Can you pass the whole of the extracted air over the motor to cool it?
Why should this cause a problem?

A series resistance (such as a high wattage filament light bulb or a heater
element) MAY produce the desired results.

One method that does wok is to use a vacuum cleaner with a series wound AC
motor (ie none of these new fangled Dyson type thing-gummies) with a series
resistor,

- Or a vacuum cleaner on lower voltage - plus a series resistor

as current will tend to increase with lowered voltage and lower speed. )

Series wound motors work on AC or DC.
I have seen a long ago application where a 230 VAC 50 Hz vacuum cleaner WAS
operated instead on 50VDC with a series resistor to act as a ventillator to
remove Hydrogen Sulphide gas from small telephone exchanges.
An H2S filter was added in the VC dust bag space as a bonus.
H2S kills in a different manner to Radon but much more quickly.
Above a certain concentration the sense of smell is disabled by the gas,
thus hiding the usual characteristic "rotten eggs" smell,  so impending
disaster is not noticed.
In our Rotorua thermal region people very occasionally die from this cause.

          Russell




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