Third, DC motor classification
The DC motor is composed of a stator magnetic pole, a rotor (armature), a commutator, a brush, a casing, a bearing, etc., and the stator magnetic pole (main magnetic pole) of the electromagnetic DC motor is composed of a core and a field winding. According to the different methods of excitation (the old standard is called excitation), it can be divided into series-excited DC motor, shunt DC motor, DC-powered DC motor and compound-excited DC motor. Due to the different excitation modes, the law of the stator magnetic flux (generated by the energization of the excitation coil of the stator pole) is also different.
The excitation winding of the series-excited DC motor and the rotor winding are connected in series by the brush and the commutator. The excitation current is proportional to the armature current. The magnetic flux of the stator increases with the increase of the excitation current. The torque is similar to the electric current. The square of the pivot current is proportional to the speed, which decreases rapidly with increasing torque or current. The starting torque can reach more than 5 times of the rated torque, the short-time overload torque can reach more than 4 times of 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 connecting (or in parallel) the series windings with external resistors or by switching the series windings in parallel.
The excitation winding of the 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 of the rated current. The speed decreases slightly with the increase of current and torque, and the short-time overload torque is 1.5 times of the rated torque. The rate of change of the speed is small, ranging from 5% to 15%. It can be adjusted by weakening the constant power of the magnetic field.
The excitation winding of the DC motor is connected to an independent excitation power supply, and the excitation current is also relatively constant. The starting torque is proportional to the armature current. The speed change is also 5%~15%. The speed can be reduced by weakening the magnetic field constant power or by lowering the voltage of the rotor winding.
In addition to the shunt windings, the stator poles of the compound-excited DC motor are also equipped with series-excited windings in series with the rotor windings (the number of turns is small). The direction of the magnetic flux generated by the series winding is the same as the magnetic flux direction of the main winding, the starting torque is about 4 times of the rated torque, and the short-time overload torque is about 3.5 times of the rated torque. The rate of change of the speed is 25% to 30% (related to the series winding). The speed can be adjusted by weakening the magnetic field strength.
