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I think the best thing is to make up a voltage regulator, use a LM 315K in a TO-3 case, with this you can adjust the voltage, if you input is AC you will need to rectify it.
There is a remarkable lack of information on the internet about this electric motor. Mabuchi HP C9000-60005 It does appear to be a 12-18v motor but that is all I can find, no rpm, amps etc. If I was reading 38v does that mean the rpm was to high and it needs to spin slower?Art