What does a higher insulation class buy you? You can run higher temperatures inside the generator head and still maintain a decent service life... that's what.
Common STs have insulation as follows:
Stator insulation is class 'E', the Rotor is class B.
Note: Class E is a European spec, 120 C or 248 F maximum temp.
Their approach to service life and longevity is building a unit with a massive frame, large ventilation openings, big fan, and wiring that is done in a way where the windings could be pulled and re-done in the field. The windings are not all glued in and sealed up, this means they don't have the highest insulation class. This appears to be part of their philosophy, a generator that can be rebuilt in the field if necessary.
What they've given up in this design is light weight, and compact size. The ST head should be the last head on your list if you plan to lug it around by hand every day. But it should run cooler than a compact unit by a whole bunch requiring less of an insulation class. I would also suggest you find another head for use outdoors in the weather, even though they claim the enclosure is 'drip proof' if you're planning to leave it outside in the rain, take the generator to a rewind shop and pay them to raise the insulation class.
Whatever head you buy, "no matter who makes it", remember the cooler you run it, the longer it will last. Never block the ventilation, if you install it in a shed, make sure it has adequate ventilation. I have heard stories of 10 kW ST heads making 13 or 14kw for a number of hours, seems to me this could shorten it's life. You should consider installing an amp meter if you are going to drive it with an engine that will allow it to be overdriven to this degree.
A last thought on this insulation class issue, I have a Honda EM5000, the engine never fails to start on the 1st or 2nd pull, but the head doesn't make juice anymore. The field winding has less resistance than it should. The cost of correcting this problem (according to the service center) is greater than the value of the generator.
I was reading a page from the ST "0peration and maintenance book". (Troubles and their elimination) section. I quote:
Problem: Open or short may occur in harmonic winding Solution: "replace winding" and they mean it!
Let's face it, the Honda is lighter, and far better suited to dragging around..., but mine might as well be a boat anchor...., if it was an ST , even I could fix it.
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