I. Classification of DC Motors
1. Brushless DC motor:
A brushless DC motor is essentially a conventional DC motor with its stator and rotor interchanged. The rotor uses permanent magnets to generate air gap flux; the stator is the armature, composed of multi-phase windings. Structurally, it is similar to a permanent magnet synchronous motor. The stator structure of a brushless DC motor is the same as that of a conventional synchronous motor or induction motor. Multi-phase windings (three-phase, four-phase, or five-phase, etc.) are embedded in the iron core. The windings can be connected in a star or delta configuration and are connected to the power transistors of the inverter for proper commutation. The rotor often uses rare-earth materials with high coercivity and high remanence, such as samarium cobalt or neodymium iron boron. Due to the different placement of the magnetic material in the poles, they can be classified as surface poles, embedded poles, and toroidal poles. Because the motor body is a permanent magnet motor, brushless DC motors are also commonly called permanent magnet brushless DC motors.
Brushless DC motors are a new type of DC motor that has emerged in recent years with the development of microprocessor technology, the application of new power electronic devices with high switching frequency and low power consumption, the optimization of control methods, and the emergence of low-cost, high-magnetic-energy permanent magnet materials.
Brushless DC motors retain the excellent speed regulation performance of traditional DC motors while also offering advantages such as no slip contact and commutation sparks, high reliability, long service life, and low noise. As a result, they have been widely used in aerospace, CNC machine tools, robots, electric vehicles, computer peripherals, and home appliances.
Depending on the power supply method, brushless DC motors can be divided into two categories: square wave brushless DC motors, whose back EMF waveform and power supply current waveform are both rectangular waves, also known as rectangular wave permanent magnet synchronous motors; and sine wave brushless DC motors, whose back EMF waveform and power supply current waveform are both sine waves.
2. Brushed DC motor
(1) Permanent magnet DC motor
Permanent magnet DC motors can be classified into three types: rare earth permanent magnet DC motors, ferrite permanent magnet DC motors, and AlNiCo permanent magnet DC motors.
① Rare earth permanent magnet DC motors: They are small in size and have better performance, but are expensive. They are mainly used in aerospace, computers, and downhole instruments.
② Ferrite permanent magnet DC motor: The magnetic poles are made of ferrite material. They are inexpensive and have good performance. They are widely used in household appliances, automobiles, toys, power tools and other fields.
③ AlNiCo permanent magnet DC motor: It requires a large amount of precious metals and is expensive, but it has good adaptability to high temperature and is used in applications with high ambient temperature or high requirements for the temperature stability of the motor.
(2) Electromagnetic DC motor.
Electromagnetic DC motors can be classified into: series-wound DC motors, shunt-wound DC motors, separately excited DC motors, and compound-wound DC motors.
① Series-wound DC motor: The current is connected in series and shunt, and the field winding is connected in series with the armature. Therefore, the magnetic field inside this type of motor changes significantly with the change of armature current. In order to avoid large losses and voltage drops in the field winding, the resistance of the field winding should be as small as possible. Therefore, DC series-wound motors are usually wound with thicker wires and have fewer turns.
② Shunt DC motor: The field winding of a shunt DC motor 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 the armature share the same power supply, and in terms of performance, they are the same as separately excited DC motors.
③ Separately excited DC motor: The field winding is not electrically connected to the armature, and the field circuit is supplied by a separate DC power source. Therefore, the field current is not affected by the armature terminal voltage or armature current.
④ Compound-wound DC motor: A compound-wound DC motor has two excitation windings, a shunt winding and a series winding. If the magnetomotive force 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 magnetomotive forces are in opposite directions, it is called differential compound excitation.
II. Working Principle of DC Motors
A DC motor contains a fixed ring-shaped permanent magnet. Current flowing through the coils on the rotor generates an Ampere force. When the coils on the rotor are parallel to the magnetic field, the direction of the magnetic field changes with continued rotation. Therefore, the brushes at the rotor end alternately contact the switching plates, causing the current direction in the coils to change as well. The resulting Lorentz force remains in the same direction, allowing the motor to maintain rotation in one direction.
The working principle of a DC generator is to convert the alternating electromotive force induced in the armature coil into a direct current electromotive force when it is drawn out from the brush terminals by the commutation action of the commutator and the brushes.
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).
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.
