I thought is was worth clarifying what I interpretted that you were first ignoring and then downplaying (the role of motor size in the discussion). As a wise man once said “Is not part of that putting up warning flags when something seems out of the ordinary”.
Sorry if somehow that came across in a bad way.
Moving on, I will state my personal opinion: Size of the motor is probably the most important single factor in selecting the “size” (operating watts) of the heater. As far as factors related to localized heating: it can be addressed by addressing the configuration of the heater in a number of ways you have mentioned: heaters operated at half voltage to reduce surface temperature, heaters standing off a safe distance from anything delicate or combustible, or just purchasing the large surface area heaters that have low max surface temperapture even when operated at their own nameplate voltage. But once this factor is addressed through selection of appropriate configuration (and however it is addressed), the heater sizing is still based on the size of the motor and other physical/geometric/environmental factors which affect the heat transfer problem of keeping the winding warm.
Just for general discussion.
Attached Slide 1 is one example selected because it is similar size motor to the one discussed here:
800hp, 900rpm, 4kv vertical outdoor open/wp2 induction motor.
Contains 6 heaters, each with rating of 240vac, 500W.
The heaters are hooked to ~120vac, so the operating watts is 6*(500/4) = 750 operating watts
We confirm by clamp-on periodically that the current is approx 6 amps, which corresponds to 720 watts at 120vac
Attached I have winding temperature traces (Yellow/Blow/Green) and ambient temperature in purple.
Daily variations in winding temperature are larger when running than while secured. This suggest that while secured, thermal time constant is longer than while running. Due to long thermal time constant of secured motor, it may not “keep up” during rapid temperature increases of warm front and therefore may go below ambient temp. In my neck of the woods, warm front is often humid front near 100% humidity where dewpoint temp ~ ambient temp. So if space heater allows motor winding temp to drop below ambient, then it is also below dewpoint. This illustrates the point “Condensation typically occurs with rising ambient temperatures”. Is it acceptable... probably a matter of opinion. The average winding temp is roughly 9F = 5C above ambient which I think was a target stated in an old IEEE standard.
Another case, slide 2.
3500 hp, 324rpm, 13.2kv vertical outdoor motor open/wp2.
24 space heaters with nameplate rating 240vac, 750 watts.
The heaters are connected in delta.. each delta leg has two parallels, with four series heaters per parallel.
Each space heater sees 480/4 = 120vac = half nameplate voltage, and has an output 750/4 = 187.5 watts. Total operating wattage: about 4,500 watts. It measures about 6 amps at 480vac, which confirms this operating wattage. It is even higher than one watt per horsepower.... but at 324rpm the physical size of this motor is much larger than we’d guess from the horespower (weight is about 50,000 pounds). Based on slide 2, this massive amount of space heaters does a better job at keeping motor temperature above ambient than the first motor. You might even argue we have more space heaters than we need. But at 13.2kv, I don’t mind the extra margin and it doesn’t hurt anything (other than space heater operating cost which is miniscule compared to cost of motor failure).
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(2B)+(2B)' ?
