When the squirrel cage rotor breaks, it is usually checked by the following method.
(1) Visual inspection
The actual maintenance experience shows that the fracture point of the cast aluminum squirrel cage rotor occurs mostly near the center of the axial length of the groove, while the fracture point of the copper bar rotor occurs mostly at the joint between the cage and the end ring. The point is mostly in the cage. At this point, take out the squirrel cage rotor, check the surface of the iron core, if there is discoloration or overheating and crack at the intersection of the cage strip and the end ring, it means that the broken strip is there.
(2) Replace the rotor check
Although it is simple and easy to replace the rotor, it is necessary to replace the good rotor of the same model for trial operation. If the motor runs back to normal after replacing the rotor, it indicates that there is a broken bar in the original squirrel cage rotor.
(3) Power check
A low-voltage current of about 10% of the rated voltage is applied to the stator winding, and an ammeter is inserted in one phase, and the rotor is slowly rotated by hand. At this time, if the reading of the ammeter is unstable, the cyclical fluctuation of the size indicates the rotor. There is a broken bar.
(4) Iron powder inspection
During the inspection, the switch S is closed, and the voltage regulating transformer is adjusted to gradually increase the voltage from zero. When the current of the current riser is gradually increased, a magnetic field is generated on the surface of the rotor, and then the iron powder is sprinkled on the rotor. If the iron powder is arranged neatly along the direction of the cage, it indicates that the rotor is intact. If the iron powder is arranged in a disorderly manner, or the iron powder cannot be sprinkled, it indicates that the rotor has a broken strip.
The wiring diagram when inspecting the rotor with iron powder is shown in Figure 1-9.
(5) Mutual sensor inspection
It is based on the principle of transformers and consists of two open cores.
The coil 1 is connected to the power source, and the iron core 1 and the iron core 2 form a closed magnetic circuit, and part of the magnetic flux is interlinked to the iron core 2. When the cage in the measured tank is intact, the current flows through the cage, forming a short-circuit coil corresponding to the core 2, which will prevent the magnetic flux from passing through the core 2, so that the induced potential of the detector coil 2 is very high. small. When moving the iron core 2 to the broken strip slot, which is equivalent to the short-circuit coil open circuit, the magnetic flux passing through the iron core 2 is increased, and the reading of the millivoltmeter is increased, indicating that the cage strip is broken.
(6) Machine inspection
A low-voltage power of 50 to 60 V is applied to the three-phase winding of the stator, so that the rotor cannot rotate, and then the rotor is slowly rotated by hand. If the three-phase current is unbalanced, its current changes with the angle of rotation, indicating that the rotor has a broken strip.
If the copper bar rotor fails, it should be repaired according to different damage conditions.
(1) Repair of partial welding of copper strips and end rings
Use a trowel to remove the old solder joints and oxide layers from the faulty part, and use a sharp chisel around the weld to clean the groove and clean it with 30% sulfuric acid solution. Then use multiple torches to heat the end rings at the same time. When the temperature reaches about 400 °C, use a torch to heat the part to be welded. When the temperature rises to 800 °C, 45% of the silver brazing filler metal can be wetted and filled with welds to cure the rear ring. The joint with the copper strip is welded firmly.
(2) Repair of a few copper strips outside the slot
When a small amount of copper strip breaks outside the tank, the end ring weld of the longer part of the copper strip should be heated first. After the flux is melted, the copper strip is punched with a hammer, and then the short copper strip is removed by heating. If there is a convex round copper strip in the straight portion, the copper strip at the end of the end ring welding hole is milled by a milling machine, and then the copper strip is removed by heating. After the broken copper strip is punched out, remove the debris in the tank, and then insert the copper strip with the same material and geometrical dimensions as the old copper strip into the groove, and then weld it firmly.
(3) Repair of a few copper strip breaks in the tank
A gap is formed in the ring at both ends of the broken strip, and the broken copper strip is chiseled with a chisel, and then replaced with a new copper strip having the same cross section as the copper strip. The ends of the new copper strip protrude from the end ring at about 15mm, and they are bent tightly against the end ring and welded firmly. The notch of the end ring is brazed, and its stacking height should be slightly higher than the end ring surface. After the end ring surface is leveled on the lathe, the rotor static balance can be calibrated.
(4) Repair of a large number of copper strip breaks
When a large number of copper bars are broken, use 4 to 6 oxygen-acetylene flame torches to heat the end rings. When all the welds are melted, use the special tools for removing the end rings shown in Figure 1-11 to remove the end rings.
Clean the end of the copper strip, pull out all the copper strips in the tank, and clean the tank. Then use a sledgehammer to push the copper strips with soft metal on the port into the groove, so that the tightness is appropriate, and the length of the copper bars protruding from the core should be equal. Then insert the end ring according to the original record, the matching gap between the copper strip and the end ring hole is about 0.1mm, and finally it is welded firmly and repaired.
(5) Repair of end ring break
When the end ring breaks, the following repair methods are usually used.
1 The area of the fracture is appropriately enlarged with a chisel, and then the portion is heated to about 450°, and then welded with 63% tin, 33% zinc, and 4% aluminum melted solder. If the area of the fracture is large and cannot be repaired, it can only be recast.
2 Heat the rotor to 650-700 ° C. When all the end rings are melted, pour them all out. After that, the slot is cleaned, and a copper rod slightly longer than the rotor core and similar in shape and slot is inserted into the slot, and then the copper rod is bent out of the core portion, and the sides are overlapped and welded by gas welding. The whole body becomes the end ring, and the end ring can be leveled on the lathe.
When the cast aluminum rotor is broken, the corresponding treatment measures should be taken according to the different fracture conditions of the rotor.
1 When there is a crack in the aluminum guide bar or the aluminum end ring, the crack can be cut out from the groove or the trapezoidal groove on both sides, and then repaired by argon arc welding.
2 When there is a crack in the aluminum guide bar or the aluminum end ring, the crack can be cut out with a sharp chisel and heated to 400-500 ° C with a blowtorch, followed by 63% tin, 33% zinc and 4% aluminum. The welded electrode can be filled by gas welding to fill the crack.
3 For small motors, the broken cage can be drilled with a long drill bit, then the residual aluminum in the tank is cleaned, and then a new aluminum strip of the same diameter is inserted, welded with the end ring, and the weld is filled.
If the cast aluminum rotor breaks severely, the new aluminum rotor is replaced in principle. The method for solving the severe fracture failure of the aluminum rotor on the spot is mostly to change the cast aluminum rotor to the copper strip rotor by the caustic soda aluminum method.
