Tech Tip: Coil Movement Issues

During startup and operation of an electric motor, the forces on the coils of an electric motor move up and down and to the sides in the slots, are pushed and pulled in and out of the slots, flex as the coils leave the slots and move in virtually all directions in the ends of the coils. In all electric motors, the movement of the coils must be controlled while allowing for flexibility in order to avoiding fracturing the insulation system. In low voltage, mush wound machines, the paper insulation, varnish (which acts as glue), and tying tape are used to reduce the movement of the coil. In form-wound coils, the tapes, blocking, surge rings, varnish and tie tapes are used to reduce movement of the coils.

When machines are not properly secured, such as when some manufactures and repair shops skimp on insulation systems, or do not tie every slot, the movement of the conductors and coils eventually wear away the turn insulation. If additional stresses of contaminants, including abrasives and chemicals which may attack the insulation system, surges and partial discharge occur, the windings will short between conductors eventually. In other cases, if the windings age and become brittle, a poorly secured winding will generate small fractures in the insulation system in which moisture and other contaminants will penetrate and then generate pressures based upon the electrical and magnetic fields in those areas. Eventually, the insulation system will either fail between conductors or to ground. A 1983 study of motor failures indicates that over 85% of winding failures occur between conductors, and not to ground.

With electrical signature analysis (motor current signature analysis) coil movement can be detected and monitored during operation. When the machine is running, there should be no movement with coil movement signatures indicating a poorly secured winding. The signature is the speed (in hertz) times the number of slots in the stator plus and minus the line frequency.

The sign of coil movement does not necessarily indicate that the winding will fail in the near future, but that the winding is weak. In some cases, this may be an issue in the original design of the machine. However, if the signature did not exist before a machine repair, then it is an indicator of motor repair defects due to either a misunderstanding of the forces in the machine or lack of insulation material.