Quote Originally Posted by adamscb View Post
Jraef, I was under the impression as the temperature of a conductive material increases, the resistance also increases? Or am I ass-backwards?
No, that is true of the CONDUCTORS, they have a Positive Temperature Coefficient (PTC) of Resistance, so the resistance increases as temperature increases. But at the same time, extreme excesses in core temperature can affect the magnetic permeability of the steel laminations (sometimes permanently) and since the overall inrush current is a combination of wire resistance AND impedance, which is affected by the magnetic flux in the core, the overall effect can show up as a slightly Negative Temperature Coefficient of Resistance, especially as motor sizes go up. When I was involved in making DC Injection Brakes, we discovered this (the hard way). A motor is PTC up to and including normal operating temperatures, but if it OVER heats, it starts to become NTC. We blew up quite a few DCIB power devices over that little nugget of knowledge... Turned out we should have read more about it first.

I realize now after re-reading my earlier post that I did say the "resistance" would decrease, so that was incorrectly worded. I was just thinking of the inrush increasing, which would be because impedance can decrease if it is over heated.