What Do You Know About Railway DC Traction motor?

With the development of urbanization and the traffic pressure brought by the increase in population, the development of urban rail transit has become a fundamental way to solve urban traffic congestion. Taking China as an example, in 1967, China developed a subway vehicle with a DC motor as the drive motor.
A DC motor is an electric machine that realizes the mutual conversion between DC power and mechanical energy. It mainly adopts the series-excitation form and adjusts the motor speed by changing the motor terminal voltage or the excitation current. It has good traction performance and reliable technology. Because DC motors have good starting and speed regulation performance, they are often used on occasions with high requirements for starting and speed regulation, such as urban trams and subway trains.

1.Basic structure of DC motors

The structure of DC motors and DC generators is basically the same. The DC motor consists of two major parts:: a stationary stator and a rotating rotor. There is a gap between the stator and the rotor, which is called an air gap. The function of the stator is to generate a magnetic field and support the motor. It mainly includes main magnetic poles, commutation magnetic poles, stator pressure rings, brush devices, end covers, etc. The function of the rotor is to generate induced electromotive force and electromagnetic torque and realize the speed of electromechanical energy. It is also usually called an armature. It mainly includes an armature core, armature winding, commutator, rotating shaft, fan, etc.

1) Main pole

The function of the main pole is to generate the main magnetic flux. It is composed of the core and the excitation winding. The core is generally made of 1mm~1.5mm low-carbon steel sheets. Small motors also use a whole piece of cast steel pole. The excitation winding on the main pole is wound with insulating copper coins to form a concentrated winding, which is insulated from the core. The coils on each main pole are generally connected in series. The main poles are always in pairs and arranged alternately with the N pole and the S pole.

2) Commutation pole

The function of the commutation pole is to generate an additional magnetic field to improve the commutation performance of the motor. Usually, the core is made of a whole piece of steel, and the winding of the commutation pole should be connected in series with the armature winding. The commutation pole is installed between the two main poles. Its polarity should be the same as the main magnetic pole that the armature conductor will enter when it is running as a generator; when it is running as a motor, it should be the same as the main magnetic pole that the armature conductor has just left.

3) Motor base

The motor base is used to fix the main magnetic pole, commutation magnetic pole end cover, etc., on the one hand, and as a part of the motor magnetic circuit, it is called a yoke. The motor base is generally made of cast steel or welded steel plates.

4) Brush device

In a DC motor, in order to connect the armature winding with the external circuit, a fixed brush device must be installed. It consists of a brush, a brush holder and a brush rod seat. The brush is a conductive block made of graphite, etc., which is placed in the brush holder and pressed against the commutator by a spring pressure finger. The brush holder is clamped on the brush rod with screws, and the brush and brush rod is connected with a copper-stranded wire. The brush rod is installed on the brush seat and insulated from each other. The brush rod seat is installed on the end cover.

5) Armature core

The function of the armature core is to pass magnetic flux and arrange the armature winding. When the armature rotates in the magnetic field, the core will generate eddy current and hysteresis loss. In order to reduce loss and improve efficiency, the armature core is generally made of silicon steel sheets. The armature core has axial cooling ventilation holes, and grooves are evenly distributed on the outer circle of the core to embed the armature winding.

6) Armature winding

The function of the armature winding is to generate induced electromotive force and electromagnetic torque through current to achieve electromechanical energy conversion. The winding is made of commonly used enameled wire winding, embedded in the armature core slot, and connected according to certain rules. In order to prevent the centrifugal force generated when the armature rotates from causing the winding to fly out, the winding is embedded in the slot and pressed with a slot wedge; the terminal part of the coil extending out of the slot is tied with a wireless glass ribbon.

2. Characteristics of DC motors

DC motors have a wide speed regulation range and smooth step-less speed regulation characteristics, which can achieve frequent step-less rapid starting, braking and reversing; large overload capacity can withstand frequent impact loads; and can meet various special operation requirements in automated production systems.
But it also has significant disadvantages: first, the manufacturing process is complicated, it consumes more non-ferrous metals, and the production cost is high; second, it is easy to generate sparks between the brush and the converter during operation, so the reliability is poor and maintenance is difficult. Therefore, it has been replaced by AC variable frequency speed regulation systems in some fields that do not require high-speed regulation performance. However, in some occasions that require a large speed regulation range, high speed, good precision, and excellent control performance, the application of DC motors currently accounts for a large proportion.