We rewound an ac motor but when we tested it,in a jog, one of the coil got burnt. We checked if there was a wrong connection but there was none. We checked also the number of turns,the size of wìre. We rewound the burnt coil,and test again,the same thing happened.We're wondering why it was happened?.Please help. . .
The basic reason is too much current, so need to find the cause of that. I have had experience with a motor that drew far too much current on no load, and continually burned out, and it turned out the field lamination were skewed so that the field was not properly aligned magnetically. This did not cause poling, but the problem was visible when the field was inspected. This was a manufacturing defect. Other reasons may be shorted turns, or the laminations are not well insulated, as each is supposed to be insulated from the next. I expect you have already checked for poling and that the shaft does not have excess play. I don't know what the effects of a bad rotor are. I guess the winding causing the problem can be tested and compared with others on the bench, without the rotor. Apply a low voltage from a transformer to the winding and measure the current, comparing with other identical windings. When you say tested in a jog: To me jogging is pulsing the motor to step it along a bit. Could this be grossly exceeding the current rating? I guess a particular coil would tend to burn out first, but the others would be hot too. In the end the rated current should not be exceeded for more than a few seconds, then wait for it to cool down. Some larger motors have starters that prevent the restart until it has cooled down. It is also possible that a supply phase is bad, possibly due to a faulty contactor or connection that has high resistance..
