What is a universal motor and why can it operate on both AC and DC?

A universal motor is a series-wound motor that operates on both AC and DC supply. In a series motor, armature and field currents are the same and reverse together, so torque direction remains constant regardless of current direction. On AC, both field and armature currents reverse simultaneously each half-cycle, producing unidirectional torque. Universal motors have high speed, high power-to-weight ratio, and are used in hand tools, vacuum cleaners, and appliances. They require commutator maintenance.