Coils, yet again

[marthur]

As I was thinking about coil failures, EI and ballast resistors, I had a question that I will throw out to the group.

1. If the primary issue with early coil failure is overheating, what is the safe temperature range for the typical oil filled coil? I am thinking about the temperature of the external can as measured by one of those point and shoot remote sensing thermometers.

2. And could we achieve the same increase in reliability by cooling the coil? No idea about how to do this easier than reducing input voltage, but still curious.

[ndutton]

It's interesting that many newer automotive alternators are water cooled and whoo boy are they ever pricey. I don't know if it applies to coils as well but in an earlier discussion with Lewco Electric about low RPM performance of alternators and insufficient cooling they suggested 200 degrees was about the point where the insulation on the internal windings begins to deteriorate and the death spiral begins.

I'm on a borrowed computer this morning and therefore away from my resources so I'm unable to attach a graph of coil heat measurements I made during the coil work. It's in one of the coil threads of a couple of years ago. If memory serves, I think my external coil temperature measured with the non-contact IR thermometer you mentioned reached around 130 degrees after 30 minutes of run time. This was after adding an external ballast resistor sized according to the recommendation. Previous temperatures before the resistor addition approached 170 degrees in the same time span. According to the graph the temperatures had not topped out at the 30 minute mark.

Before resistor
coil = 3.3 ohms, voltage ~ 14 volts, amps = 4.24 amps

Post resistor
Coil + resistor = 4.2 ohms, voltage ~ 14 volts, amps = 3.3 amps