The problem with all these temp specs and sensors is that there is little to no information about WHERE the measurement is taken. Is it ambient air temp, ie. the temp of the cooling air being drawn into the alternator? Or is it the rectifier temp, and if so, measured where? At one of the output terminals, such as where balmar's temp sensor attaches? Or something else? Or is it the case temp, and if so, where on the case? By the bearings to guard against cooking out the bearing grease? Or is it the stator temp to guard against damage to the winding insulation? It's anyone's guess.
And there is zero info on the relationship between any of the temp measurement points when the alternator is operating. If we knew the relationship between the various measurement points, then we could translate from one to the other.
And last, there is zero info available about duty cycle for alternators. They list an output value, but what does that mean? What we are all finding is that it's actually a short-term peak or surge output value, but it can't be sustained for more than a handful of minutes before the alternator overheats, or at least until we think it over heats because after all, we really don't know what constitutes overheating, other than when it goes up in smoke.
The few temp specs I have found for alternators I 'think' are for the engine compartment ambient. But I haven't seen it stated explicitly. I have asked alternator manufacturers about max allowed temps, and where they are measured, and at best received wishy washy answers. The fact that anyone is asking seems to be a new thing for them. But when you have a regulator with a temp probe, you kind of need to know where to place it, and what temp to expect or allow before de-rating the output.
So all this leaves us with only empirical knowledge. Here's what I think we know:
- Balmar's temp prob attaches to the power post on the back of the alternator. As such, it's mostly measuring the rectifier temp. That's reasonable, but important to know when to set a temp limit.
- With the possible exception of "large frame" (something that is really undefined), and "school bus" alternators (another thing that is undefined), alternators can only output full rated amperage for minutes. How many minutes depends on the alternator, and you can only guess or observe how long that will be.
- There is a huge difference in the temperature of a rectifier and a stator, with the stator being much hotter. I have personally seen a stator running at 325F and rectifier at around 140F. Is that alternator just fine, or massively overheating? Well, it all depends on where you measure, and what that particular temp should be. I ultimately got word back from Prestolie that it was "just fine", but it sure had me concerned for quite a while.
- Measuring the stator temp can be elusive. For a long time I couldn't resolve why at sustained full output some people were measuring cool "case" temps (150F range) while others like me were measuring high temps (300F + range), all for similarly rated alternators in the 200A, 24V range, made by the same manufacturer. I finally figured it out. There are different case designs from the same manufacturer, one with an outer case around the stator, and one with the stator directly exposed. So we were measuring different things.
- You are on your own to figure out what your alternators continuous duty rating is. Have fun.
