When run from DC there are less losses because the current doesn't reverse there will be no hysteresis losses in the field coils and the eddy loss will also be lower.As far as AC or DC series wound universal motors generealy dont have any change in efficiency from one to the other. AC does tend to give some a hum or growl when under load. But at the same input voltage there is no real measurable difference.
I found that out for my self by removing the fan from a series wound vacuum cleaner motor. Luckily the motor wasn't too badly damaged, just one of the brushes suffered a bit of wear.CAREFUL WITH THE SERIES WOUND MOTOR - they tend to speed up and blow themselves to hell if you run them unloaded.
That may be true.My experience is they are usually parallel wound. Could it be they are series for 220V and parallel for 110V?
No, I just discovered I could power a 230V vacuum cleaner motor well off 48V, actually it was probably closer to 56V and it might not have been running at full power.Hero999, do you have a formula for relating the DC equivalent voltage of a universal motor to the AC rating? I looked around, couldn't find one, be a handy thing to know.