Brake energy recovery is realized by using the principle of motor/generator reversibility of electric motors. Its basic working principle is: the wheel under the action of inertia through the speed reducer to the motor rotor speed will be greater than the motor synchronous speed, so that the rotor reverse cut the magnetic inductance, generating higher than the stator coil electromotive force of the reverse electromotive force, at this time the drive motor equipment into the energy recovery mode, and the towing motor will be reversed to act on the drive shaft, the formation of regenerative braking force on the wheels to form the braking[8].
Although motor regenerative braking can recover braking energy and provide part of braking force to the wheels, but it can not make the wheels stop rotating completely, the braking effect is limited by many conditions such as motor, battery and speed, and it can not complete the braking requirements independently under the conditions of emergency braking and high-intensity braking, so in order to ensure the braking safety performance of the mine car[9], in order to ensure the braking performance of the mine car, at the same time, motor regenerative braking, it must be used as the auxiliary locomotive braking system, so as to achieve not only ensure the braking safety of the mine car but also to recycle the purpose of the energy of the considerable[10].
2.1 Selection of brake energy recovery method
In the current research on brake energy recovery at home and abroad, according to whether the brake pedal force is decoupled from the hydraulic braking force or not, the brake energy recovery system can be categorized into iterative regenerative braking system (RBS) and collaborative regenerative braking system (CRBS), and by comparing these two types of regenerative braking systems, a more efficient and concise brake energy recovery system is selected[11].
In an iterative regenerative braking system (RBS), also called a parallel brake energy recovery system, the brake pedal force and brake wheel cylinder hydraulic pressure are non-decoupled, and during braking, as long as the brake pedal is pressed down, the brake wheel cylinder generates hydraulic braking force, and the electric braking is only superimposed on this braking force (Fig. 2). So there is still a part of energy lost, and the energy recovery rate is low[12].
Collaborative Regenerative Braking System (CRBS), also called series brake energy recovery system, decouples the brake pedal force and the brake wheel cylinder hydraulic pressure, and after depressing the brake pedal, the controller recognizes the driver's braking demand by capturing the current brake pedal opening and its rate of change and calculates the demanded braking force[13]. The electric braking force is dominated by the electric braking force, supplemented by the hydraulic braking force, and the two are dynamically coordinated and controlled in order to increase the proportion of electric braking and increase energy recovery. (Fig. 3)
Figure 3. Decoupling of brake pedal force and hydraulic braking force
As seen above, the biggest difference between the two is whether the brake pedal is decoupled from the brake actuator. In a braking condition of an electric vehicle, one of the sources of braking torque is the hydraulic braking generated by the brake wheel cylinder, and the other source is the electric motor that provides negative torque to achieve deceleration through the drive shaft, i.e. electric braking[14].
Figure 4. Relationship between braking force and brake strength
As shown in Fig. 4, when the brake pedal and brake actuator are non-decoupled, the locomotive carries out motor braking on top of hydraulic braking when the braking force is small, but the motor braking accounts for a small amount of the brake force, so a part of the energy is consumed, and the full recovery of braking energy cannot be carried out[15].
When the brake pedal and brake actuator decoupled, the locomotive braking process, the first motor braking, in the demand for braking power over the motor itself braking force F1, hydraulic braking and then auxiliary braking, motor braking exists in the entire braking process, so the energy generated by the braking can be maximized recovery.
After comparing two types of brake energy recovery systems, in order to fully recover the energy wasted during the braking process, we chose to retrofit a collaborative brake energy recovery system to the mine car.
2.2 Selection of energy storage methods for mine cars
At present, domestic and foreign research on the energy storage system of the regenerative energy recovery system is more abundant, according to the energy storage components are mainly divided into flywheel energy storage system, hydraulic energy storage system and electrochemical energy storage system, through the comparison of these types of energy storage methods, choose to install the convenient, low-cost energy storage method[16].
Flywheel energy storage system is to accelerate the rotating flywheel to store energy in the flywheel, in the process of braking energy recovery, the kinetic energy of the mine car is transferred to the flywheel energy recovery system through the drive wheel, and the energy is recovered through the energy manager controlling the flywheel box, the higher the mass of the flywheel the higher the inertia, and the higher the storage capacity, and the flywheel is accelerated through the control of the clutch state, accelerating the flywheel to a certain rotational speed. Realize the energy storage, after disconnecting the clutch, the flywheel continues to rotate movement, there is a certain amount of energy loss in the process. (Fig. 5) in the operation process, the need to match the speed of the flywheel with the speed of the transmission, there is a certain instability in the mechanical connection, and because of the working characteristics of the flywheel can be seen, the speed of the flywheel to a higher speed, there is a large centrifugal force, the flywheel box is in an unstable state, and at the same time, may lead to the resonance phenomenon of the entire mechanical system, and at the same time, high-speed rotation of the flywheel will be connected with mechanical components of intense friction, the flywheel box is in an unstable state. At the same time, the high-speed rotating flywheel will have intense friction with the connected mechanical components, and the friction coefficient can be reduced by magnetic levitation or vacuum in the manufacturing of the flywheel box, but at the same time, the cost increases dramatically[17].
Figure 5. Flywheel Energy Storage System Principles
The hydraulic energy storage system works by converting kinetic energy into hydraulic energy, as shown in Fig. 6, in which the hydraulic pump acts as an energy conversion element, and in the braking energy recovery process, the driving wheels drive the mechanical system, and the transmission is connected to the hydraulic pump, and the recovered kinetic energy drives the hydraulic pump to work, which converts the kinetic energy into the pressure energy of the hydraulic oil, realizing the recovery of braking energy, and the energy stored by the hydraulic system is long in time, and it will be released in the mine car when it accelerates or The hydraulic system stores energy for a long time, and when the mine car accelerates or climbs a slope, the hydraulic energy storage system releases the energy to improve the larger power and increase the driving capacity of the drive system of the mine car. In braking energy recovery, the hydraulic energy storage system is more stable than the flywheel system, but it occupies a large space, which is not suitable for installation and use on the mine car[16].
The electrochemical energy storage system takes the motor as the energy conversion element, as in Fig. 7, by adding the motor into the drive system, in the process of braking energy recovery, the vehicle controller cuts off the connection between the engine and transmission, at this time by the kinetic energy of the mining truck drives the transmission mechanism to drive the motor to rotate, through a series of transformations, the kinetic energy will be converted into electrical energy, with the battery as a storage element of the braking energy recovery system, to receive the The battery is used as the energy storage element of the brake energy recovery system to receive the electric energy generated by the generator and increase the battery power. In the stage of accelerating or climbing, the battery supplies power to the motor, and provides power with the engine at the same time, releasing the recovered energy. In summary, using the battery as the energy storage element has the advantages of small space occupation, high energy density and long storage time[18].
Figure7. Battery storage system principle
Summarize and compare the advantages and disadvantages of the above energy storage methods, draw a comparison chart as in Table I, and choose a suitable energy storage method for mine car locomotives. Because the electrochemical energy storage system with batteries occupies less space than the hydraulic energy storage system and has a lower cost compared to the flywheel energy storage system, as well as having the advantages of high energy density and long storage time, we choose to use the batteries as the energy storage element.
Table 1
Comparison of advantages and disadvantages of three energy storage methods
projects | Hydraulic energy storage | electrochemical energy storage | flywheel energy storage |
energy density | poor | excellent | favorable |
power density | excellent | poor | excellent |
Energy storage efficiency ( long period ) | favorable | medium | poor |
Energy storage efficiency ( short period ) | favorable | excellent | favorable |
Energy conversion efficiency | poor | poor | favorable |
dependability | favorable | poor | favorable |
2.3 Collaborative regenerative braking system design for underground mine cars
This paper takes the CTY5/6GB explosion-proof electric locomotive of Xiangtan Yutong Company as the research object, builds a collaborative regenerative braking system model, takes the battery as the energy storage element, and the system is arranged as shown in Fig. 8. The electric motor provides power for the traction locomotive through the drive shaft, and when the vehicle starts up, the battery provides power for the electric motor to start up the vehicle quickly. When the vehicle is braked, the electric motor converts to a generator state and recovers the energy generated by braking and stores it in the battery. In this way, the system enables energy recovery and storage to improve the energy efficiency of the locomotive. In addition, in situations where forced braking is not required, such as long downhill grades, the electric motor is in the generator state and converts the inertial energy of the vehicle into electrical energy and stores it in the battery. This approach not only helps to reduce energy consumption, but also extends the life of the battery.
Figure 8 Internal structure of regenerative braking system