Inter-turn Insulation Faults: Characteristics And Cause Analysis

Inter-turn insulation fault is one of the common electrical faults of motor windings. It occurs in both single-phase and three-phase motors, except for windings with only one single turn. When a single conductor has a relatively large cross-sectional area, the coil consists of fewer turns; in contrast, conductors with small cross-sectional areas usually form coils with a large number of turns, which leads to a higher probability of inter-turn faults in windings.

For coils wound with multiple parallel conductors, inter-turn insulation risks do not exist among wires within the same bundle of a single coil, but occur between wires of different coil turns.

Form-wound coils generally have fewer turns, while random-wound coils usually contain a large number of turns. Therefore, form-wound windings are recommended for low-voltage high-power motors, high-voltage motors and motors operating under special working conditions.

1. Causes of Inter-turn Insulation Fault During Manufacturing

For insulated wires such as enameled wires and mica wires, their intrinsic inter-turn insulation can theoretically withstand the impulse of turn-to-turn voltage. Nevertheless, certain procedures in the actual winding manufacturing process may damage the insulation layer of magnet wires. Some damages cannot be repaired or sufficiently reinforced even through the impregnation and baking process, eventually triggering inter-turn faults during factory testing or field operation.

Apart from mechanical damage to magnet wires during production, the inherent quality of magnet wires plays a decisive role. The adhesion and uniformity of the insulation layer as well as the surface smoothness of conductors directly determine the reliability of motor windings. For form-wound windings, the forming process imposes high mechanical requirements on wire insulation. Poor-quality magnet wires are prone to insulation cracking during shaping, resulting in severe inter-turn insulation faults.

To address this issue, proper selection of magnet wires in the design phase and strict compliance with winding processing specifications are essential to avoid mechanical damage to wire insulation during production. During varnish impregnation and curing, full filling of insulating varnish must be ensured. Key production control points include suitable equipment selection, performance parameters of insulating varnish, technical process parameters and strict process implementation.

2. Analysis of Inter-turn Insulation Fault During Motor Operation

Any insulation damage to magnet wires incurred during manufacturing becomes a latent defect in subsequent service. In addition, the first coil directly connected to the power supply bears the severest voltage impulse during motor startup, and it also suffers the most severe deformation during coil embedding. Actual fault statistics show that most inter-turn faults occur at the first coil of windings without reinforced insulation protection. For this reason, many motor manufacturers and repair workshops adopt special insulation reinforcement on the first coil to effectively reduce the occurrence of inter-turn faults.

Inspection on faulty high-voltage motor windings reveals that inter-turn faults frequently occur in coils with poor varnish impregnation and loose arrangement between turns, which may result from inadequate impregnation, improper coil wrapping or curing procedures.

Electrical failures of motors are partially determined by material performance, yet most originate from manufacturing process standards. Variations in production techniques among different manufacturers and motor types make targeted reinforcement measures effective in preventing such faults. Learning lessons from failed motors is a more practical way to improve product quality than evading responsibility.