Permanent Magnet DC Motors And Electromagnetic DC Motors

Electromagnetic DC motor: a motor that relies on DC operating voltage

Structure: It is composed of stator poles, rotor (armature), commutator (commonly known as commutator), brush, casing, bearing, etc.

Working principle: The alternating electromotive force induced in the armature coil is relied on the commutator to cooperate with the commutation effect of the brush, so that when it is drawn from the brush end, it becomes a DC electromotive force. The direction of the induced electromotive force is determined according to the rule of the right hand (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 direction of the conductor force is determined by the left-hand rule. This pair of electromagnetic force forms a moment acting on the armature. This moment is called electromagnetic torque in the rotating electrical machine. The direction of the torque is counterclockwise, which attempts to rotate the armature counterclockwise. If this electromagnetic torque can overcome the resistance torque on the armature (such as resistance torque caused by friction and other load torque), the armature can rotate in a counterclockwise direction.

Division: The excitation method of the DC motor refers to the problem of how to supply power to the excitation winding and generate the excitation magnetomotive force to establish the main magnetic field. According to the different excitation methods, DC motors can be divided into the following types. 1. He encourages. The excitation winding has no connection relationship with the armature winding, and the DC motor powered by the other DC power supply to the excitation winding is called a separately excited DC motor. Permanent magnet DC motors can also be viewed as separately excited DC motors. 2, and encourage. The excitation winding of the parallel excitation DC motor is connected in parallel with the armature winding. As a parallel-excited generator, the terminal voltage from the motor itself supplies power to the excitation winding; as a parallel-excited motor, the excitation winding and the armature share the same power supply, and the performance is the same as that of the DC motor. 3. Series excitation. After the excitation winding of the series-excited DC motor is connected in series with the armature winding, it is then connected to the DC power supply. The excitation current of this DC motor is the armature current. 4. Compounding. The compound excitation DC motor has two excitation windings in parallel excitation and series excitation. If the magnetomotive force generated by the series excitation winding is the same as the direction of the magnetomotive force generated by the parallel excitation winding, it is called product complex excitation. If the two magnetomotive force directions are opposite, it is called differential compound excitation. DC motors with different excitation methods have different characteristics. In general, the main excitation methods of DC motors are parallel excitation, series excitation and compound excitation, and the main excitation methods of DC generators are other excitation, parallel excitation and compound excitation.

Applications: recorders, video recorders, DVD players, electric shavers, hair dryers, electronic watches, toys, etc.

 

Permanent magnet DC motor:

Structure: It is also composed of stator poles, rotors, brushes, housings, etc. The stator poles are made of permanent magnets (permanent magnetic steel) with ferrite, aluminum nickel cobalt, neodymium iron boron and other materials. According to its structure, it can be divided into cylinder type and tile type. Most of the electricity used in recorders is cylindrical magnets, and most of the motors used in power tools and automotive electrical appliances use special magnets.

The rotor is generally formed by laminating silicon steel sheets, which has fewer slots than the rotor of an electromagnetic DC motor. Most of the low-power motors used in the recorder are 3 slots, and the higher ones are 5 slots or 7 slots. The enameled wire is wound between the two slots of the rotor core (three slots have three windings), and its joints are welded to the metal sheet of the commutator. The brush is a conductive component connecting the power supply and the rotor winding, and has both conductive and wear-resistant properties. The brushes of permanent magnet motors use unisex metal sheets or metal graphite brushes and electrochemical graphite brushes. The permanent magnet DC motor used in the recorder and player uses an electronic speed-stabilizing circuit or a centrifugal speed-stabilizing device.

 

Brushless DC motor:

Definition: It uses semiconductor switching devices to achieve electronic commutation, that is, to replace traditional contact commutators and brushes with electronic switching devices. It has the advantages of high reliability, no reversing sparks, and low mechanical noise. It is widely used in high-end recording seats, video recorders, electronic instruments and automated office equipment.

Structure: composed of permanent magnet rotor, multi-pole winding stator, position sensor, etc. The position sensor converts the current of the stator winding along a certain order according to the change of the rotor position (that is, detects the position of the rotor magnetic pole relative to the stator winding, and generates a position sensor signal at the determined position, which is processed by the signal conversion circuit To control the power switch circuit and switch the winding current according to a certain logical relationship). The working voltage of the stator winding is provided by the electronic switch circuit controlled by the output of the position sensor.

There are three types of position sensors: magnetic sensitive, photoelectric and electromagnetic. A brushless DC motor using a magneto-sensitive position sensor, the magneto-sensitive sensor device (for example, Hall element, magnetodiode, magneto-electrode tube, magneto-resistor or special integrated circuit, etc.) is installed on the stator assembly, To detect the change of the magnetic field generated when the permanent magnet and the rotor rotate.

Working principle: A brushless DC motor with a photoelectric position sensor is equipped with a photoelectric sensor device at a certain position on the stator assembly. The rotor is equipped with a shading plate and the light source is a light-emitting diode or a small light bulb. When the rotor rotates, due to the effect of the shading plate, the photosensitive components on the stator will generate pulse signals intermittently at a certain frequency.

The brushless DC motor with electromagnetic position sensor is equipped with electromagnetic sensor components (such as coupling transformer, proximity switch, LC resonance circuit, etc.) on the stator assembly. When the position of the permanent magnet rotor changes, the electromagnetic effect will make the electromagnetic sensor Generate high-frequency modulation signals (the amplitude of which varies with rotor position).

 

Comparison between DC motor and AC motor:

It has fast response, large starting torque, and can provide rated torque from zero speed to rated speed, but the advantages of DC motors are also its shortcomings, because DC motors must produce constant torque under rated load. For performance, the armature magnetic field and rotor magnetic field must be maintained at 90°, which requires carbon brushes and commutators. The carbon brush and commutator will produce sparks and carbon powder when the motor rotates. Therefore, in addition to causing damage to the components, the use occasions are also limited. AC motors do not have carbon brushes and commutators. They are maintenance-free, sturdy, and widely used. However, to achieve performance equivalent to that of DC motors, complex control techniques are required. Nowadays, semiconductors are developing rapidly, and the switching frequency of power components is much faster, which improves the performance of driving motors. The speed of the microprocessor is also getting faster and faster, which can realize the control of the AC motor in a rotating two-axis orthogonal coordinate system to properly control the current component of the AC motor on the two axes to achieve similar DC motor control and equivalent to the DC motor. performance.