Direct current machine refers to a rotating machine that can convert DC electric energy into mechanical energy (DC motor) or mechanical energy into DC electric energy (DC generator).It is a motor that can realize the conversion between DC electric energy and mechanical energy.When it operates as a motor, it is a DC motor, which converts electrical energy into mechanical energy;When operating as a generator, it is a DC generator, which converts mechanical energy into electrical energy.[1]
The structure of DC motor shall beStator and rotorIt consists of two major parts.The part that is stationary when the DC motor is running is calledstatorThe main function of the stator is to generatemagnetic field, from the engine base, mainmagnetic pole, commutator, end cap, bearing andbrushDevice, etc.The part that rotates during operation is calledrotor, its main function is to produceelectromagnetic torque And induced electromotive force are the hubs of energy conversion for DC motors, so they are usually calledarmature, composed of rotating shaft, armature iron coreArmature winding、commutatorAnd fan.
stator
(1) Main magnetic pole
DC motor (Figure 1)
The main magnetic pole is used to generateAir gap magnetic field。The main magnetic pole is composed of the main magnetic pole iron core andExcitation windingIt consists of two parts.The iron core is generally formed by laminating and riveting 0.5mm-1.5mm thick silicon steel plates, and is divided into two parts: pole body and pole shoe. The part covered with excitation winding above is called pole body, and the part widened below is called pole shoe. The pole shoe is wider than the pole body, which can adjust the distribution of magnetic field in the air gap and fix the excitation winding.The excitation winding is made of insulated copper wire and is sheathed on the core of the main magnetic pole.For the whole main polescrewFixed on the base,
(2) Commutator pole
The role of the reversing pole is to improve the commutation and reduce the commutation spark that may occur between the brush and the commutator when the motor is running. It is generally installed between two adjacent main poleswindingform.For reversing pole windingInsulated conductorIt is wound and sheathed on the iron core of the commutating pole. The number of commutating poles is equal to that of the main magnetic pole.
(3) Stand
The housing of the motor stator is called the frame.The base has two functions:
One is used to fix the main magnetic pole, reversing pole and end cover, and to support and fix the whole motor;
Second, the seat itself is alsomagnetic circuitA part of, which forms the magnetic path between the magnetic poles,fluxThe part that passes through is called the yoke.In order to ensure that the base has sufficient mechanical strength and good magnetic conductivity, it is generally made of steel castings or welded by steel plates.
(4) Brush gear
The brush device is used to introduce or lead out DC voltage andDC currentOf.The brush device is composed of a brush, a brush holder, a brush rod, a brush rod seat, etc.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 holder is fixed on the brush rod. The brush rod is installed on the 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. The circumferential position can be adjusted and fixed after adjustment.
rotor
(1) Armature core
DC motor (Figure 2)
The armature core is the main part of the main magnetic circuit, and it is also used to embed the armature winding.Generally, the armature iron core is formed by laminating the punched pieces made of 0.5mm thick silicon steel sheets to reduce the eddy current loss and hysteresis loss generated in the armature iron core during motor operation.The stacked iron core is fixed on the rotating shaft or rotor support.The outer circle of the iron core is provided with an armature slot in which the armature winding is embedded.
(2) Armature winding
The function of armature winding is to generate electromagnetic torque and induced electromotive force, which is the energy conversion of DC motorKey components, so it is called armature.It is composed of manycoil(hereinafter referred to as element) is connected according to certain rules, and the coil is of high strengthEnameled wireOr glass fiber wrapped flat copper wire is wound. The coil edges of different coils are embedded in the armature slot in two layers. The coil and iron core as well as the upper and lower coil edges must be properly insulated.To prevent the coil edge from being thrown out of the slot by centrifugal force, the slot is fixed with a slot wedge.The termination part of the coil protruding out of the slot shall be bound with thermosetting weft free glass tape.
(3) Commutator
DC motor (Figure 3)
In the DC motor, the commutator is equipped with electric brush, which canDC power supplyIt is converted into the alternating current in the armature coil, so that the direction of the electromagnetic torque is constant;stayDC generatorThe commutator is equipped with electric brush, which can change the alternating current induced in the armature coilemfIt is converted into DC electromotive force from positive and negative brushes.Commutator is a cylinder composed of many commutating plates, which are insulated by mica sheets.
(4) Rotating shaft
The rotating shaft plays a supporting role in rotor rotation and needs a certain mechanical strength and stiffness. It is generally made of round steel.
Main categories
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DC generator
DC motor (Figure 6)
The DC generator ismechanical energyConverted into DC electric energymachine。It is mainly used as DC motor, electrolysis, electroplating, electric smelting, charging andalternatorDC motor required for excitation power supply, etc.Although in places where DC power is neededpowerRectifier element, handlealternating currentConverted to DC, but from some aspects of working performance, the AC rectifier power supply can not completely replace the DC generator.
DC motor
A rotating device that converts direct current energy into mechanical energy.The motor stator provides the magnetic field, the DC power supply provides the current to the rotor winding, and the commutator keeps the direction of the rotor current and the torque generated by the magnetic field unchanged.According to whether there is a common brush commutator, DC motors can be divided into two categories, including brushless DC motors and brushless DC motors.
DC motor (Figure 7)
Brushless DC motor is a new type of DC motor developed in recent years with the development of microprocessor technology and 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.
Brushless DC motors not only maintain the good speed regulation performance of traditional DC motors, but also have the advantages of no sliding contact and reversing spark, high reliability, long service life and low noise. Therefore, they are widely used in aerospace, CNC machine tools, robots, electric vehicles, computer peripherals and household appliances.
According to different power supply modes, brushless DC motors can be divided into two categories: square wave brushless DC motors, whose back EMF waveform and supply current waveform are rectangular waves, also known as rectangular wave permanent magnet synchronous motors;The back EMF waveform and the supply current waveform of the sine wave brushless DC motor are sine waves.
Excitation mode
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Excitation mode refers to the mode of generating magnetic field in rotating motor. There are four excitation modes for DC motor:
Separately excited DC motor
The excitation winding has no connection with the armature winding, and the DC motor powered by other DC power supplies to the excitation winding is called separately excited DC motor,Permanent magnet DC motorIt can also be regarded as separately excited or self-excited DC motor, which is generally called permanent magnet excitation mode.
Shunt excited DC motor
The excitation winding of the shunt excited DC motor is in parallel with the armature winding. As a shunt excited generator, it is the end of the motor itselfVoltagePower supply for excitation winding;As a shunt wound motor, the excitation winding and armature share the samePower SupplyIt is the same as the separately excited DC motor in terms of performance.
Series excited DC motor
The excitation winding of the series excited DC motor is connected in series with the armature winding, and then connected to the DC power supply.The excitation current of this DC motor isArmature current。
Compound excitation DC motor
Compound excitation DC motorThere are two excitation windings, parallel excitation and series excitation.ifSeries excited windingThe generated magnetic flux potential is compared withShunt windingThe same direction of the generated magnetic flux potential is called product compound excitation.If two magnetic fluxes are in opposite directions, it is called differential compound excitation.
DC motors with different excitation modes have different characteristics.Generally, the main excitation modes of DC motor are parallel excitation, series excitation and compound excitation, and the main excitation modes of DC generator are separate excitation, parallel excitation and compound excitation.[2]
Model designation
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DC motor (Figure 4)
The model of domestic motor is generally in EnglishChinese Phonetic AlphabetThe format is: the first part uses capital alphabetic letters to represent the product code, the second part uses Arabic numerals to represent the design serial number, the third part uses Arabic numerals to represent the frame code, and the fourth part uses Arabic numerals to represent the armature core length code.
working principle
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A ring permanent magnet is fixed inside the DC motor, and the current generates an ampere force through the coil on the rotor. When the coil on the rotor is parallel to the magnetic field, the direction of the magnetic field that continues to rotate will change. Therefore, the brush at the end of the rotor contacts with the conversion plate alternately, so the current direction on the coil also changes, and the direction of the Lorentz force generated will not change,So the motor can rotate in one direction.
DC motor (Figure 5)
The working principle of the DC generator is to change the AC electromotive force induced in the armature coil into DC electromotive force when it is led out from the brush end by the commutator with the commutation effect of the brush.
The direction of induced electromotive force isRight-handed ruleConfirm (the magnetic induction line points to the palm of the hand, the thumb points to the direction of the conductor movement, and the other four fingers point to the direction of the induced electromotive force in the conductor).
The force direction of conductor is determined by the left hand rule.This pairElectromagnetic forceIt forms a torque acting on the armature, which is called electromagnetic torque in the rotating motor. The direction of the torque is counterclockwise, trying to make the armature rotate counterclockwise.If this electromagnetic torque can overcome the resistance torque on the armature (such as the resistance torque caused by friction and other load torque), the armature can rotate counterclockwise.
Control principle
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For the control principle of DC brushless motor, in order to make the motor rotate, the control unit must first determine the order of opening (or closing) the power transistors in the inverter according to the position of the motor rotor sensed by the hall sensor, and then determine the order of opening (or closing) the power transistors in the inverter according to the stator winding. AH, BH, CH (these are called upper arm power transistors) and ALBL, CL (these are called lower arm power transistors) make the current flow through the motor coil in sequence to generate a forward (or reverse) rotating magnetic field, and interact with the rotor magnet, so that the motor can rotate clockwise/counterclockwise.When the motor rotor rotates to the position where another group of signals is sensed by the hall sensor, the control unit will turn on the next group of power transistors, so that the circulating 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);To reverse the motor rotor, the power transistor is turned on in reverse order.
Basically, the power transistor can be opened as follows: AH, BL group → AH, CL group → BH, CL group → BH, AL group → CH, AL group → CH, BL group, but it can never be opened as AH, AL or BH, BL or CH, CL.In addition, because the electronic parts always have the response time of the switch, the response time of the parts should be taken into account when the power transistor turns off and turns on. Otherwise, when the upper arm (or lower arm) is not completely turned off, the lower arm (or upper arm) has been turned on, resulting in the short circuit of the upper and lower arms and burning of the power transistor.
When the motor rotates, the control department will compare the command composed of the speed set by the driver and the acceleration/deceleration rate with the speed of the hall sensor signal change (or calculate by software), and then decide whether the next group of switches (AH, BL or AH, CL or BH, CL or...) will be turned on and how long the switch will be turned on.If the speed is not enough, the opening will be longer, and if the speed is too high, the opening will be shorter. This part of the work will be completed by PWM.PWM is the way to determine whether the motor speed is fast or slow. How to generate such PWM is the core to achieve more accurate speed control.
High speed speed control must consider whether the CLOCK resolution of the system is enough to grasp the time to process software instructions. In addition, the data access mode for the change of the hall sensor signal also affects the processor efficiency, correctness of judgment and real-time performance.As for speed control at low speed, especially for low speed starting, because the returned hall sensor signal changes more slowly, how to capture the signal mode, processing time and properly configure the control parameter values according to the motor characteristics are very important.Or the change of speed return is based on the change of encoder to increase the signal resolution for better control.The motor can run smoothly and respond well, and the proper control of P.I.D. cannot be ignored.As mentioned earlier, the DC brushless motor is controlled in a closed loop, so the feedback signal is equal to telling the control unit how far the motor speed is from the target speed, which is error.If you know the error, you will naturally have to compensate it, in the way of traditional engineering control, such as P1. D. Control.However, the control state and environment are complex and changeable. If the control is robust, the factors to be considered may not be completely mastered by traditional engineering control, so fuzzy control, expert system and neural network will also be incorporated into the intelligent P1. D. Important theory of control.
