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Principle and structure of DC motor

Views: 1     Author: mcmaster     Publish Time: 2023-05-05      Origin: https://www.mcmaster-electric.com


DC motors feature a wide speed range, smooth speed regulation characteristics, high overload capacity, and large starting and braking torque. Why?

First of all, we need to know how a DC motor works?

According to the law of electromagnetism: a current-carrying conductor in a magnetic field is subjected to electromagnetic force, the direction of which is determined by the left-hand rule, i.e., the magnetic lines of force pass vertically through the palm, the direction of the four fingers point in the direction of the current (or the direction of positive charge movement), and the direction of the thumb is the direction of the electromagnetic force.


Our goal is to make the rotor of a DC motor turn, and a single conductor in a constant magnetic field cannot achieve this, so we add another conductor to the magnetic field and an opposite current flows, so that we get the torque we want.


Can a coil made of two conductors rotate all the time with an applied DC power supply?


According to Ampere's law in the energized solenoid: hold the energized solenoid with the right hand, so that the four fingers bend in the same direction as the current, then the end pointed by the thumb is the N pole of the energized solenoid.


When the coil is moved by electromagnetic force to a specific position, the torque is zero, according to the electromagnet same-sex repulsion, opposite-sex attraction, at this time the coil will not turn again. If you want the coil to rotate continuously, you must switch the wires at this point. In order to facilitate switching, DC motor adds commutator to armature, commutator and armature winding are welded together and rotate together with armature, commutator is insulated from each other, and carbon brush and external DC power supply are used to form a circuit. The purpose of commutation is achieved, thus realizing the continuous rotation of the motor rotor.


When a turn of coil movement to the vertical magnetic wire, this time the torque is zero, if you use this DC motor, the motor movement will be very unstable, how to solve?


The solution to this problem is to increase the number of coils and commutators. When one turn of the coil moves to the position of the perpendicular magnetic wire before the carbon brush disengages from the commutator of the original coil and touches the commutator of the other coil, the new coil forms a current circuit and creates torque. Therefore, the greater the number of coils and commutators, the smoother the motor rotation.


After understanding the principle of DC motor rotation, let's see how the actual motor structure looks like. What is the role of each of them?


The actual motor mainly consists of main pole, base, armature core, armature winding, commutator and carbon brush.


The main pole: to generate the air gap magnetic field, its structure is the main core jacket excitation winding (centralized winding), because the N and S poles in the motor can only appear in pairs, so the number of poles of the main pole must be an even number, and to alternate polarity in a uniform arrangement along the inner circle of the chassis, the polarity of the two adjacent poles opposite to each other is called a pair of poles.


Base: In addition to supporting the role of fixed, the main body is also part of the magnetic flux path between the poles.


Commutator and brush: Simply put, they are used to switch the wires.


Armature core: used to form the flux path and embed the armature winding. In order to reduce eddy current losses, the armature core is generally made of laminated silicon steel sheets coated with insulating paint 0.35mm to 0.5mm thick.


Armature winding: The core component that passes current and generates electromagnetic force or electromagnetic torque, enabling the motor to achieve electromechanical energy conversion. The armature winding consists of several coils wound with insulated wires. Each coil is welded to the commutator with certain laws. Reliable insulation is required between conductor and conductor, between coil and coil, and between coil and core; the armature winding structure is much more complex than the excitation winding, so we will focus on the analysis.


Why DC motor armature winding must form a closed circuit? Why should DC motor be made into double winding? How should the carbon brushes of DC motor be arranged? To solve these questions, it is natural to understand the method of armature winding.


Earlier we initially understood the composition of the DC motor structure, DC motor armature winding is not connected to the external circuit by a fixed point, but by the commutator plus carbon brush structure and the external power supply connected, so in the motor rotation process of each branch components in constant change. In order to make each branch potential and current stable and constant, the winding must be closed. Combined with the DC motor rotation conditions, the winding of the armature winding resistance must meet these requirements.


A relatively simple winding method that comes to mind is to wind the armature winding directly on the armature core, called a ring armature winding. Although the ring armature winding meets the requirements to get the motor turning, it has obvious disadvantages. Since the magnetic resistance of the armature core is much smaller than that of the rotor shaft and air, the magnetic lines of force almost always flow through the armature core, and the part of the conductor inside the armature core only plays the role of connection, which does not produce electromagnetic torque, and the conductor utilization rate is low. And the armature core to be connected to the shaft, so the internal part of the conductor is also not easy to place.


Since the conductor can not be placed inside the armature core, we take it out and put it on the surface of the armature core? We found this winding form to overcome the shortcomings of the ring winding, called drum winding. The actual motor uses drum winding.