Working Principle of Universal motor : It is modified DC series motor which can work on AC as well as DC supply. According to basic motor principle, whenever a current carrying conductor is placed in magnetic field, force is exerted on it. In Universal motor, when supply is given, the current flows through both field and armature winding. The magnetic field is produced by the field current. The current carrying armature conductors are in this magnetic field, so there is production of force or torque on the armature conductors and motor rotates. The force is directly proportional to the product of main flux and armature current.
F (or torque) α (flux)x(Armature current)
They have nearly same operating characteristic for both AC and DC supply.