1.4.2 Turn Insulation

The purpose of the turn insulation in both random- and form-wound stators is to prevent
shorts between the turns in a coil. If a turn short occurs, the shorted turn will appear as the
secondary winding of an autotransformer. If, for example, the winding has 100 turns between
the phase terminal and neutral (the “primary winding”), and if a dead short appears across
one turn (the “secondary”), then 100 times normal current will flow in the shorted turn. This
follows from the transformer law:
npIp = nsIs (1.1)
where n refers to the number of turns in the primary or secondary, and I is the current in the
primary or secondary. Consequently, a huge circulating current will flow in the faulted turn,
rapidly overheating it. Usually, this high current will be followed quickly by a ground fault
due to melted copper burning through any groundwall insulation. Clearly, effective turn insulation
is needed for long stator winding life.