Electromagnetic DC motor consists of a stator pole and rotor (armature), commutator (commonly known as the commutator, brush, casing, bearing),
The stator pole (main magnetic pole) of an electromagnetic DC motor consists of an iron core and an excitation winding. According to the difference of excitation (old standard called excitation), it can be divided into series excited DC motor, shunt excited DC motor, separately excited DC motor and compound DC motor. Because of the different excitation modes, the law of stator magnetic pole flux (generated by the excitation coil of the stator pole) is different.
Between the DC motor excitation windings and rotor windings are connected in series through brush and commutator, the excitation current and the armature current is proportional to the stator flux increases with the increase of excitation current, armature current and torque approximately proportional to the square of the speed with the torque or current decreased quickly. Its starting torque can reach more than 5 times of the rated torque, the overload torque can reach 4 times of rated torque, the speed change rate is large, the no-load speed is very high (generally not allowed to run under no-load). By using external resistors and series excitation windings are connected in series (or parallel), or will the series field winding in parallel to achieve speed.
The excitation current of parallel excited DC motor is parallel to the rotor winding, and the excitation current is constant. The starting torque is proportional to the armature current, and the starting current is about 2.5 times of the rated current. The speed is slightly decreased with the increase of the current and torque, and the short-time overload torque is 1.5 times of the rated torque. The speed change rate is small, which is 5%~15%. The constant power can be regulated by weakening the magnetic field.
The exciting winding of the separately excited DC motor is supplied by the independent excitation power supply, and the excitation current is also constant, and the starting torque is proportional to the armature current. The speed change is also 5%~15%. The rotor speed can be reduced by reducing the constant power of the magnetic field to increase the speed or by reducing the voltage of the rotor winding.
On the stator pole of the compound excitation DC motor, besides the shunt winding, a series of winding connected with the rotor winding (less turns) is also installed. The flux direction of the series winding is the same as the flux direction of the main winding, the starting torque is about 4 times of the rated torque, and the short time overload torque is about 3.5 times of the rated torque. The speed change rate is 25%~30% (related to series winding). The speed can be adjusted by weakening the magnetic field intensity.
The commutator of the commutator is made of silver, copper, cadmium, copper and other alloy materials, molded by high strength plastic. The brush contacts with the commutator to provide armature current for the rotor winding. The brush of electromagnetic DC motor is usually made of metal graphite brush or electro graphite brush. The core of the rotor is made of silicon steel lamination, generally 12 slots, embedded with 12 sets of armature windings, each winding is connected in series, and then connected with 12 pieces of commutator piece.