Electric Locomotive Electric Drive

Author: Chief Editor: Zhao Jingchao et al.
Publisher:
Publish Date: 2003-06-01
Features:
Introduction This book focuses on the domestic Dongfeng 4B type diesel locomotive as the main subject, providing a detailed introduction to the motor, electrical equipment, electric transmission principles, circuit control, two-stage resistance braking, and water resistance testing of diesel locomotives. It also offers an in-depth explanation of the infinite-speed regulation circuit of the Dongfeng 4B type diesel locomotive DLJ6-05-00-000XLA currently used in various railway stations and sections. Additionally, it provides a comparative introduction to the upgraded product—the Dongfeng 4C type diesel locomotive. This book serves as a teaching material for intermediate railway vocational schools in the field of diesel locomotives and can also be used as a reference for engineering technicians and workers engaged in diesel locomotive work.
Excerpt:
Chapter DC Motor Principles
Section Overview Devices that convert electrical energy into other forms of energy and vice versa are primarily motors. Based on the type of energy conversion, motors can be categorized as follows:
1. Devices converting mechanical energy into electrical energy—generators;
2. Devices converting electrical energy into mechanical energy—electric motors;
3. Devices that change the form of electrical energy—converters that transform alternating current (AC) into direct current (DC), frequency converters that change AC frequency, and transformers that change AC voltage.
According to the form of electrical energy, the working characteristics, and the purpose of the motor, they can be classified as DC motors, AC motors, and control motors. Electrically powered diesel locomotives are one of the main power types in China's railway system. The mechanical energy of the diesel engine, the power source of the electrically powered diesel locomotive, is transmitted to the locomotive through an intermediate power device—the traction motor. The traction motor includes the generator, traction motor, and other related motors.
The electric transmission of diesel locomotives can be divided into three types: DC-DC, AC-DC, and AC-AC. In a DC-DC transmission system, both the generator and the traction motor are DC motors. In an AC-DC transmission system, the generator is an AC motor, while the traction motor is a DC motor. In the early-stage AC-AC transmission system, both the generator and the traction motor are AC motors. Compared to AC motors, DC motors have a more complex structure, consume more non-ferrous metals, have more faults during operation, require more maintenance, and cannot handle high power. Therefore, DC generators in diesel locomotives are gradually being replaced by AC generator-rectifier units. However, DC motors offer better starting performance and speed regulation, so most traction motors in modern diesel locomotives are DC motors.
Section 2 Basic Working Principle of DC Motors
To facilitate the explanation of the working principle of DC motors, the working principle of an AC generator will first be described. Figure 1-1 shows the schematic diagram of a single-coil AC generator. N and S are two poles with opposite polarities, and the coil abcd can rotate between the two poles (the iron core and shaft of the fixed coil are not shown in the figure for simplicity). The two ends of the coil are fixedly connected to two insulated copper rings (slip rings) I and II, which rotate with the coil. Two stationary brushes A and B make sliding contact with the copper rings I and II, respectively. The coil is connected to the load (consumer) through the slip rings and brushes, forming a circuit.
If the prime mover drives the coil to rotate at a constant speed υ (speed n), the induced electromotive force (EMF) ex on one side of the coil is:
ex = Bxlυ
where l—the effective length of the coil side (the length of the conductor in the magnetic field).
At the position shown in Figure 1-1, both sides ab and cd of the coil are at the center lines of the magnetic poles. If the coil rotates counterclockwise, according to the right-hand rule, the a end of the ab side under the N pole has a positive polarity, and the c end of the cd side under the S pole has a positive polarity. The EMFs of the two effective sides add up to form the coil EMF. At this time, slip ring I and brush A are positive, while slip ring II and brush B are negative.
When the load is connected, the coil outputs current from slip ring I and brush A, passes through the load, and returns to the coil from brush B and slip ring II. When the coil rotates 90°, both effective sides are at the center lines between the two magnetic poles (called the geometric neutral line). At this position, the magnetic flux density is zero, and the coil has no induced EMF. When the coil rotates 180°, the positions of ab and cd sides are exchanged, and the coil EMF is opposite to the initial state. The d end becomes positive, and the a end becomes negative.
Under load, the coil outputs current from slip ring II and brush B, passes through the load, and returns to the coil from brush A and slip ring I. Clearly, when the coil rotates 180°, compared to the initial position, both the effective sides and the coil EMF are reversed, the polarities of brushes I and II are exchanged, and the voltage and current in the load are also reversed. When the coil rotates 270°, both ab and cd sides are again at the geometric neutral line, and the coil has no EMF. When the coil rotates 360°, the induced EMF is the same as the initial position (i.e., Figure 1-1), with brush A being positive and brush B being negative.
When the coil rotates continuously, it repeats the above process. It can be seen that:
- The coil completes one rotation, and the coil EMF alternates once;
- The polarities of brushes A and B, as well as the output voltage and current, all alternate once.
According to Equation (1-1), the induced EMF of the coil is related to the magnetic flux density B. If the magnetic flux density in the air gap is sinusoidal, under constant speed conditions, the induced EMF of the coil is also a sine wave (Figure 1-2).
If the two slip rings of the above AC generator are modified into two opposite and insulated semi-rings I and II, forming a single slip ring, the a end and d end of the coil are fixedly connected to the two semi-rings I and II, respectively. Brushes A and B are placed at the center lines of the two magnetic poles, respectively. In this way, the AC generator is transformed into a DC generator (Figure 1-3).
The main difference between a DC generator and an AC generator lies in the slip ring mechanism. The slip rings of a DC generator are called commutators.
At the position shown in Figure 1-3, the induced EMF of the coil is the same as that in Figure 1-1. Slip ring I and brush A are positive, while slip ring II and brush B are negative. When the coil rotates 180°, the positions of ab and cd sides are exchanged, and the coil EMF is reversed. The positions and polarities of the two semi-rings I and II of the commutator rotating with the coil also change. Slip ring II becomes positive, and slip ring I becomes negative. At this time, the stationary brush A contacts slip ring II and remains positive, while brush B contacts slip ring I and remains negative.
When the coil rotates 360°, the coil EMF is the same as the initial position, with slip ring I and brush A being positive, and slip ring II and brush B being negative.