From University of Michigan found this:
Universal Motor Construction
Universal motors can operate on either AC or DC power. The rotor has coils that are connected to an external circuit through a commutator on the shaft, as shown in the schematic at right.
Note that the field in the rotor will oppose the field in the stator at any given moment, regardless of whether or not the polarity of the supply current changes with time. This feature gives the universal motor its unique characteristic.
The price of versatility is efficiency; universal motors are not as efficient as similarly-constructed AC and DC series motors
