Question

Why are universal motor performs better under dc operation then is ac operation?

Answer 1

Universal motor is nothing but a DC series motor. In a DC series motor, both Armature and field windings are in series. On application of DC, it works on the principles of DC motor. On application of AC supply, which alternates in both directions, it still works by delivering the torque in one direction. This happens because, when applied voltage reverses, both armature and field windings of DC series motor gets reversed and so the torque direction remains same.

Please note that motor works with reduced efficiency on AC supply