The effect of winding stresses on the pulse endurance of corona resistant magnet wire

Past studies have indicated reliability advantages of using corona resistant magnet wire (CRMW) for insulation systems in motors operated with pulse width modulated (PWM) adjustable speed drives. However, twisted pair testing of CRMW after the application of mechanical stress indicates that these life advantages of CRMW rapidly diminish, where a 60 - 70% reduction in life is observed for each 9% elongation stress. This contrasts with an observed reduction of only ∼25% for standard wire. Hence, it is important to understand how stresses induced in this wire during winding, insertion, and end turn blocking operations can affect magnet wire pulse resistance performance. To this end, pulse endurance life testing of magnet wire twisted pairs was compared with pulse endurance testing of a wound stator model. These tests indicate that the winding stresses observed for stators can be simulated by inducing a mandrel pull stress in the magnet wire prior to forming a twisted pair. The use of this correlation can not only improve the studies of winding stresses on CRMW, but also assess the impact of varnish treatment on winding life. These studies will facilitate further improvement in the insulation system reliability.