Motor winding repair 1. Short circuit of stator winding

Winding short circuit refers to the direct collision of two copper wires caused by insulation damage to the coil wire, resulting in the current flowing directly into the circuit without passing through the load. The main reasons are high power supply voltage, excessive current, moisture and vibration wear on the winding, and accidental scratching of the outer insulation of the wire during repair of the embedded wire. After a short circuit fault occurs in an electric motor, high temperatures will be generated at the fault location, causing more insulation to burn out. Therefore, when checking for a short circuit fault in a running electric motor, carefully observe the motor coil for any burnt marks or smell after the motor stops running. It is also possible to run the motor without load for a period of time, immediately remove the end cover after stopping, and touch the coil with your hand to see if the heating is uniform. Areas with inter turn short circuits generally have higher temperatures. But the most effective method is still to use the induction method, which utilizes the principle of electromagnetic induction to span a specially designed short-circuit transformer - short-circuit detector across the stator slot. After the short circuit detector is powered on, if the thin iron sheet or saw blade placed on the slot it crosses vibrates, it indicates that there is a short circuit in the winding. If the winding is connected in multiple or triangular ways, the joints of each winding or phase winding need to be disassembled before inspection, otherwise it is difficult to detect. After finding the location of the short circuit, if it is found that the coil short circuit is not serious and the winding is not burnt out, local repairs should be carried out regularly. The short circuit can be re wrapped with insulation material. If only one coil of the entire winding is burned out due to a short circuit and is urgently needed, temporary measures can be taken, such as removing the coil or using a jumper to remove the short circuited coil from the winding circuit. However, at this time, all the wires at one end of the short circuited coil should be cut off (otherwise a short circuit current will be generated), and then the wire ends on both sides should be twisted together and wrapped with insulation material. If most of the insulation of the winding is burned out, a new winding needs to be replaced.

2. Inspection and repair of stator winding grounding

Winding grounding refers to the connection between the winding and the casing or iron core. The reasons for this include the coil being damp, the insulation material of the winding losing its insulation function, the motor running under overload for a long time causing the coil to overheat and the insulation material to become brittle and damaged, and careless operation during wire embedding causing the wire to collide with the iron core or end cap. If one phase of the winding is grounded, it will make the motor casing live, and if two phases are grounded, it will cause a short circuit and burn out the winding.

The commonly used light bulb method is to check whether the winding is grounded. Before inspection, the joints of each phase winding need to be disconnected so that each phase winding is not connected to each other. Then, connect the light bulb in series with the low-voltage power supply (12-36v), with one end connected to the motor housing and the other end alternately connected to the joints of each phase winding. If the light bulb lights up, it indicates that this phase has a winding grounding phenomenon. If it is not directly grounded (touching the shell), and there is leakage between the winding and the iron core due to insulation moisture, the high leakage resistance between the winding and the iron core may not be detected by the light bulb method. At this time, several dry batteries and a speaker (earphone) can be connected in series. When there is a fault, a "click" sound will be heard, and good windings do not have this sound. If there is only one phase winding and the grounding point is in the end slot without serious burns, there is no need to remove the coil. Simply place insulation paper on the grounding point and apply insulation paint. If the grounding point is in the slot, the wire needs to be gently pulled out of the slot, repaired, and then placed in the slot or replaced with a new coil. At this point, it is necessary to use insulating paper to pad the insulation groove (especially at the groove) to prevent the wire from directly touching the two ports of the groove and causing grounding again. If there are two windings grounded or multiple grounding points, and the coil is severely damaged, the entire winding can be replaced according to the specific situation. In addition, sometimes the insulation of the wire may be cut and grounded due to one or several protruding silicon steel sheets in the stator core of the slot. In this case, simply remove or flatten the protruding silicon steel sheet, and then re insulate the cut wire.

3. Inspection and repair of stator winding open circuit

Winding open circuit refers to the disconnection of one or several coils due to wire breakage or loose end connections. The main reasons are motor overload, high voltage, excessive current, poor welding or detachment due to mechanical vibration. If there is a phase break in the winding, the motor will become single-phase and cannot start; If the stator is a winding with multiple connections, and one of the phases is disconnected, it will cause the three-phase current of the motor to be unbalanced, causing the winding to heat up, the speed to decrease, and unable to reach the rated speed and power. At the same time, it will also cause vibration and abnormal noise in the motor. The commonly used light bulb method is to check the open circuit of the winding. Connect the light bulb in series in one phase winding and connect the low-voltage power supply (12-36V). If the light bulb does not light up, it indicates that there is an open circuit in the winding.

If the disconnection is caused by poor contact due to joint detachment or the disconnection point at the end, it can be re welded firmly, then polished clean and wrapped with insulation. If the breaking point occurs in the slot, the broken coil should be removed and rewound.

4. Inspection and repair of rotor winding

For wound rotor asynchronous motors, common faults in the rotor winding include short circuits and loose joints. A short circuit in the winding can cause the rotor winding to heat up, even burn out, and result in an unbalanced current in the stator, while also producing abnormal noise. If the winding joint is loose, it will cause the motor speed to decrease (usually 1/2 of the rated speed when unloaded), reduce the torque, and cause fluctuations in the stator winding current (when measuring the stator current with a clamp ammeter, the needle will oscillate back and forth). When checking, first open the connection between the stator winding and the slip ring to make the rotor winding in an open circuit state. Then, the three-phase current is passed into the stator winding. If the rotor rotates at a high speed (without reaching the rated speed), it indicates that there is a short circuit in the rotor winding. In addition, a short circuit detector can also be used to check whether there is a short circuit in the rotor winding, using the same method as checking the stator winding. Loose joints in windings generally occur at the connection between the lead wires and slip rings, often caused by loose screws or poor welding. This type of malfunction can be detected by careful observation.

Classification of windings - Centralized windings

Centralized winding is applied to convex stator, usually wound into rectangular coils, wrapped and shaped with warp tapes, and then immersed in paint and dried before being embedded on the iron core of convex magnet. The excitation coil of general commutator type motors (including DC motors and general motors) and the main winding of single-phase hood type convex motors all use centralized windings.

Centralized windings usually have one coil per ji, but there are also cases where a hidden ji form is used, such as frame type covered ji motors, which use one coil to form two ji motors.

Distributed winding

The stator of a distributed winding motor does not have a convex shape, and each magnetic coil is composed of one or several coils embedded and wired according to a certain rule to form a coil group. After being energized, different magnetic coils are formed, so it is also called an implicit type. According to the different arrangement forms of embedded wiring, distributed windings can be divided into two types: concentric and stacked.

Concentric winding

Concentric winding is a coil group consisting of several coils with similar shapes but different sizes, embedded in a zigzag shape at the same center position. Concentric windings can be formed into dual plane or tri plane windings according to different wiring methods. This type of stator winding is commonly used for single-phase motors and some three-phase asynchronous motors with small power or large span coils.

Stacked winding

Stacked windings are generally composed of coils of the same shape and size, with one or two coil edges embedded in each slot, and evenly distributed by stacking them one by one at the outer end of the slot. Stacked windings are divided into two types: single-layer stacked and double-layer stacked. A single-layer stacked winding, also known as a single stack winding, is one slot with only one coil edge embedded; When two coil edges belonging to different coil groups are embedded in each slot, they are placed in the upper and lower layers of the slot, forming a double-layer stacked winding, also known as a double stacked winding. Depending on the variation of embedded wiring methods, stacked windings can also be derived into cross type, concentric cross type, and single/double layer hybrid types. At present, the stator winding of three-phase asynchronous motors with high power generally adopts a double-layer stacked structure; Small motors often use derived forms of single-layer stacked windings, but rarely use single-layer stacked windings.

Rotor winding

The rotor winding of asynchronous motors can be divided into two types: squirrel cage type and winding type.

Rat cage type

The squirrel cage structure is relatively simple, usually made by pouring aluminum water into the rotor iron core groove and casting the end rings at both ends; It is also possible to embed copper strips and then weld copper end rings. In order to improve the starting performance, the squirrel cage type can also be made into special forms such as deep groove type and double squirrel cage.

Winding type

The wound rotor winding is the same as the stator winding. In addition to the various types of windings mentioned above, it can also use waveform windings. The waveform winding is composed of a single or several turn rod-shaped unit coil, which is embedded and welded into a single coil at the end by two components, forming the entire winding. Its wiring principle is different from the above winding, but its appearance is similar to that of a double-layer winding. Waveform winding is commonly used in the rotor winding of large AC motors and the armature winding of DC motors.