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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.
The AC/DC universal motor is found in portable tools such as electric drills, saws, sanders, etc., and in home appliances such as vacuum cleaners, electric mixers, blenders, etc., where high speed, power and small size are an advantage. However, it is closer in concept to the DC motor than to the AC motor and, therefore, has some inherent disadvantages, which could be avoided in purely AC induction motors; chiefly, the need for commutation and brushes. The universal motor is basically a series DC motor which is specially designed to operate on AC as well as on DC. A standard DC series motor has very poor characteristics when operated on AC, mainly due to two reasons: a) The high reactance of both the armature and field windings limits AC current to a much lower value than DC current (for the same line voltage). b) If solid steel is used for the stator frame, AC flux will produce large eddy currents in the frame with consequent heating. To insure satisfactory operation of the universal motor from an AC power source, some modifications are necessary. The reactance of the series field and armature windings must be reduced as much as practicable. The reactance of the series field winding can be somewhat reduced by using fewer turns of heavier wire. However, it would not be practical to eliminate the reactance voltage drop due to the series field since that would also eliminate the magnetic field. The reactance voltage drop due to the armature winding can be practically eliminated by use of a compensating winding. The compensating winding is connected in series with the armature winding (conductive compensation) and arranged such that the ampere-turns of the compensating winding oppose and neutralize the ampere-turns of the armature. To realize this compensation, the compensating winding is displaced by90 electrical degrees from the field winding. Since the motor used in this experiment us a4-pole motor, the mechanical displacement is450 . The compensating winding also improves commutation considerably. This is a great adventure since the field of a universal motor is weakened by lowering the reactance of the series field winding. If the compensating winding is short circuited (inductive compensation), the alternating currents in the armature are induced by transformer action into the shorted compensating winding, thus, effectively cancelling the reactive armature currents. To reduce losses due to hysteresis and eddy currents, the field structure is laminated. Few universal motors operate at the same speed on AC as on DC. Whether it runs faster on AC or DC is a matter of design.
