A couple of days ago we learned about the working principle of asynchronous motors . To recap, an asynchronous motor works by having a rotating magnetic field continuously cut the closed conductors in the rotor, generating induced electromotive force and induced current. The interaction between the induced current in the rotor and the rotating magnetic field then produces electromagnetic torque, causing the rotor to rotate in the same direction as the rotating magnetic field.
Yesterday we discussed synchronous motors, specifically permanent magnet synchronous servo motors. Today, let's talk about DC motors. We'll mainly share three aspects of DC motors: their basic structure, working principle, and classification information.
DC motors are motors that operate on DC voltage and are widely used in radios, video recorders, DVD players, electric shavers, hair dryers, electronic watches, toys, and more. The basic principle of a DC motor is based on electromagnetic induction, and like an asynchronous motor, it consists of a stator and a rotor.
I. Basic Structure of DC Motors
A DC motor consists of two parts: a stator and a rotor, with a certain air gap between them.
The stator of a DC motor consists of components such as the frame, main magnetic poles, commutating magnetic poles, front and rear end covers, and brush holders. Among them, the main magnetic poles are the main components that generate the air gap magnetic field of the DC motor, and are composed of permanent magnets or laminated iron cores with DC excitation windings.
The rotor of a DC motor consists of components such as the armature, commutator (also known as a rectifier), and shaft. The armature consists of two parts: the armature core and the armature winding. The armature core is made of stacked silicon steel sheets with evenly distributed slots on its outer circumference, and the armature winding is embedded in these slots.
A commutator is a mechanical rectification component. It consists of commutator segments stacked into a cylindrical shape and then molded as a single unit using metal clamps or plastic. The commutator segments are insulated from each other. The quality of the commutator has a significant impact on its operational reliability.
Basic structure diagram of a DC motor
II. Working Principle of DC Motors
Working principle of DC motor
As shown in the figure, direct current flows from brush B through the semi-circular commutator into the coil (armature winding) cdba. According to the left-hand rule, the force situation of the coil segments cd and ab is shown in Figure F, generating a clockwise torque T. The coil rotates clockwise at a speed of n. Regardless of the rotation of the coil, the current direction on the effective side of the S pole is always inward, and the current direction on the effective side of the N pole is outward. After the motor armature winding is energized, it continues to rotate clockwise.
As the coil rotates continuously, although the coil sides in contact with the two brushes change constantly, brush A remains at a positive potential, and brush B remains at a negative potential. When segment ab of the coil is at the N pole, the current direction is cdba; when segment ab is at the S pole, the current direction is abdc. In other words, the current direction in coil abcd constantly alternates. These two semi-circular copper segments are called commutator segments, and together they form a commutator.
III. Classification of DC Motors
A DC motor consists of stator poles, a rotor (armature), a commutator, brushes, a housing, and bearings. The stator poles (main poles) of an electromagnetic DC motor consist of an iron core and excitation windings. Based on their excitation (formerly called magnetization) method, DC motors can be further classified into series-wound DC motors, shunt-wound DC motors, separately excited DC motors, and compound-wound DC motors. Due to the different excitation methods, the magnetic flux through the stator poles (generated by energizing the excitation coils of the stator poles) also differs.
In a series-wound DC motor, the excitation winding and rotor winding are connected in series via brushes and a commutator. The excitation current is proportional to the armature current, and the stator flux increases with the increase of the excitation current. The torque is approximately proportional to the square of the armature current, and the speed decreases rapidly with the increase of torque or current. Its starting torque can reach more than 5 times the rated torque, and its short-term overload torque can reach more than 4 times the rated torque. The speed variation rate is large, and the no-load speed is very high (generally, it is not allowed to operate under no-load conditions). Speed regulation can be achieved by connecting an external resistor in series (or in parallel) with the series winding, or by switching the parallel connection of the series winding.
In a shunt-wound DC motor, the excitation winding is connected in parallel with the rotor winding. Its excitation current is relatively constant, and the starting torque is proportional to the armature current, which is approximately 2.5 times the rated current. The speed decreases slightly with increasing current and torque, and the short-term overload torque is 1.5 times the rated torque. The speed variation rate is small, ranging from 5% to 15%. Speed can be adjusted by using constant power to weaken the magnetic field.
In a separately excited DC motor, the excitation winding is powered by an independent excitation power supply, and its excitation current is relatively constant. The starting torque is proportional to the armature current. The speed variation is also 5%~15%. The speed can be increased by weakening the magnetic field and maintaining constant power, or the speed can be decreased by reducing the voltage of the rotor winding.
In addition to the shunt winding, the stator poles of a compound-wound DC motor also have a series winding (with fewer turns) connected in series with the rotor winding. The magnetic flux generated by the series winding is in the same direction as that of the main winding. The starting torque is approximately four times the rated torque, and the short-term overload torque is approximately 3.5 times the rated torque. The speed variation rate is 25%~30% (related to the series winding). The speed can be adjusted by weakening the magnetic field strength.
