For squirrel-cage asynchronous motors with cast aluminum rotors, the most common problem with the rotor is broken bars. Broken rotor bars will make it difficult to start the motor. When the motor is working, it will emit intense periodic electromagnetic noise and sensation, and the pointer of the three-phase ammeter will vibrate. The motor can only be reduced with load and the speed will drop. So master Rotor Repair The way is important! So how to check and repair the broken strip of the motor rotor?

The way to check the broken rotor bar is as follows:

1. Appearance view. Remove the rotor from the motor and observe carefully. If it is found that the iron core is overheated and discolored, it is the broken part. If no discoloration is found, carefully observe the cage bars for fine cracks. Sometimes the cracks cannot be found by this inspection, and the broken bars may occur in the rotor slots, which can be checked with a broken bar detector.

2. Check with the broken strip detector. The method is: put the broken bar detector on a cage bar, and then put a hacksaw blade on the cage bar. If the cage bar is intact, the broken bar detector will induce a current in the cage bar and cause the hacksaw blade to vibrate; if the saw blade does not vibrate, it means that the cage bar spanned by the circuit breaker detector has been cracked. Then look for cracks. If the crack cannot be seen with the naked eye, place the broken strip detector and hacksaw blade in place, and attach one end of a wire to the end ring and move the other end forward against the cage bar. When moving to a certain point, if the hacksaw blade vibrates, it means that the cracking point has been short-circuited by the wire, which is the cracking point.

3. Check with iron powder. Pass low-voltage and high-current on the two end rings of the rotor (so that it can absorb iron powder), and sprinkle the iron powder on the surface of the rotor. The current passes through the cage bars to generate magnetic lines of force, which will attract iron powder. If there is very little iron powder around a cage bar, it is a broken bar.

The repair methods for broken rotor bars are as follows:

1. Drill holes in the cracks, use a tap to screw up the threads, screw on the same material (aluminum) as the cage bars, and connect the cracks.

2. Aluminum should be cast from scratch when conditions permit. The rotor can be heated to about 700 ℃ with a coal furnace to melt the aluminum strip, remove the residual aluminum in the groove and the iron core, and then re-cast aluminum. When there is no cast aluminum condition, copper bars can also be used instead of aluminum bars.

3. Use copper bars instead of aluminum bars. The cross-sectional area of ​​the replaced copper strip should account for about 70% of the slot area (do not fill the slot, otherwise the starting torque will be small, and the current will increase, etc.), and the two ends are welded with a short-circuit ring. After the rotor is welded, a static balance test is performed. For high-speed motors, a dynamic balance experiment is also required.

The above are the methods of rotor repair and inspection when the motor fails. After mastering it well, we will no longer be afraid of the motor failure. Therefore, it is still necessary for us to understand some related methods of rotor repair, which will be used in our use. Helped a lot in the process!