive measures during the manufacturing process, such as during the manufacture of the iron core, the transportation of the iron core with windings, and the insertion of the rotor into the stator, to avoid damage to the windings.

Under normal circumstances, after the motor is assembled, there should be a certain space between the ends of the windings and the motor housing and end covers, and there should be no substantial contact. However, for motors with compact specifications or irregularly shaped windings, it is possible that there may be substantial contact between the windings and the motor housing or end covers after the windings are inserted into the motor housing and assembled, causing damage to the ground insulation and resulting in interturn failures. This type of failure is concentrated in specific motor specifications, and the location of the ground fault is relatively fixed, often concentrated at the welding point between the lead wires and the main wires, and at the outlet of the lead wires from the motor housing. To prevent this type of problem from occurring, it is necessary to strictly control the technical aspects of winding processing, set appropriate positions and sizes for the winding openings, and ensure that the distribution of the lead wires is reasonable. Additionally, it is necessary to avoid oversized dimensions at the ends of the windings after the windings are formed.

Conclusion

The stator is a key core component of a motor, and the quality of the stator windings directly affects the service life of the entire motor. Due to the fact that the stator is prone to collisions and scratches during the dipping, winding, and assembly processes, causing damage to the insulation between the windings and the ground or between the turns, it is necessary to perform insulation performance tests on the stator windings, which are a key part of motor testing. These tests typically include measurement of the insulation resistance, power frequency withstand voltage testing, and high voltage pulse testing of the interturns to test the insulation between the turns, between the windings, and between the windings and the ground. The insulation performance of the windings is determined by comparing the measured insulation resistance, the leakage current during the withstand voltage testing, and the absolute and differential values of the interturn waveforms with the standard values that have been set.