Motor start & operation

Once upon a time:
A ventilation fan company needed a direct drive motor for a new highway tunnel. The state government specifications required that the motor must continue to run in the event of a fire for at least one hour (it was long ago, and it might have been two hours). The circumstances of which were that the tunnel would have doors at each end that would close at the onset of the fire. It was certain that some of the fuel was gasoline or diesel, but I don't remember if there were any others (ie: a propane truck or chemicals).

In any case, the specifics did require the temperature of the exhaust going through the fan and around the motor was at least 300°F (maybe even 350°). After that period, it was not necessary that the motor ever run again (like the rocket motor above).
Not only did the motor manufacturer have to consider winding and rotor temperatures, but that of the grease and bearings too.

The successful bidder had to demonstrate this capability by three live tests, for which the state was going to provide an old abandoned railroad tunnel.

It was decided not to bid for both technical and legal reasons. Who would want to be in highway tunnel in such a fire and have the doors closed at both ends??

I never did find out who, or how they solved that conundrum.

Consider the starter motor on an automobile. It is at ambient temperature, is expected to run for less than 30 seconds and run for hours and hours. They need to be cheap and small. A byproduct of that is that they get very, very hot in that short time. It amazes me to hear the people in a parking lot continuing to grind and grind away hoping to start their flooded engine.

Another interesting application: refineries have jet pump motors that start three to four times an hour, run for 5 minutes then shut off. 24 hours per day, 8000 hours per year. The rating is generally around 2000 Hp 3600 rpm and 4 kV. Understandably, despite these units having special windings and copper bar rotor design, have relatively short lives. At this rate, the API recommended value of 5000 starts would go by quickly.