Is there any advantage in running a universal motor on DC power?
I was told in the 1970s that the Universial   motor runs better due to less sparking at the brushes .I tried this on a vacuum cleaner motor and a sewing machine motor and found that it was true .Bridge rectifiers suitable for mains were starting to become cheap .A Si bridge rectifier on 230VAC mains wastes about 1% so it is plausible that you are better off with DC .I did not use a filter cap because I was concerned about peak charging currents and did not want to give the motor 315VDC .The electro caps in my junk box in 1976 were not big enough being more suited for valve radios.These days you could do a simple boost PFC stage giving the motor fairly pure DC which can be bucked down for speed control .When the barefoot universial motor is stalled on its AC mains supply the resistance and the reactance of the windings add vectorialy to      limit the current .When the stalled motor is given DC it is just the resistance that is limiting current .This increased stall current issue makes the motor less idiot proof on DC unless some other form of current limiting is employed .The universial motors that I have mucked around with are two pole .This means that operation below about 3000rpm from a theory viewpoint wont be as smooth .I found that controlled DC gave much better low speed operation than the cheap orthodox triac controlled AC .