ELECTRIC MACHINES

Many of you may have heard about rewinding a motor. It is necessary to rewind a motor when its winding get defective. It happen in many ways. But the common fault is burning of winding due to insulation failure or overload. Here I am trying to explain the rewinding process of a slip-ring induction motor. This is a 435kW induction motor rotor. This is the rotor that we are going to rewind. All the coils are removed from the slots and it is cleaned. After that two coats of red varnish is applied. This is to improve the insulation resistance value and to protect core from corrosion. After all the above process the rotor looks as above. Now it has a pleasing look. Now the next part start. The above picture shows the winding overhang support. It is insulated using Fibre Glass tape. This has to be before starting rewinding. Now we are going to make the coils. Here we are using annealed copper strips of electrical grade for making rotor coils. These strips in various size comes in spools and

Here is a 3 phase alternator stator winding. This winding was burnt due to overload in one phase. We can clearly see all coils of a phase winding is overheated and blackened. Normally this type of winding failure do not get warranty repair.

Here is an another example of burnt winding due to Insulation Failure. If you look at the first picture the winding failure will not be clear. The portion where the winding failed is marked here. Next picture is a closeup view of that portion. Here winding failure is clearly visible.

Sometimes it is very easy to find the cause of winding failure. Here this picture shows an alternator stator winding which is burnt due to the insulation failure of separation. Two nearby phase coils are separated using an insulation paper depending the class of insulation. If the IR value of that paper goes below a minimum value there will be a winding failure. It can happen in slot also. Here the insulation failure is at coupling side winding overhang. Next picture shows a close-up image of the winding were it failed.

This picture is of a defective stator winding of welding alternator. The stator winding was failed due to insulation failure. This picture shows the bunt coils at connection side of winding.

This Picture is of a new motor. But due to mishandling it's body was broken. We can clearly see the stator core and some portion of stator winding.