Issue 100: Control Principle of Hengyu Guoqiang DC Retarder Motor
Time：2018-11-24 | Source：恒宇国强 | Browse：5415859
We know that motor is generally divided into motor and generator. Motor is a device that converts electrical energy into mechanical energy, and generator is a device that converts mechanical energy into electrical energy. The operation principle of motor is based on Faraday's law of electromagnetic induction. In the process of energy conversion, the motor must have two parts which can make relative motion
The components that establish the excitation field,
An induced component that induces an electromotive force and flows through a working current.
In these two parts, the static one is called the stator, and the rotating one is called the rotor. The electromagnetic torque is generated by the interaction between the exciting magnetic field in the air gap and the magnetic field established by the current in the induced part. Through the action of electromagnetic torque, the generator absorbs mechanical power from the mechanical system, and the motor outputs mechanical power to the mechanical system. Different ways of establishing the above two magnetic fields form different kinds of motors. Today we are going to talk about the DC motor which is commonly used in maker field, DIY production of science and technology or some conventional projects. It is used to be called DC motor or DC motor.
Internal structure of DC motor
Its fixed part (stator) is equipped with a pair of static main magnetic poles N and s excited by DC, and its rotating part (rotor) is equipped with armature core. There is an air gap between stator and rotor. An armature coil connected by two conductors A and X is placed on the armature core. The head end and end end of the coil are respectively connected to two arc-shaped copper sheets, which are called commutating sheet. The commutators are insulated from each other, and the whole composed of commutators is called commutator. The commutator is fixed on the shaft, and the commutator and the shaft are insulated from each other. A pair of fixed brushes B1 and B2 are placed on the commutator. When the armature rotates, the armature coil is connected with the external circuit through the commutator and the brush.
Operation principle of DC motor
If the use of single-chip microcomputer to control the rated input voltage of 5V motor, there is such a phenomenon: to the two wires of the motor directly connected to the power supply, the motor will move up, and then the two wires connected in reverse, the motor will reverse. Why?
If DC power is added to the two brushes, DC current will flow in from brush a, through coil ABCD and out from brush B. according to the knowledge of electromagnetic induction, current carrying conductors AB and CD will be affected by Ampere force, and their direction can be determined by the left-hand rule. The force on the two conductors forms a torque, which makes the rotor rotate counterclockwise. Note that the external power supply is DC, but due to the effect of the brush and commutator, the current flowing through the coil is AC, and the direction of the torque generated is unchanged. If the direction of rotation is defined as positive, the reversal when we adjust VCC and GND can be well explained. In fact, the direction of Ampere force on the conductor rod is different.
There are so many simple principles.
For DC motor, in fact, we use the controller to control it. In fact, we only care about two things:
How to adjust the speed?
How to control the direction of rotation?
Now let's talk about it in detail.
Speed regulation and direction of DC motor
Direction control is one of the core points of motor control. In order to realize forward and reverse rotation, it is impossible for us to exchange VCC and GND frequently, which is certainly unrealistic. Is there a circuit to help us realize this function? The answer is yes. This typical motor drive circuit is called "H-bridge" drive circuit. It can not only control the rotation direction of the motor, but also adjust the speed. There is another reason. We know that the IO driving ability of ordinary single-chip microcomputer is insufficient. It may be OK to drive LEDs and buzzers, but not motors. In order to drive high-power loads, we have to use some other electronic components, which are the most important switch transistor in the hardware field, In order to achieve the purpose of small current control large current. However, the driving ability of the triode is still relatively weak, and the maximum current allowed is only 800mA, which is only suitable for driving some toy cars. If you want to drive a higher power motor, you need to replace the triode with a MOS transistor with a higher working current.
We know that the motor speed control using PWM technology, that is, pulse width modulation technology, PWM pulse signal after the general optocoupler isolation, drive field effect transistor or triode, and then drive the motor. The motor receives pulse voltage. Because the inductance of the motor has filtering effect, the effective voltage of the motor is the average value of the pulse voltage. Changing the pulse width changes the average value of the voltage of the motor, thus adjusting the speed of the motor.
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