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Techniques that must be mastered in motor maintenance

Date:2023-09-26   Author:XINDA MOTOR

When the motor is running, one or other faults may occur, causing the motor to malfunction or even burn out. There are many reasons for motor malfunction or burnout, such as: high losses during starting and braking; overloading during continuous operation; too many repeated short-term operations; rotor stall; three-phase motor operation with phase loss; low voltage operation . ; Surrounding environment temperature is too high, humidity is too high, dust, acidic gas corrosion, etc. Common repair tasks for electric motors include motor winding replacement and repair, core damage repair, and rotor failure treatment.

1Repair  of motor stator winding

1.1 Winding requirements for motor stator windings

The winding is one of the main components of the motor and a key part of energy conversion It is composed of many coils connected. All the wound coils are placed in the stator core slots, and then connected in series or parallel according to certain rules to become the motor windings. Whether the winding design is reasonable or not will affect the torque, speed, loss, efficiency and temperature rise of the motor. Therefore, the following requirements are put forward for the motor winding: ① The electric potential and magnetic potential of each phase winding must be symmetrical, and the resistance and reactance must be balanced . That is to say, the winding structures of the three phases are exactly the same; ② shorten the connection part, save copper, and reduce the short-circuit loss of the winding; ③ the winding heat dissipation must be good, and the insulation and mechanical strength must be reliable; ④ the construction process of the winding structure is good.

1.2 Several important parameters of motor windings

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

1.2.2 Pitch. The number of slots spanned between two active sides of a winding element is called the pitch.

1.2.3 Electrical angle Electrical angle = number of pole pairs × 360°.

1.2.4 Number of slots per pole and phase. The number of slots occupied by each phase winding under each magnetic pole.

1.3 Types of motor windings

1.3.1 Composition principles of three-phase stator winding:

The number of coils in each phase of the three-phase stator winding is equal and the layout is the same. The phases are separated by 120° electrical angle in the slot. This conforms to these two principles, that is, it constitutes a three-phase symmetrical winding. This can only be achieved in a three-phase symmetrical winding. Induction produces a balanced three-phase electromotive force .

1.3.2 The types of three-phase stator windings include single-layer windings and double-layer windings.

There is no interlayer insulation problem in the single-layer winding slot , so interlayer and interphase insulation breakdown failure will not occur in the slot. Both winding and embedding are more convenient; the disadvantage is that the selection of short-pitch coils is limited and the electromagnetic waveform is not ideal, so it is generally used in small- power motors. Its coils are wound with round electromagnetic wires into multi-turn scattered embedded coils. The advantage of double-layer winding is that you can choose any suitable short-pitch coil. The technical indicators of the motor are better than those of single-layer winding. It is generally used for large and medium-sized motors.

1.4 Replacement and repair of motor stator windings

1.4.1 Coil winding

①Lower coil: The loose coil is wound with insulated wire. The geometric size is guaranteed by the winding mold. The winding mold must be made of appropriate size and the pulling force should not be too large. The wires must be arranged sequentially in the wire mold; the wires are not crossed neatly, which not only makes it difficult to embed the wires, but also easily causes short-circuit faults between turns; during the winding process, the broken wires should be welded to the hypotenuse of the end of the coil, and the joints are not allowed to remain in the coil groove part.

② Formed coils: Formed coils are more complicated to manufacture than loose coils. Generally, when repairing and replacing coils, spare coils are used or coils are ordered from the manufacturer according to the motor model.

1.4.2 Disconnect, connect and inspect. Preparation work before offline: including the preparation of insulation materials and coils, slot wedges, welding materials, slot insulation, cleaning of the iron core, cleaning of the work site, etc. At the same time, you must be familiar with the drawings and clearly understand the number of motor poles and winding sections. distance, lead direction, etc. to avoid errors in the production line. The offline process of embedded windings is:

① Remove the lower edge of the first pitch coil. Determine the position of the first slot based on the buckle slot of the iron core or the outlet hole of the machine base, embed the edge of the lower coil one slot at a time, and pad the upper coil with cardboard;

②Embed the remaining coils. After placing the lower coil edge, pad the interlayer insulation, lower the upper coil edge into the groove with the corresponding pitch, use a scribing board to straighten the coil edge in the groove, and gently press the end with your hands to prevent it from warping, and then Fold over the trench insulation and drive in trench wedges;

③ Remove the last pitch coil, lift the upper edge of the first pitch coil, lower it into the lower edge of the last pitch coil, then lower the upper edge of the next pitch coil, fold the groove insulation, and drive in slot wedge;

④ Shaping the end, use a rubber hammer or a bamboo board to make the end into a bell mouth. The diameter should be appropriate. If it is too small, it will affect the insertion of the rotor and ventilation and heat dissipation. If it is too small, it will affect the insertion of the rotor and ventilation and heat dissipation. If the end is too close to the casing, it will affect the insulation of the motor. ;

⑤ Bandage the ends. Trim the ends of the same insulation paper neatly;

⑥Insulation test. Check the tightness of the protruding wedge at the end and whether the insulation is damaged, and conduct a withstand voltage test on the phase-to-phase insulation and ground insulation;

⑦Connection between pole groups. Connect the coils of the same group according to the "forward string" or "reverse string" regulations, lead out the cables, weld them tightly, wrap them with insulation, and then tie the connecting wires tightly;

⑧Inspection test. Check wiring and welding quality;

⑨ Dip paint. Dip and dry the paint and spray insulating paint on the winding surface. Wiring

① Connect the individual coils into a pole phase group according to the 60° phase band distribution;

② Connect the pole phase groups of the same phase to form each phase winding;

③ Lead the beginning and end of the three-phase winding to the junction box with wires. All connectors must be welded and insulated. Inspection

Before dipping in paint, the stator winding should be inspected, including: ① Whether the winding is grounded; ② Whether the winding is short-circuited; ③ Whether the winding is open-circuited; ④ Whether the winding is connected incorrectly or reversely.

After confirming that there are no above-mentioned problems, proceed with dipping and drying.

1.4.3 Dip paint and drying

After rewinding or partially replacing the stator winding, the process of dipping and drying can fill the gaps between the winding and the iron core, and between the wires and wires with insulating paint, so that the winding and the iron core form a whole. It enhances the moisture resistance of the winding and improves the insulation strength, heat dissipation capacity and mechanical strength of the winding. Therefore, the paint dipping and drying of the stator winding is a very important process in motor repair. There are roughly the following procedures; Pre-baking:

The function of pre-baking is to drive away moisture in the winding. It is usually carried out in an oven with the temperature controlled at about 120° and the time is about 4h-8h. Use a 500V megger to measure the insulation resistance of the winding to ground every 1 hour. When the insulation resistance value stabilizes, pre-baking can be completed. Dip paint:

After the stator core temperature drops to 60℃--70℃, it can be dipped in paint. When dipping in paint for the first time, the viscosity of the paint should be lower so that the insulating paint can penetrate into the winding as much as possible. For the second dip, the viscosity should be higher so that a thicker paint film can be formed on the surface of the winding. Under normal circumstances, the method of pouring paint can be used, and the pouring should be repeated several times evenly. Drying:

Drying is to evaporate the solvent and moisture in the paint so that a stronger paint film can be formed on the winding surface. The drying process is best divided into two processes. ① Low temperature stage. The temperature is controlled at 70℃--80℃, about 2h--4h; ② High temperature stage. The temperature is around 130°C for about 8h to 16h in order to form a solid paint film. At this time, the insulation resistance of the winding to ground should be measured every 1 hour with a 500V megger. Until the last 3 hours, the insulation resistance value is stable above 5MΩ, and the winding The drying is complete.

1.4.4 Repair of stator winding end wear

The stator winding of an AC motor is acted upon by electromagnetic forces with alternating directions during operation, because the magnitude of the electromagnetic force generated between adjacent windings is proportional to the square of the current flowing through it. Therefore, when the motor is started, short-circuited, or blocked, the maximum electromagnetic force sometimes reaches dozens of times the normal value. Therefore, the end structure of the winding with the end painted and cured can easily cause insulation wear. This is a very common fault of AC motors. The reasons are:

① The amount of heat shrinkage of the end binding rope after heat curing is not enough, so it cannot tighten the ends of each phase winding after the paint is cured, so that the winding ends cannot effectively bind and fix the windings in the direction of force vibration. . Therefore, once the motor is subjected to alternating tangential force and axial force, the force point is added to the contact point between the winding and the end binding, causing vibration displacement. This is one of the main reasons for accelerated insulation wear there;

② Binding the ends when taking off the line causes damage to the end insulation crimping wire;

③The paint pouring and curing process at the end cannot guarantee the rigidity of the rope after curing;

④The gap in the slot increases, and the coil vibrates in the slot, causing the coil to wear at the slot.

1.4.5 Local repair of stator winding. If one or part of the motor winding is burned out, local maintenance should be considered. First of all, it is necessary to find out the exact fault point of the faulty coil and heat the motor winding to 100℃--130℃ to soften the winding insulation. Then remove the slot wedge and faulty coil while they are hot. When removing the old coil, be careful not to damage the good coil. . After the faulty coil is removed, a new coil is replaced, and then the slot wedge is driven in and the paint is dipped and dried.

 Motor core troubleshooting

2.1 The role of motor core

The iron core of the motor is not only a key component for electromagnetic energy conversion, but also withstands the combined effects of mechanical vibration, electromagnetic force, and heat in the motor. In order to reduce the eddy current loss in the iron core, a certain insulation resistance value is required between the iron core punching sheets.

2.2 Repair of common core faults

2.2.1 Individual teeth of the core tooth pressure plate open outward along the axial direction

This is because the teeth of the tooth pressure plate were not leveled when the iron cores were stacked. After the iron cores were pressed, the punching sheets appeared wavy, the iron core teeth were warped, or they hit the stator iron core when extracting the rotor. Pry up the individual teeth of the tooth pressure plate. During repair, you only need to use a copper rod to flatten the individual teeth that open outwards.

2.2.2 The iron core is partially burned out

The arc generated when a phase-to-phase short circuit or a ground short-circuit fault occurs in the winding, sometimes partially burns out the core. If the burnt area is not large, you can repair it without dismantling the iron core. First use files, chisels, grinding wheels and other tools to flatten or trim the partially burned areas of the iron core, and then use a knife to remove the silicon steel sheets piece by piece. Coat the surface of the iron core with a layer of self-drying insulating paint, and then press the iron core tightly. If the burnt area is large, the iron core should be replaced or the damaged punching sheet should be replaced.

2.2.3 Radial looseness of iron core

There are two situations when the iron core is radially loose:

①For small motors that use external press-fit core technology, the fit between the outer circle of the iron core and the inner hole of the base is too loose, or the positioning screws between the iron core and the base are loose. The repair method is to tighten the positioning screws. If necessary, you can add additional positioning screws to the machine base, or you can weld the iron core and the machine base;

② For large and medium-sized motors that use the internal pressure-installed iron core process, it is caused by the radial positioning device of the iron core being open or loose, or the welding between the iron core and the machine base being desoldered. The repair method is to tighten the positioning device and install anti-loosening screws or repair welding at the desoldered area.

 Troubleshooting of squirrel cage rotor

Broken bars are a common fault of squirrel-cage asynchronous motors. Most of them are caused by desoldering of the welding joint between the copper bar and the end ring. The desoldering joint can be cleaned and soldered firmly with silver solder.

In addition, there are often some defects in the rotating shaft. For example: the rotating shaft is bent; the keyway is worn, the bearing gear is worn, etc. Depending on the situation, these can be solved by welding, straightening, re-milling a keyway, plating a layer of chromium on the bearing files by electroplating or spraying.