What are the main types of DC motors?

DC motors are mainly divided into two categories, namely DC generators and DC motors.
1. DC generators
DC generators are machines that convert mechanical energy into DC electrical energy. They are mainly used as DC motors required for DC motors, electrolysis, electroplating, electrosmelting, charging, and excitation power supplies for AC generators. Although power rectifier elements are also used to convert AC power into DC power where DC power is required, AC rectifier power supplies cannot completely replace DC generators in terms of certain working performance.

DC generators are mainly used as DC motors required for DC motors, electrolysis, electroplating, electrosmelting, charging, and excitation power supplies for AC generators. Although power rectifier elements are also used to convert AC power into DC power where DC power is required, DC motors cannot be compared with AC generators in terms of ease of use, reliability of operation, and certain working performance. The potential waveform of DC generators is better and the electromagnetic interference is smaller, but due to the presence of commutators, their manufacturing and maintenance are complex and the price is higher.

The working principle of a DC generator is to convert the alternating electromotive force induced in the armature coil into a DC electromotive force when it is led out from the brush end by the commutation action of the commutator and the brush. Because the electromotive force led by brush A through the commutator is always the electromotive force in the coil side that cuts the N-pole magnetic field line. Therefore, brush A always has a positive polarity. Similarly, brush B always has a negative polarity. Therefore, the brush end can lead out a pulsating electromotive force with a constant direction but a changing magnitude. The induced electromotive force in the coil is an alternating electromotive force, while the electromotive force at the A and B ends of the brush is a DC electromotive force. When the armature of the generator is driven by other machines to rotate counterclockwise at a uniform speed, the coil abcd moves to cut the magnetic field lines. Using the right-hand rule, it can be determined that the direction of the induced electromotive force generated by the ab segment conductor is b→a; the direction of the induced electromotive force generated by the cd segment conductor is d→c, then the brush A in contact with the slider 1 is the positive pole, and the brush B in contact with the slider 2 is the negative pole. When the coil rotates to the neutral plane, the induced electromotive force gradually decreases from the maximum value to zero. When the coil rotates through the neutral plane, the direction of the induced electromotive force generated by the ab segment conductor changes from a→b; the direction of the induced electromotive force of the cd segment conductor changes from c→d. At this time, brush A changes to contact with the commutator slider 2, and brush B contacts with the slider 1. As the coil rotates continuously in the magnetic field, the induced electromotive force between the commutator sliders 1 and 2 is an alternating electromotive force whose magnitude and direction change with time, but brushes A and B alternately contact the commutator sliders 1 and 2 that rotate simultaneously with the coil, so a pulsating DC electromotive force is generated between brushes A and B, and the output from A and B is DC power.

2. DC motor
A rotating device that converts DC power into mechanical energy. The motor stator provides a magnetic field, the DC power supply provides current to the rotor winding, and the commutator keeps the rotor current and the torque generated by the magnetic field unchanged in direction. DC motors can be divided into two categories according to whether they are equipped with a commonly used brush-commutator, including brushed DC motors and brushless DC motors.

Brushless DC motor is a new type of DC motor developed in recent years with the development of microprocessor technology and the application of new power electronic devices with high switching frequency and low power consumption, as well as the optimization of control methods and the emergence of low-cost and high-magnetic energy level permanent magnetic materials.

Brushless DC motors not only maintain the good speed regulation performance of traditional DC motors, but also have the advantages of no sliding contact and commutation sparks, high reliability, long service life and low noise. Therefore, they have been widely used in aerospace, CNC machine tools, robots, electric vehicles, computer peripherals and household appliances.

According to the different power supply methods, 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; sine wave brushless DC motors, whose back EMF waveform and power supply current waveform are both sine waves.

The performance of DC motors is closely related to their excitation methods. Usually, there are four excitation methods for DC motors: DC separately excited motors, DC shunt excited motors, DC series excited motors and DC compound excited motors.

(1). DC separately excited motor: There is no electrical connection between the excitation winding and the armature, and the excitation circuit is supplied by another DC power supply. Therefore, the excitation current is not affected by the armature terminal voltage or armature current.

(2). DC shunt motor: The circuit is connected in parallel and divided, and the voltage across the shunt winding is the voltage across the armature, but the excitation

winding is wound with thin wires and has many turns, so it has a large resistance, making the excitation current passing through it small.

(3). DC series motor: The current is connected in series and the voltage is divided. The excitation winding is connected in series with the armature, so the magnetic field in this motor changes significantly with the change of the armature current. In order to avoid large losses and voltage drops in the excitation winding, the smaller the resistance of the excitation winding, the better, so the DC series motor is usually wound with thicker wires and has fewer turns.

(4). DC compound motor: The magnetic flux of the motor is generated by the excitation current in the two windings.