1. Repair of Motor Stator Winding 

 

1.1 Winding Requirements for Motor Stator Windings The winding is one of the main components of the motor and the key part of energy conversion. It is composed of many coils connected together. All the wound coils are placed in the slots of the stator core, and then connected in series or parallel according to a certain rule to form the motor winding. The rationality of the winding design will affect the torque, speed, loss, efficiency, and temperature rise of the motor. 

 

Therefore, the following requirements are proposed for the motor winding:

 

①The voltage and magnetic potential of each phase winding should be symmetrical, and the resistance and reactance should be balanced, that is, the structure of the three-phase windings should be exactly the same;

 

②  Shorten the connecting part to save copper, reduce winding short-circuit losses; 

 

③The winding should have good heat dissipation, reliable insulation, and mechanical strength;

 

④ The winding structure should have good construction workmanship.

 

1.2 Several Important Parameters of Motor Windings

 

 1.2.1 Pitch. The range occupied by each magnetic pole along the inner circle of the motor stator core is called the pitch.

 

 1.2.2 Slot Pitch. The number of slots spanned by two effective sides of a winding element is called the slot pitch.

 

 1.2.3 Electrical Angle. Electrical Angle = Number of Pole Pairs × 360°.

 1.2.4 Slots per Pole per Phase. The number of slots occupied by each phase winding under each magnetic pole.

 

1.3 Types of Motor Windings 

 

1.3.1 The construction principle of three-phase stator windings: In the three-phase stator windings, the number of coils per phase is equal, and the arrangement is the same. The phase-to-phase interval is 120° electrical angle in the slot, which meets these two principles and constitutes a three-phase symmetrical winding. Only in a three-phase symmetrical winding can a balanced three-phase electromotive force be induced. 

 

1.3.2 Types of three-phase stator windings include single-layer windings and double-layer windings. Single-layer windings do not have inter-layer insulation problems in the slot, so inter-layer and inter-phase insulation breakdown faults will not occur in the slot. Winding and winding are more convenient; the disadvantage is that the selection of short-distance coils is limited, and the electromagnetic waveform is not ideal, so it is generally used for small power motors, and its coils are made of round electromagnetic wires wound into multi-turn scattered windings. Double-layer windings have the advantage of being able to choose suitable short-distance coils at will, and the technical indicators of the motor are better than single-layer windings, generally used for large and medium-sized motors.

 

1.4 Disassembly and Repair of Motor Stator Windings

 

 1.4.1 Winding of Coils 

 

① Scattered Winding: Scattered windings are wound with insulated circular conductors, and the geometric dimensions are ensured by the winding die. The winding die must be made appropriately sized, and the pulling force should not be too large. The conductors should be arranged in order in the die; if the conductors are not neatly crossed, it will not only be difficult to embed the wire but also easy to cause turn-to-turn short-circuit faults; during the winding process, the conductor breaks should be welded at the oblique edge of the coil terminal, and joints are not allowed to remain in the slot. 

 

② Formed Coils: Formed coils are more complex to manufacture than scattered windings. Generally, when replacing coils, spare coils are used or coils are ordered from the manufacturer according to the motor model.

 

1.4.2 Embedding, Wiring, and Inspection. Preparatory work before embedding: including the preparation of insulating materials and coils, slot wedges, welding materials, slot insulation, core cleaning, workplace cleanliness, etc., and it is also necessary to be familiar with the drawings to clearly understand the motor pole number, winding pitch, lead direction, etc., to avoid mistakes in the embedding process. 

1.4.4 Repair of Stator Winding End Wear The stator winding of an AC motor is subject to the action of alternating electromagnetic forces during operation, the magnitude of which is proportional to the square of the current passing through. Therefore, during startup, short-circuit, and blockage, the maximum electromagnetic force can sometimes reach several times the normal value. As a result, the end structure of the winding, which is solidified with poured paint, is prone to insulation wear, which is a very common fault in AC motors. The reasons are: 

 

① The thermal shrinkage of the end lashing rope after heat solidification is not enough, so it cannot tighten the ends of the windings of each phase after the paint solidifies, making it impossible to securely tie and fix the winding ends in the direction of vibration under force. Therefore, once the motor is subjected to alternating tangential and axial forces, the force point is added at the contact between the winding and the end lashing, causing vibration displacement. This is one of the main reasons for accelerating insulation wear at this point; 

 

② Insulation line damage at the end during winding;

 

 ③ The pouring and solidification process of the end cannot guarantee the rigidity of the lashing rope after solidification; 

 

④ The gap in the slot increases, causing the coil to vibrate in the slot, leading to wear at the slot mouth.

 

1.4.5 Local Repair of Stator Winding If a motor winding or a part of a coil is burned out, local repair should be considered. First, the exact fault point of the faulty coil must be identified, and the motor winding should be heated to 100°C-130°C to soften the winding insulation, and then remove the slot wedges and faulty coils while the insulation is still soft. Be careful not to damage the good coils when removing the old coils. After the faulty coils are removed, replace them with new ones, and then drive in the slot wedges and perform impregnation and baking treatment.

 

2 Motor Core Fault Repair

 

 2.1 The Role of the Motor Core The core of a motor is not only a key component for electromagnetic energy conversion but also bears the combined effects of mechanical vibration, electromagnetic force, and heat in the motor. To reduce eddy current losses in the core, the core laminations need to have a certain insulation resistance value.

 

2.2 Common Core Fault Repair 

 

2.2.1 Individual teeth of the core tooth pressure plate protrude outward along the axis. This is due to the fact that the teeth of the tooth pressure plate are not leveled during the stacking of the core, and after the core is pressed, the laminations appear wavy, and the teeth of the core are raised, or the teeth pressure plate is pried up by the stator core when the rotor is pulled out. The repair only needs to use a copper rod to knock flat the individual teeth that protrude outward.

 

 2.2.2 Local burning of the core. The arc produced when the winding has a phase-to-phase short circuit or ground short circuit fault sometimes burns a part of the core. If the burned area is not large, the repair can be done without disassembling the core. First, use files, chisels, grinding wheels, and other tools to level or repair the locally burned part of the core, then use a knife to peel off the silicon steel sheets one by one, apply a layer of self-drying insulating paint on the core surface, and then press the core tightly. If the burned area is large, the core or the damaged laminations should be replaced. 

 

2.2.3 Radial loosening of the core. There are two situations for radial loosening of the core:

 

 ① For small motors using the external pressure core assembly process, it is caused by the loose fit between the outer circle of the core and the inner hole of the machine seat, or the loosening of the positioning screws between the core and the machine seat. The repair method is to tighten the positioning screws, add additional positioning screws on the machine seat if necessary, or weld the core to the machine seat; 

 

② For large and medium motors using the internal pressure core assembly process, it is caused by the radial positioning device of the core being welded open, loosened, or the welding point between the core and the machine seat being dewelded. The repair method is to tighten the positioning device, add anti-loosening screws, or repair the dewelded area.

 

3. Rotor Fault Handling of Cage Rotor Breakage is a common fault in cage-type asynchronous motors, and most of them are due to the welding points of copper bars and end rings being unwelded, which can be cleaned and firmly welded with silver solder. In addition, there are often some defects on the shaft. For example: shaft bending; 

 

keyway wear, bearing seat wear, etc. These can be solved by methods such as straightening with a welding torch, re-milling a keyway, plating or spraying to plate a layer of chromium on the bearing seat, etc., depending on the situation.