The working principle of a DC generator is to convert the alternating electromotive force induced in the armature coil.
Actual picture of DC motor
Figure 1
The principle is that the commutation effect of the commutator and the brushes is used to convert the current into a DC electromotive force when it is drawn from the brush end.
The direction of the induced electromotive force is determined by the right-hand rule (the magnetic field lines point to the palm, the thumb points to the direction of the conductor's motion, and the other four fingers point to the direction of the induced electromotive force in the conductor).
Working principle
The direction of the force on a conductor is determined by the left-hand rule. This pair of electromagnetic forces creates a torque acting on the armature, which is called electromagnetic torque in a rotating electrical machine. The direction of the torque is counterclockwise, attempting to make the armature rotate counterclockwise. If this electromagnetic torque can overcome the resistive torque on the armature (such as the resistive torque caused by friction and other load torques), the armature can rotate counterclockwise.
DC motors are electric motors that operate on DC working voltage and are widely used in radios, video recorders, DVD players, electric shavers, hair dryers, electronic watches, toys, etc.
Electromagnetic
An electromagnetic DC motor consists of stator poles, rotor (armature), commutator (commonly known as rectifier), brushes, housing, bearings, etc.
The stator poles (main poles) of an electromagnetic DC motor consist of an iron core and excitation windings. Based on their excitation (formerly known as magnetization) method, they can be further classified into series-wound DC motors, shunt-wound DC motors, separately excited DC motors, and compound-wound DC motors. Due to the different excitation methods, the magnetic flux of the stator poles (generated by energizing the excitation coils of the stator poles) also differs.
In a series-wound DC motor, the excitation winding and rotor winding are connected in series via brushes and a commutator. The excitation current is proportional to the armature current, and the stator flux increases with the increase of the excitation current. The torque is approximately proportional to the square of the armature current, and the speed decreases rapidly with the increase of torque or current. Its starting torque can reach more than 5 times the rated torque, and its short-term overload torque can reach more than 4 times the rated torque. The speed variation rate is large, and the no-load speed is very high (generally, it is not allowed to operate under no-load conditions). Speed regulation can be achieved by connecting an external resistor in series (or in parallel) with the series winding, or by switching the parallel connection of the series winding.
In a shunt-wound DC motor, the excitation winding is connected in parallel with the rotor winding. Its excitation current is relatively constant, and the starting torque is proportional to the armature current, which is approximately 2.5 times the rated current. The speed decreases slightly with increasing current and torque, and the short-term overload torque is 1.5 times the rated torque. The speed variation rate is small, ranging from 5% to 15%. Speed can be adjusted by using constant power to weaken the magnetic field.
In a separately excited DC motor, the excitation winding is powered by an independent excitation power supply, and its excitation current is relatively constant. The starting torque is proportional to the armature current. The speed variation is also 5%~15%. The speed can be increased by weakening the magnetic field and maintaining constant power, or the speed can be decreased by reducing the voltage of the rotor winding.
In addition to the shunt winding, the stator poles of a compound-wound DC motor also have a series winding (with fewer turns) connected in series with the rotor winding. The magnetic flux generated by the series winding is in the same direction as that of the main winding. The starting torque is approximately four times the rated torque, and the short-term overload torque is approximately 3.5 times the rated torque. The speed variation rate is 25%~30% (related to the series winding). The speed can be adjusted by weakening the magnetic field strength.
The commutator segments are made of alloy materials such as silver-copper and cadmium-copper, molded from high-strength plastic. The brushes slide in contact with the commutator, providing armature current to the rotor windings. Electromagnetic DC motors typically use metal-graphite brushes or electrographite brushes. The rotor core is made of laminated silicon steel sheets, typically with 12 slots, containing 12 sets of armature windings. These windings are connected in series and then individually connected to the 12 commutator segments.
DC motor
The excitation method of a DC motor refers to how the excitation winding is powered to generate an excitation magnetomotive force to establish the main magnetic field. Based on different excitation methods, DC motors can be classified into the following types.
He encouraged
A DC motor in which the field winding is not connected to the armature winding, but is powered by another DC power source, is called a separately excited DC motor. In the schematic diagram, M represents the motor, and G represents the generator. Permanent magnet DC motors can also be considered separately excited DC motors. See the diagram below:
Figure 2
And encourage
In a shunt-wound DC motor, the field winding is connected in parallel with the armature winding. As a shunt generator, the terminal voltage generated by the motor itself supplies power to the field winding; as a shunt motor, the field winding and armature share the same power supply, and in terms of performance, they are the same as a separately excited DC motor. See the diagram below:
Figure 3
Series excitation
In a series-wound DC motor, the field winding and armature winding are connected in series and then to a DC power supply. The field current of this type of DC motor is the armature current. See the diagram below:
Figure 4
Reinforcement
A compound-wound DC motor has two excitation windings: a shunt winding and a series winding. If the magnetomotive force (MF) generated by the series winding is in the same direction as that generated by the shunt winding, it is called cumulative compound excitation. If the two MFs are in opposite directions, it is called differential compound excitation. See the diagram below:
Figure 5
DC motors with different excitation methods have different characteristics. Generally, the main excitation methods for DC motors are shunt-wound, series-wound, and compound-wound, while the main excitation methods for DC generators are separately excited, shunt-wound, and compound-wound.
Permanent magnet
Permanent magnet DC motors also consist of stator poles, rotor, brushes, and housing. The stator poles use permanent magnets (permanent steel), made of materials such as ferrite, AlNiCo, and NdFeB. Based on their structural form, they can be divided into several types, including cylindrical and block-type. Most motors used in VCRs and players use cylindrical magnets, while motors used in power tools and automotive electrical appliances mostly use block-type magnets.
The rotor is generally made of laminated silicon steel sheets, and has fewer slots than the rotor of an electromagnetic DC motor. Most low-power motors used in VCRs and players have 3 slots, while higher-end models have 5 or 7 slots. Enameled wire is wound between two slots of the rotor core (three slots means three windings), and each joint is welded to the commutator's metal plates. Brushes are conductive components that connect the power supply to the rotor windings, possessing both conductivity and wear resistance. Permanent magnet motors use single-element metal sheets, metal-graphite brushes, or electrographite brushes.
The permanent magnet DC motor used in the recorder employs an electronic speed stabilization circuit or a centrifugal speed stabilization device.
Brushless DC
Brushless DC motors use semiconductor switching devices to achieve electronic commutation, replacing traditional contact commutators and brushes with electronic switching devices. They offer advantages such as high reliability, no commutation sparks, and low mechanical noise, and are widely used in high-end tape recorders, video recorders, electronic instruments, and automated office equipment.
A brushless DC motor consists of a permanent magnet rotor, a multi-pole winding stator, and a position sensor. The position sensor, based on changes in rotor position, commutates the current in the stator windings in a specific sequence (i.e., it detects the position of the rotor poles relative to the stator windings, generates a position sensing signal at a defined position, processes the signal through a signal conversion circuit, and then controls the power switching circuit to switch the winding current according to a specific logic). The operating voltage of the stator windings is provided by an electronic switching circuit controlled by the position sensor output.
Position sensors come in three types: magnetic, photoelectric, and electromagnetic. In a brushless DC motor that uses a magnetic position sensor, the magnetic sensor element (such as a Hall element, magnetic diode, magnetic transistor, magnetoresistive element, or dedicated integrated circuit) is mounted on the stator assembly to detect changes in the magnetic field generated by the permanent magnet and the rotor during rotation.
A brushless DC motor employing photoelectric position sensors has photoelectric sensor components positioned at specific locations on the stator assembly, and a light-shielding plate mounted on the rotor. The light source is a light-emitting diode or a small bulb. As the rotor rotates, due to the effect of the light-shielding plate, the photosensitive components on the stator will intermittently generate pulse signals at a certain frequency.
A brushless DC motor that uses an electromagnetic position sensor has an electromagnetic sensor component (such as a coupling transformer, proximity switch, LC resonant circuit, etc.) installed on the stator assembly. When the position of the permanent magnet rotor changes, the electromagnetic effect will cause the electromagnetic sensor to generate a high-frequency modulation signal (the amplitude of which changes with the rotor position).
Superiority
DC motors have the characteristics of fast response and large starting torque.
Motor actual picture
Figure 6
DC motors offer rated torque from zero speed to rated speed, but this advantage is also their disadvantage. To generate constant torque under rated load, the armature and rotor magnetic fields must maintain a constant 90° angle, requiring carbon brushes and a commutator. These brushes and commutators generate sparks and carbon dust during motor rotation, potentially damaging components and limiting their applications. AC motors, on the other hand, are brushless, robust, and widely applicable, but achieving DC-like performance requires complex control techniques. Rapid advancements in semiconductor technology have significantly increased the switching frequency of power components, improving drive motor performance. Microprocessors are also becoming increasingly faster, enabling AC motor control within a rotating two-axis orthogonal coordinate system. By appropriately controlling the current components of the AC motor along both axes, similar control methods to DC motors can be achieved, resulting in comparable performance.
Furthermore, many microprocessors now integrate the necessary motor control functions into chips, and their sizes are becoming increasingly smaller; examples include analog-to-digital converters and pulse width modulation. A brushless DC motor is an application that uses electronic control to commutate an AC motor, achieving characteristics similar to a DC motor without the structural deficiencies of a DC motor.
control structure
Figure 7
A brushless DC motor is a type of synchronous motor, meaning that the rotor speed is affected by the speed of the rotating magnetic field of the stator and the number of rotor poles (p).
n=120. f/p. With a fixed number of rotor poles, changing the frequency of the stator rotating magnetic field can change the rotor speed. A brushless DC motor is essentially a synchronous motor with added electronic control (driver). This controls the frequency of the stator rotating magnetic field and feeds the rotor speed back to the control center for repeated correction, aiming to achieve characteristics close to a DC motor. In other words, a brushless DC motor can maintain a certain rotor speed within its rated load range even when the load changes.
A brushless DC driver includes a power supply section and a control section. The power supply section provides three-phase power to the motor, while the control section converts the input power frequency as needed.
The power supply can accept direct DC input (typically 24V) or AC input (110V/220V). If the input is AC, it must first be converted to DC by a converter. Regardless of whether the input is DC or AC, the DC voltage must be converted to a three-phase voltage by an inverter to drive the motor coils. The inverter typically consists of six power transistors (Q1-Q6), divided into upper arms (Q1, Q3, Q5) and lower arms (Q2, Q4, Q6), connected to the motor as switches controlling the flow through the motor coils. The control unit provides PWM (Pulse Width Modulation) to determine the switching frequency of the power transistors and the timing of inverter commutation. DC brushless motors generally require speed control that maintains a stable speed at a set value without significant fluctuations when the load changes. Therefore, the motor is equipped with a Hall sensor that senses the magnetic field, serving as a closed-loop speed control and also as a basis for phase sequence control. However, this is only used for speed control and cannot be used for positioning control.
Control Principles
Figure 8
To make the motor rotate, the control unit must determine the sequence of power transistors in the inverter based on the position of the motor rotor sensed by the Hall sensor and the stator windings. This causes current to flow sequentially through the motor coils, generating a clockwise (or counterclockwise) rotating magnetic field that interacts with the rotor's magnets, thus making the motor rotate clockwise or counterclockwise. When the motor rotor rotates to a position where the Hall sensor senses another set of signals, the control unit activates the next set of power transistors. This cycle continues, allowing the motor to rotate in the same direction until the control unit decides to stop the motor rotor by turning off the power transistors (or only turning on the lower arm power transistors); to reverse the motor rotor's direction, the power transistor activation sequence is reversed.
