The working principle of DC Motor

.A direct current machine (DC machine) refers to a rotating motor capable of converting electrical energy from direct current into mechanical energy (as a DC motor) or converting mechanical energy into direct current electrical energy (as a DC generator). It is a type of motor that facilitates mutual conversion between electrical and mechanical energy. When operating as a motor, it functions as a DC motor, converting electrical energy into mechanical energy; when operating as a generator, it functions as a DC generator, converting mechanical energy into electrical energy.

 

Composition structure

The structure of a DC motor should consist of two main parts: the stator and the rotor. The stationary part of a DC motor during operation is called the stator. The main function of the stator is to generate a magnetic field, which is composed of the motor base, main magnetic poles, reversing poles, end caps, bearings, and brush devices. The part that rotates during operation is called the rotor, which mainly generates electromagnetic torque and induced electromotive force. It is the hub for energy conversion in DC motors, so it is usually called the armature, consisting of a shaft, armature core, armature winding, commutator, and fan.

 

Main magnetic pole

The function of the main magnetic pole is to generate an air gap magnetic field. The main magnetic pole consists of two parts: the main magnetic pole iron core and the excitation winding. The iron core is generally made of 0.5mm to 1.5mm thick silicon steel plates, which are pressed and riveted together. It is divided into two parts: the pole body and the pole shoes. The part on top of the excitation winding is called the pole body, and the part on the bottom that is widened is called the pole shoes. The pole shoes are wider than the pole body, which can adjust the distribution of magnetic field in the air gap and facilitate the fixation of the excitation winding. The excitation winding is made of insulated copper wire and wrapped around the main magnetic pole core. The entire main magnetic pole is fixed to the machine base with screws,

Reversing pole

The function of the reversing pole is to improve commutation and reduce the possible commutation sparks between the electric brush and the commutator during motor operation. It is generally installed between two adjacent main magnetic poles and consists of a reversing pole iron core and a reversing pole winding. The reversing pole winding is made of insulated wires wound around the reversing pole iron core, and the number of reversing poles is equal to the main magnetic poles.

 

Machine base

The outer shell of the motor stator is called the base. The function of the machine base is twofold:

One is used to fix the main magnetic pole, reversing pole, and end cover, and to support and fix the entire motor;

Secondly, the machine base itself is also a part of the magnetic circuit, which forms the magnetic path between the magnetic poles. The part through which the magnetic flux passes is called the magnetic yoke. To ensure that the machine base has sufficient mechanical strength and good magnetic conductivity, it is generally made of cast steel or welded from steel plates.

 

Electric brush device

Electric brush device is used to introduce or extract DC voltage and DC current. The electric brush device consists of an electric brush, a brush holder, a brush rod, and a brush rod holder. The electric brush is placed in the brush holder and compressed with a spring to ensure good sliding contact between the brush and the commutator. The brush holder is fixed on the brush rod, which is mounted on a circular brush rod seat and must be insulated from each other. The brush rod seat is installed on the end cover or bearing inner cover, and the circumferential position can be adjusted. After adjustment, it is fixed.

 

(1)Armature iron core

The armature core is the main part of the main magnetic circuit and is used to embed the armature winding. The general armature iron core is made of 0.5mm thick silicon steel sheets punched and laminated to reduce eddy current and hysteresis losses generated in the armature iron core during motor operation. The stacked iron core is fixed on the shaft or rotor bracket. The outer circle of the iron core has armature slots, in which armature windings are embedded.

(2)Armature winding

The function of the armature winding is to generate electromagnetic torque and induced electromotive force, and it is a key component for energy conversion in DC motors, so it is called the armature. It is composed of many coils (hereinafter referred to as components) connected according to certain rules. The coils are wound with high-strength enameled wire or glass fiber wrapped flat copper wire. The coil edges of different coils are divided into upper and lower layers and embedded in the armature slot. The insulation between the coil and the iron core, as well as between the upper and lower coil edges, must be properly maintained. To prevent centrifugal force from throwing the coil edge out of the slot, the slot is fixed with slot wedges. The end connection part of the coil extending out of the slot is tied with thermosetting non-woven glass tape.

 

(3)Commutator

In a DC motor, the commutator is equipped with electric brushes, which can convert the external DC power into alternating current in the armature coil, keeping the direction of electromagnetic torque constant; In a DC generator, the commutator is equipped with electric brushes, which can convert the alternating electromotive force induced in the armature coil into direct electromotive force drawn from positive and negative electric brushes. A commutator is a cylindrical body composed of many commutator segments, which are insulated with mica sheets between them.

 

(4)Rotating shaft

The shaft plays a supporting role in the rotation of the rotor and requires a certain degree of mechanical strength and stiffness. It is generally processed from round steel

Main Categories

 

(5)DC generator

A DC generator is a machine that converts mechanical energy into DC electrical energy. It is mainly used as the excitation power source for DC motors, electrolysis, electroplating, electric smelting, charging, and AC generators. Although power rectifier components are also used in places where direct current is needed to convert alternating current into direct current, in terms of certain performance aspects, alternating current rectifier power sources cannot completely replace direct current generators.

(6)DC motor

A rotating device that converts direct current energy into mechanical energy. The stator of the electric motor provides a magnetic field, the DC power supply provides current to the winding of the rotor, and the commutator keeps the direction of the rotor current and the torque generated by the magnetic field constant. DC motors can be divided into two categories based on whether they are equipped with commonly used brush commutators, including brushed DC motors and brushless DC motors.

Brushless DC motor is a new type of DC motor developed in recent years with the development of microprocessor technology, the application of new power electronic devices with high switching frequency and low power consumption, as well as the optimization of control methods and the emergence of low-cost, high magnetic energy level permanent magnet materials.

(7)Control Principle

The control principle of a brushless DC motor is that in order to make the motor rotate, the control unit must first determine the position of the motor rotor sensed by the hall sensor, and then decide the order of turning on (or off) the power transistors in the inverter according to the stator winding. The AH, BH, CH (these are called upper arm power transistors) and AL, BL, CL (these are called lower arm power transistors) in the inverter make the current flow through the motor coil in sequence to generate a forward (or reverse) rotating magnetic field, and interact with the magnet of the rotor, so as to make the motor rotate clockwise/counterclockwise. When the motor rotor rotates to the position where another set of signals is sensed by the hall sensor, the control unit turns on the next set of power transistors, so that the cyclic motor can continue to rotate in the same direction until the control unit decides to stop the motor rotor and turn off the power transistors (or only turn on the lower arm power transistors); If the motor rotor is to be reversed, the order of power transistor activation is reversed.

 

Basically, the opening method of power transistors can be exemplified as follows: AH, BL group → AH, CL group → BH, CL group → BH, AL group → CH, AL group → CH, BL group, but it must not be opened as AH, AL or BH, BL or CH, CL. In addition, because electronic components always have switch response time, the response time of the power transistor should be taken into account when switching off and on. Otherwise, when the upper arm (or lower arm) is not completely turned off, the lower arm (or upper arm) will already be turned on, resulting in a short circuit between the upper and lower arms and burning out the power transistor.

 

When the motor rotates, the control unit will compare the command composed of the speed and acceleration/deceleration rate set by the driver with the speed of the hall sensor signal change (or calculated by software) to determine whether the next set of switches (AH, BL or AH, CL or BH, CL or...) should be turned on and the duration of the turn on time. If the speed is not enough, it will be extended; if the speed is too high, it will be shortened. This part of the work is completed by PWM. PWM is the way to determine whether the motor speed is fast or slow, and how to generate such PWM is the core to achieve more precise speed control.

 

The high-speed control must consider whether the CLOCK resolution of the system is sufficient to grasp the processing time of software instructions. In addition, the data access method for hall sensor signal changes also affects processor performance, judgment accuracy, and real-time performance. As for low-speed speed control, especially low-speed starting, the changes in the hall sensor signal returned become slower. It is very important to extract the signal, process it at the right time, and configure the control parameter values appropriately according to the characteristics of the motor. Alternatively, the speed feedback can be adjusted based on the encoder changes to increase the signal resolution for better control. The motor can operate smoothly and respond well, and the appropriateness of P.I.D. control cannot be ignored. As mentioned earlier, the brushless DC motor is a closed-loop control, so the feedback signal is equivalent to telling the control department how much the motor speed is still different from the target speed, which is the error. Knowing the error naturally requires compensation, which can be achieved through traditional engineering controls such as P.I.D. control. However, the state and environment of control are actually complex and ever-changing. To achieve robust and durable control, factors that need to be considered may not be fully mastered by traditional engineering control. Therefore, fuzzy control, expert systems, and neural networks will also be included as important theories for intelligent P.I.D. control.