Renovation and operation analysis of speed control system of battery electric locomotive

1 Introduction

Mining electric locomotives are important means of transportation in the coal mining industry. Due to the extremely harsh working environment in coal mines, the electric drive system of electric locomotives is very demanding. However, the current mining electric locomotives are driven by DC motors with complex structures, high costs, high failure rates, and high maintenance costs. The speed control system still uses the primitive and backward resistance step-down speed control method. This multi-stage contact variable resistance speed regulator often burns out due to strong sparks generated by the contacts, so the maintenance volume is large. At the same time, the operation of electric locomotives with resistance leads to a large waste of electric energy. For high-gas mines, explosion-proof battery electric locomotives are generally used as tools for transporting materials and gangue. For many years, battery electric locomotives have been using DC motor series resistance speed control, so that 20-25% of the electric energy is consumed on the resistance, resulting in a waste of electric energy. In particular, battery electric locomotives, due to the excessive consumption of electric energy, the charging interval of the battery is shortened and the battery life is reduced. In recent years, with the development of science and technology, the speed control problem of AC motors has been successfully solved. The speed control system of AC motor not only has the same performance as that of DC motor, but also has lower cost and maintenance fee and higher reliability than DC motor system. It is imperative to replace DC motor with AC motor for speed control, and AC motor has unmatched advantages over DC motor. Since DC speed control runs with resistors, it consumes a lot of power. AC variable frequency speed control can save up to 35% of power because it does not use high energy consumption resistors. If the electric locomotive is set to regenerative feedback braking, the electric energy generated by the motor can be fed back to the battery when the electric locomotive slows down or goes downhill, which can greatly save power and extend the charging time.

2 Four-quadrant characteristics and energy regeneration of asynchronous motors

The three-phase AC asynchronous motor drive system has the advantages of simple structure, reliable operation, low cost, high efficiency and energy saving, so it is widely used in the transformation project of battery electric locomotives.

Since mining electric locomotives work in harsh underground environments, the speed control system is in frequent starting, braking, acceleration and deceleration states. In this way, we can make full use of the four-quadrant characteristics of three-phase AC asynchronous motors for speed control and braking: the first and third quadrants are the running states of the motor, which are forward and reverse respectively; the second and fourth quadrants are the power generation states of the motor, which are forward and reverse respectively. Since battery electric locomotives use direct torque for variable frequency speed control.

Energy regeneration will occur in electric locomotives during deceleration or emergency braking. First, assume that a three-phase current is passed through the stator winding of a three-phase asynchronous motor, with polarities of i1a>0, i1b<0, and i1c<0 (subscript: "1" represents stator, "2" represents rotor, and dots on letters represent vectors). This current will form a magnetic motive force f1 in the air gap of the motor that is distributed according to the sine law and rotates at the synchronous speed, as shown in Figure 1. f1 first establishes the main magnetic field bm (фm) of the air gap, and bm is a rotating magnetic field. When the inverter drives the asynchronous motor to decelerate, the synchronous speed no of the rotating magnetic field always decreases before the rotor speed n, that is, n0m cuts the stator and rotor windings, and induces stator electromotive forces e1a, e1b, e1c and rotor electromotive forces e2a, e2b, and e2c in the stator and rotor windings, so there are three-phase currents i2a, i2b, and i2c in the rotor circuit. Interacting with the air gap magnetic flux фm, an electromagnetic braking torque is generated, which is opposite to n0 and stops the rotor from rotating. The excitation magnetic motive force f1 formed by i1 and the excitation magnetic motive force f2 formed by i2. Assuming that the rotating magnetic field generated by the stator current with a phase sequence of a-b-c is counterclockwise, due to n0o. Since the rotor rotates counterclockwise at a speed n, the actual speed of f2 is n2=n-△n=no, and the direction is counterclockwise, that is, both f1 and f2 rotate counterclockwise, and the speed is n2. In other words, f1 and f2 remain relatively stationary, and there is no relative motion between them.

When the asynchronous motor is loaded, the rotating magnetic field bm generated in the air gap is the result of the combined magnetic motive force fm of the two relatively static magnetic motive forces f1 and f2, that is, f1=fm+(-f2). This formula means that the stator magnetic motive force of the asynchronous motor contains two components: the excitation magnetic motive force fm that generates the main magnetic flux φm in the air gap and the part (-f2) that offsets the rotor magnetic motive force f2 generated by the rotor current.

Because n00-n/n＜0, the rotor induced electromotive force se'2 is in the opposite direction. The angle of φ2 that i'2 lags behind e'2 is between 90° and 180°, the active component i'2a of the rotor current is less than 0, and the reactive component i'2r of the rotor current is less than 0.

When the asynchronous motor is in deceleration, it is equivalent to an asynchronous generator, that is, its vector diagram is shown in the figure. The phase angle between u1 and i1 is φ1>90°. At this time, the active power of the stator is negative, that is, the stator winding feeds back electrical energy to the DC side, and the reactive power of the stator is positive. From the perspective of the motor, the asynchronous motor absorbs negative active power and lagging reactive power. The former is output to the power grid, and the latter is used for excitation.

3 Control of electric locomotive regenerative braking

The motor equivalent circuit is shown in Figure 2.

Assuming that the mechanical energy of the load carried by the motor is basically consumed by the motor, we have

When the negative slip rate of the motor is within the range, the mechanical energy of the system is converted into electrical energy through the motor and fed back to the battery. At the same time, the motor does not have overcurrent, that is, the part of the feedback electric energy that cannot be absorbed by the battery can be consumed by the motor itself without overcurrent. When the slip rate changes beyond this range, the electrical energy converted by the motor cannot be effectively fed back to the battery, and the remaining energy cannot be completely consumed in the internal resistance of the motor coil, so overcurrent is prone to occur. Therefore, from the perspective of simplified control, when the control slip rate changes within the range of s1≥s≥s2, overcurrent can be avoided during regenerative braking.

The regenerative braking method proposed in this paper is in direct torque control (DTC). First, observe the stator flux, control the amplitude of the stator flux to be constant, and then select zero vector and non-zero vector to adjust the instantaneous slip and control the output torque to be constant. Therefore, the amplitude and phase of the stator flux are observed by the flux link, and the vector is selected to control the magnitude of the flux amplitude; the torque control link (combined with the flux control link) selects the vector to control the rotation speed of the flux. The system adopts the direct torque control method, and its flux control link can effectively control the deviation of the flux. In this case, it can ensure that the output current harmonics are small and the operation is stable. The regenerative braking control realized on the basis of direct torque control is to make the stator frequency track the change of the rotor speed. As long as the range of the slip rate can be guaranteed, the system can be realized without overcurrent during the braking process. Moreover, the strength of the braking effect can be achieved by adjusting the slip rate and the negative value of the stator flux.

4 Simulation Results

The above-established new direct torque control asynchronous motor variable frequency speed regulation system is simulated. The three-phase inverter switch device uses IGBT, anti-parallel feedback diode, IGBT buffer resistor RS = 10kω, buffer capacitor CS = 10-3μF. The asynchronous motor parameters used in the simulation experiment are: rated power PN = 12kw, rated frequency Fn = 50hz, ψn = 0.95wb, RS = 0.16891ω, RR = 0.13973ω, LS = 0.02877h, LM = 0.02777h, LR = 0.0289h, NP = 2, J = 0.1349kg.m2. Control system parameters: ψ*s=0.95wb, speed setting is 150rad/s, load torque setting tm=15n.m, torque limit value is 80n.m, proportional coefficient kp=50 of PI regulator, integral coefficient ki=130, DC side voltage udc=600v.

First, set the speed reference to 150rad/s. After the system stabilizes, set the speed reference to -150rad/s and observe the magnetic flux, speed, torque, line current, and DC side current waveforms.

It can be seen from the waveform in Figure 3 that the asynchronous motor is in the operating states of acceleration, deceleration, and reversal, from forward acceleration to feedback braking, and finally to reverse acceleration to a stable state, realizing the four-quadrant characteristics of the asynchronous motor.

5 Conclusion

Most of the battery-electric locomotives used in mines in my country use DC motors with series resistance for speed regulation. If the success of the project can be successfully applied to the actual coal mines, the speed regulation performance, traction capacity, load capacity and maintenance volume of the electric locomotive will be comprehensively improved. In addition, from the perspective of economic benefits, the battery-electric locomotive uses braking to charge the battery, which saves a lot of electricity, extends the working time of the battery-electric locomotive, reduces the average annual charge number of the battery, extends the service life of the battery, and greatly improves the economic benefits.