The structure principle of DC motor

DC motor is a motor that converts DC electrical energy into mechanical energy. Because of its good speed regulation performance, it is widely used in electric drives.
(1) Structure
The DC motor is mainly composed of a frame, an armature, a main magnetic pole, a commutation magnetic pole, a commutator, a brush holder, an end cover, a fan, and an outlet box, as shown in Figure 1. The structure diagram is shown in Figure 2. Among them, the stationary part is called the stator; the rotating part is called the armature or rotor.

The structure principle of DC motor
Figure 1 Structure of DC motor
The structure principle of DC motor
Figure 2 Schematic diagram of the structure of a DC motor

① Stator. The stator is composed of a base, a main magnetic pole, an excitation winding, a terminal benefit and a brush device.
a. Base. The base is used to fix the main magnetic pole, brush holder and end cover and other parts of the base, supporting and protecting, and together with the main magnetic pole core, yoke, and armature core to form the magnetic circuit of the motor, the magnetic flux passes through the entire magnetic circuit The situation is shown by the dotted line in Figure 3. The base is made of cast iron, cast steel or steel plate.

The structure principle of DC motor
Figure 3 Main magnetic pole flux

b. Main magnetic pole. The role of the main pole is to generate an air gap magnetic field. The main magnetic pole is composed of the main magnetic pole core and the excitation winding. The iron core is generally made of 0.5~1.5mm thick silicon steel plate punched and laminated, and is divided into two parts: the pole body and the pole palm. The part on which the excitation winding is fixed is called the pole body, and the widened part below is called the pole palm. , The pole palm is wider than the pole body, which can not only adjust the distribution of the magnetic field in the air gap, but also facilitate the fixation of the excitation winding. The field winding is made of insulated copper wire and sleeved on the core of the main magnetic pole. The entire main magnetic pole is fixed on the base with screws, as shown in Figure 4.

The structure principle of DC motor
Figure 4 Schematic diagram of the main magnetic pole structure

c. Electric brush device. The brush device is used to introduce or draw DC voltage and DC current. It is composed of a brush holder, a brush, a compression spring, and a copper wire braid, as shown in Figure 5. The brush is placed in the brush holder and compressed with a spring to make a good sliding contact between the brush and the commutator. The brush box is fixed on the brush rod, and the brush rod is installed on the circular brush rod seat, which must be insulated from each other. Often several brush boxes are mounted on the same insulated brush rod. In the circuit connection, the brush boxes on the same insulated brush rod are connected in parallel, which is called a group of brushes. In a general DC motor, the number of brush groups can be represented by the number of brush rods, and the number of brush rods is equal to the number of main magnetic poles of the motor.

The structure principle of DC motor
Figure 5 The structure of the brush device

The brush rods are evenly distributed along the circumferential direction on the outer surface of the commutator. During normal operation, the brush rod has a correct position relative to the surface of the commutator. If the position of the brush rod is unreasonable, it will directly affect the performance of the motor. The brush rod seat is installed on the end cover or the inner cover of the bearing, and the circumferential position can be adjusted and fixed after adjustment. The brush device is shown in Figure 6.

The structure principle of DC motor
Figure 6 Brush device diagram

② Rotor. The rotor (armature) of the DC motor is mainly composed of armature core and armature winding, commutator, shaft and fan, and its structure is shown in Figure 7.

The structure principle of DC motor
Figure 7 The structure of the armature

a. Armature core. The armature core has two functions: one is as the main part of the main magnetic circuit; the other is the embedded discharge armature winding. Since the relative movement between the armature core and the main magnetic field will cause eddy current loss and hysteresis loss in the core (the two parts of the loss together are called core loss, referred to as “iron loss”), in order to reduce the iron loss, the armature core It is usually laminated with a 0.5mm thick silicon steel sheet coated with insulating varnish and fixed on the shaft. The armature core has evenly distributed slots along the circumference, in which the armature winding can be embedded, as shown in Figure 8.

The structure principle of DC motor
Figure 8 The structure of the armature core

b. Armature winding. The armature winding is composed of many coils arranged and connected according to a certain rule. It is the main circuit part of a DC motor and is a key component that generates electromotive force through induction after electrification to achieve electromechanical energy conversion.
The coil is made of round and rectangular cross-section wires wrapped with insulating materials, also called “components”, and each component has two outlet ends. The armature coil is embedded in the slot of the armature core, and the two outlet ends of each element are connected with the commutator segments of the commutator in a certain rule to form an armature winding, as shown in Figure 9.

The structure principle of DC motor
Figure 9 The armature winding is connected to the commutator

c. Commutator. The commutator converts the direct current passing through the brush into an alternating current in the winding. The commutator is installed on the rotating shaft and has an interference fit with the rotating shaft. The commutator is mainly composed of many commutating segments, which are insulated with mica between the segments, and the number of commutating segments is equal to the number of coils, as shown in Figure 10.

The structure principle of DC motor
Figure 10 The structure of the commutator

(2) Working principle
DC motors are made using the basic principle that energized conductors are subjected to force in a magnetic field.
The physical model of a DC motor is shown in Figure 11. In the figure, N and S are stator poles, and abcd is a coil fixed on a rotatable magnetic permeable cylinder. The coil and the permeable cylinder are called the rotor or armature of the motor. The first and end ends a and d of the coil are connected to two commutating segments that are insulated from each other and can rotate with the coil. The connection between the rotor coil and the external circuit is carried out through fixed brushes placed on the commutator segment.

The structure principle of DC motor
Figure 11 The physical configuration of a DC motor

The brush A.B is connected to a DC power supply, so a current flows in the coil abcd, and the direction of the current is shown in Figure 11. According to the law of electromagnetic force, the electromagnetic force F received on the current-carrying conductor ab and cd is
Where: B—the air gap magnetic density where the conductor is located, Wb/㎡:
l—The length of conductor ab or cd, m
I—current in the conductor, A
The direction of the conductor’s force is determined by the left-hand rule, the direction of the conductor ab is from right to left, and the direction of the conductor cd is from left to right (see Figure 11). This pair of electromagnetic forces forms a torque acting on the armature. This torque is called electromagnetic torque in a rotating electric machine. The direction of the torque is counterclockwise, trying to make the armature rotate in the counterclockwise direction. If the resistance torque on the armature (such as resistance torque caused by friction and other load torques) can be repaired according to the electric breaking torque, when the armature rotates 180°, the conductor cd turns to the N pole, and the conductor When ab turns to S-level, because the direction of the current supplied by the DC power supply remains unchanged, the current flows in from brush A, and flows out from brush B after passing through conductors cd and ab. At this time, the force direction of conductor cd changes from Right to left. The direction of the force on the conductor ab changes to the electromagnetic torque generated from left to right is still counterclockwise, so once the armature rotates, due to the converter cooperates with the brush to convert the current, the DC current is alternately transferred from the conductor ab With the inflow of sum cd, as long as the coil side is under the N pole, the direction of the overcurrent is always the direction in which the brush A flows in, and when it is under the S pole, it always flows out from the direction of the brush B. This ensures that the current in the coil side of each pole is always in one direction, thereby forming a torque with a constant direction, so that the motor can continuously rotate.

(3) Classification
According to the excitation mode, DC motors are divided into three types: permanent magnet, separate excitation and self-excitation. Among them, self-excitation is divided into three types: parallel excitation, series excitation and compound excitation. Shunt-excited DC motors and series-excited DC motors are commonly used in electric vehicles.
① Shunt-excited DC motor. The field winding of this motor is connected in parallel with the armature winding, as shown in Figure 12, and the field winding is called a shunt winding. Since the shunt winding bears all the voltages at both ends of the armature, its voltage value is relatively high. In order to reduce its copper loss, the shunt winding must have a larger resistance to reduce the excitation current. Therefore, the shunt winding has more turns and is wound with a thinner wire.

The structure principle of DC motor
Figure 12 The electrical principle of a shunt-excited DC motor

Shunt-excited DC motors were widely used in early electric vehicles, such as Isuzu ELF/Resort, Daihatsu Hijet Van, Suzuki Alto, etc.
②Series-excited DC motor. The field winding of this motor is connected in series with the armature winding, as shown in Figure 13, and the field winding is called a series winding. In order to reduce its voltage drop and copper loss, the series winding should have a smaller resistance. Therefore, it is always wound with a wire with a larger cross-sectional area, and the number of turns is small.

The structure principle of DC motor
Figure 13 The electrical principle of a series-excited DC motor

The series-excited DC motor is a motor used in early electric vehicles. The torque is larger at low speed, and the excitation becomes weaker at high speed. The motor can be controlled by torque only by using armature control, which is very simple. Since the torque characteristic of this motor is similar to the transmission output characteristic of a vehicle using an engine, approximately equivalent driving comfort can be obtained. But because its speed range is too small, it must have a transmission device. For example, the Subaru Samber EV Classic electric car uses a constant-torque series-excited DC motor with a rated power of 25kW as the drive motor.

(4) Starting, speed regulation and reverse rotation
① Start. Connecting the circuit of the stationary motor to the power supply causes the rotating part of the motor to rotate, and finally achieves normal operation, which is called the starting of the motor. If you don’t use any starting equipment but connect the motor directly to the power supply, this starting method is called direct starting, and its starting current is very large. When the motor is just connected to the power supply, since the armature has not yet rotated, the back electromotive force is equal to zero. At this time, the current through the armature (that is, the starting current) should be
In the formula: Iq—starting current, A;
U—starting voltage, V;
Ef-reaction electric potential, V;
Rs—The internal resistance of the armature, Ω.
Because the internal resistance of the armature is very small and the applied voltage is the rated value, the armature current when the motor is directly started will be more than ten times, or even hundreds of times, larger than the rated current. Such a large current will cause strong sparks on the commutator, which may burn the commutator. Therefore, when the DC motor is started, a starting varistor must be connected in series in the armature circuit to reduce the starting current, as shown in Figure 14. In order to obtain a larger starting torque without damaging the commutator, the starting current is usually limited to 1.5 to 2.5 times the rated current of the armature.

The structure principle of DC motor
Figure 14 Starting principle of series starting rheostat

During the starting process, as the motor speed increases, the armature current gradually decreases, and the starting resistance should also gradually decrease. When the motor speed reaches the rated value, the starting resistance should be reduced to zero.
In addition, when starting, the magnetic field rheostat Rs in the excitation circuit should be placed at the position with the smallest resistance to maximize the magnetic flux. In this way, the motor can generate a large enough starting torque and increase the back-EMF. Fast to shorten the starting process.
② Speed ​​regulation. There are three ways to adjust the speed of a shunt-excited DC motor as follows.
a. Change the voltage U of the power supply line. This method has a wide range of speed regulation, but it must have a dedicated DC power supply. Adjustable voltage can be obtained by using a motor/generator set and a silicon controlled rectifier circuit.
b. Change the voltage drop of the armature circuit. Connecting a variable-speed rheostat Rq in series in the armature circuit can reduce the voltage applied to the armature, as shown in Figure 15. If the resistance value of the Rq varistor is increased, the voltage drop of the resistor will increase and the speed will decrease. This method is not economical because of the large armature current, which makes the variable-speed rheostat itself consume a large amount of power.

The structure principle of DC motor
Figure 15 The principle of speed regulation of series speed regulation rheostat

c. Change the magnetic pole flux. Connect a magnetic field rheostat in series in the excitation circuit to adjust the motor speed, as shown in Figure 16. If the resistance value of the magnetic field rheostat is increased, the excitation current decreases, the magnetic flux also decreases, and the motor speed increases. Generally, the current in the excitation circuit is very small, and the energy loss of the magnetic field rheostat during the speed adjustment process is also small, which is more economical. Therefore, this speed adjustment method is widely used in the power system.
If the series-excited motor also adopts the method of changing the energy to adjust the speed, the magnetic field rheostat must be connected in parallel with the series-excited winding, as shown in Figure 17. When the resistance value of the field transformer is reduced, the current through the varistor increases, and the current through the series winding decreases, the magnetic flux generated by it also decreases, and the speed increases.

The structure principle of DC motor
Figure 16 The principle of the series magnetic field rheostat to adjust the speed
The structure principle of DC motor
Figure 17 Principle of speed regulation by parallel magnetic field rheostat

③ Reverse. The direction of rotation of the motor is determined by the force direction of the conductor of the armature winding in the magnetic field. Changing the direction of the armature current or changing the direction of the field current can make the DC motor reverse. Specific method: reversely connect both ends of the armature connected to the power supply, or reversely connect both ends of the field winding. If the directions of the two currents are changed at the same time, the direction of rotation remains unchanged.

(5) Features
①Good speed regulation performance. The DC motor can realize uniform and smooth stepless speed regulation under heavy load conditions, and the speed regulation range is wide.
②Large starting torque. DC motors can achieve speed regulation evenly and economically. Therefore, any machinery that starts under heavy loads or requires uniform speed regulation, such as electric cars, can be driven by DC motors.
③The control is relatively simple. The DC motor is generally controlled by a chopper, which has the advantages of high efficiency, flexible control, light weight, small size, and fast response.
④There are vulnerable parts. Due to the existence of wear-prone components such as brushes and commutators in DC motors, regular maintenance or replacement must be carried out.