Introduction of DC motor
DC motors are motors that rely on DC working voltage and are widely used in tape recorders, video recorders, DVD players, electric shavers, hair dryers, electronic watches, toys, etc.
1. Electromagnetic DC motor is composed of stator magnetic pole, rotor (armature), commutator (commonly known as commutator), brush, casing, bearing, etc. The stator magnetic pole (main magnetic pole) of electromagnetic DC motor is composed of iron core and excitation winding. According to the different ways of excitation (the old standard is called excitation), it can be divided into series-excited DC motors, shunt-excited DC motors, separately-excited DC motors and compound-excited DC motors. Due to the different excitation methods, the laws of the stator pole magnetic flux (generated by the excitation coil of the stator pole after being energized) are also different.
2. The excitation winding and the rotor winding of the series-excited DC motor are connected in series through brushes and commutators. The excitation current is proportional to the armature current. The magnetic flux of the stator increases with the increase of the excitation current, and the torque is approximately the same as the electric current. The pivot current is proportional to the square, and the speed decreases rapidly with the increase of torque or current. The starting torque can reach more than 5 times the rated torque, the short-term overload torque can reach more than 4 times the rated torque, the speed change rate is large, and the no-load speed is very high. (It is generally not allowed to run under no load). Speed regulation can be achieved by using an external resistor in series (or parallel) with the series winding, or by switching the series winding in parallel.
3. The excitation winding of the shunt DC motor is connected in parallel with the rotor winding, the excitation current is relatively constant, the starting torque is proportional to the armature current, and the starting current is about 2.5 times the rated current. The speed decreases slightly with the increase of current and torque, and the short-term overload torque is 1.5 times of the rated torque. The speed change rate is small, 5%~15%. The speed can be adjusted by a constant power that weakens the magnetic field.
4. The excitation winding of the separately excited DC motor is connected to an independent excitation power supply, and its excitation current is relatively constant, and the starting torque is proportional to the armature current. The speed change is also 5%~15%. The rotational speed can be increased by weakening the field constant power or by reducing the voltage of the rotor windings.
5. In addition to shunt windings, the stator poles of compound-excited DC motors are also equipped with series-excited windings (which have fewer turns) in series with the rotor windings. The direction of the magnetic flux generated by the series winding is the same as that of the main winding, the starting torque is about 4 times the rated torque, and the short-time overload torque is about 3.5 times the rated torque. The speed change rate is 25%~30% (related to the series winding). The rotational speed can be adjusted by weakening the magnetic field strength.
6. The commutator segment of the commutator is made of alloy materials such as silver copper and cadmium copper, and is molded with high-strength plastic. The brushes are in sliding contact with the commutator to provide armature current to the rotor windings. The brushes of electromagnetic DC motors generally use metal graphite brushes or electrochemical graphite brushes. The iron core of the rotor is made of laminated silicon steel sheets, generally with 12 slots, and 12 sets of armature windings are embedded.
