Universal Motor is a type of motor in which the stator coils (i.e. excitation windings) and the rotor coils (i.e. armature windings) are connected in series to the power supply through brushes and commutator. Single-phase series-excited motors are commonly known as series-excited motors or general-purpose motors.

Since the basic structure of the Universal Motor is similar to that of a DC series-excited motor, but can be used in AC, the principle of generating rotating torque can also be expressed in terms of the DC motor principle. A DC voltage is applied to the carbon brushes A and B. The current in the winding flows in from the carbon brushes A, through the conductor under the N-pole, through the conductor under the S-pole, and out from the carbon brushes B. The magnetic field is applied to the conductor under the N-pole and out from the S-pole. As the magnetic field interacts with the magnetic field generated by the current in the conductor, the winding is forced to move. If the conductor is transferred to another polarity magnetic field, the direction of the current in the winding is changed accordingly through the action of brushes and commutator, i.e., the direction of the current in the conductor located under the magnetic field of a certain polarity remains unchanged, so that the direction of the force on the winding is unchanged. Therefore, the torque produced by the winding is always unidirectional.

As Universal Motor, in the case of AC power supply, the stator current and rotor current will change at the same time, that is to say, the stator flux and armature current will change at the same time, and according to the left hand rule of motor, it can be seen that, in the case that the flux and armature current change direction at the same time, the rotor steering remains unchanged. Although the polarity of the power supply changes repeatedly, the rotor always maintains a constant steering, which is the working principle of the series excitation motor.

Single-phase series excitation motor mainly consists of stator, armature, bracket, brush grip and commutator.

Stator: The stator is the mechanical support of the whole motor, which is used to generate the main magnetic flux, and it mainly consists of stator core and excitation winding.

Armature: The armature, also known as the rotor, is the rotating part of the motor and is the most important part of the motor. The role of the rotor is to generate electromagnetic torque, so as to convert electrical energy into mechanical energy, by the rotor shaft, iron core, armature winding, commutator and cooling fan.

Commutator: The commutator consists of a number of trapezoidal commutator plates surrounded by each other. Commutator and commutator piece with commutator piece of general insulation with mica, commutator piece of the lower two ends of the half-moon slot in the pressed glass fibre phenolic plastic, so that the commutator can constitute a whole.

Carbon brush holder: Carbon brush holder consists of chassis, brush grip, spring and carbon brush.