Research on PID Control of Auxiliary Motors in Electric Locomotives-EEWORLD

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The auxiliary motor of electric locomotive is mainly an auxiliary device designed to ensure the normal operation of the main circuit. Whether it is put into operation depends on the working state of the main circuit. Even when the ambient temperature is very low, although the main circuit is working in traction or braking conditions, the temperature rise of the main circuit electrical equipment served by these auxiliary devices is not high, which not only causes energy waste, increases the loss of equipment, and reduces its service life, but also generates a lot of mechanical and electromagnetic noise, affecting the comfort of crew members. If a control system with a closed-loop control function is developed, the auxiliary motor is controlled to work when it is needed, and shut down when it is not needed. The auxiliary motor PID control system based on new sensors, PLC and HMI is the research content of this topic.
The auxiliary motor is an important device in the auxiliary circuit of electric locomotive. It includes asynchronous splitter phase, air compressor motor, traction fan motor, brake fan motor, transformer fan motor and submersible pump motor. The splitter phase provides three-phase power for the auxiliary circuit, the compressor provides wind source for the locomotive, the traction fan forces cooling of the rectifier, smoothing reactor and traction motor, and the transformer fan and submersible pump dissipate heat for the transformer. The power of these motors ranges from 10 kW to 35 kW. They are the most powerful energy-consuming equipment in electric locomotives except for traction motors. For example, for a 6-axle electric locomotive, the total power of the auxiliary motors is 300 kW. The total power of the auxiliary motors of 8-axle electric locomotives such as Shaoshan 4 is even greater. More importantly, the control of these high-power energy-consuming equipment is completely controlled manually by the driver. Among these auxiliary motors, the power of the traction fan motor is second only to the phase splitter. For the sake of the safety of electric locomotive operation, an interlock is set in the logic control link. In the advanced position of the braking condition and the traction condition, even in the cold winter, when the ambient temperature is very low and there is no need to force cooling of the traction motor, smoothing reactor, main transformer and other equipment, the traction fan must be turned on. In other words, the conditions and timing for turning on the fan are not based on the temperature rise of the above equipment, but are determined by the working conditions of the locomotive. Such a control strategy has obvious shortcomings, which is neither scientific nor energy-saving. In the context of advocating energy conservation and emission reduction and low-carbon economy becoming a social consensus, it is of practical economic significance to study and optimize the locomotive auxiliary motor control system according to the specific conditions of the locomotive.
For the control of electric locomotive auxiliary motors, we should first clarify the control object, analyze their characteristics and control requirements, study the control strategy, design the control system scheme, and determine the specific implementation path. In the above-mentioned auxiliary motors, the role of the phase splitter is to convert the single-phase industrial frequency AC into three-phase industrial frequency AC, which is the prerequisite for the normal operation of other auxiliary motors. Its control is manually controlled. In addition, the air compressor is controlled by the pressure controller according to the pressure of the main air cylinder, and the brake fan is controlled by the driver when it is a resistance brake condition. Their control methods do not change. The control of the traction fan and the transformer fan is manually controlled by the driver. Their control strategies need to be optimized, and the control method is changed to PID control. Therefore, their control is the object of this paper.

1. Selection of control strategy
The function of the traction fan is to cool the traction motor, and the function of the transformer fan is to cool the transformer. Therefore, the selection of the control strategy should not only ensure the normal and reliable operation of the above-mentioned electrical equipment, but also consider energy saving. Safety, reliability and rationality are the basic requirements. At the same time, factors such as high degree of automation, low cost and easy technical implementation should also be taken into account. The essence of the fan control problem is to scientifically and reasonably control the start and stop of the fan according to the temperature rise of the control object and the change of ambient temperature, that is, to start when the control object needs ventilation and stop when it does not need ventilation. However, the characteristic of the traction motor temperature rise control is that it has a large hysteresis. When the temperature rise is too high, ventilation and cooling will cause the motor temperature to be too high, affecting the reliable operation of the motor, causing certain harm to the motor itself, and even causing damage to the motor. This phenomenon is called under-regulation. As the motor is ventilated and cooled, the motor temperature drops accordingly. When it drops to the safe operating temperature of the motor, the fan is controlled to stop. The temperature of the motor may be very low, and the motor itself no longer needs ventilation. This will cause waste and fail to play a reasonable control role. This phenomenon is called over-regulation. The above problem is the hysteresis problem of the temperature control system.
Based on the above control requirements and characteristics, we decided to use the PID controller, which is currently widely used and has excellent control performance. PID is the abbreviation of proportional integral differential controller. It can effectively prevent undershoot and overshoot in the temperature adjustment process by setting the correct parameters and obtain more ideal control performance. The main parameters of the PID controller are: measured value, set value, proportional coefficient, integral time, differential time, sampling time, adjustment dead zone, etc. The measured value is the temperature value of the traction motor, the set value is the reference value determined according to the specific model of the motor, and the value of the proportional coefficient is a percentage, which is a proportional response to the deviation of the system. Once the system has a deviation, the proportional adjustment immediately produces a regulating effect to reduce the deviation. A large proportional effect can speed up the adjustment and reduce the deviation, but an excessively large proportion will reduce the stability of the system and even cause system instability. The regulating function of the integral time is to eliminate the steady-state error of the system and improve the degree of zero difference. Because there is an error, the integral regulation is carried out until there is no difference, the integral regulation stops, and the integral regulation outputs a constant value. The strength of the integral action depends on the integral time. The larger the integral time, the stronger the integral action and the slower the regulation response. On the contrary, the weaker the integral action, the faster the regulation response. Adding integral regulation can reduce the stability of the system and slow down the dynamic response. The differential time reflects the rate of change of the system deviation signal. It has predictability and can predict the trend of deviation change. Therefore, it can produce advanced control. Before the deviation is formed, it has been eliminated by the differential regulation. Therefore, the dynamic performance of the system can be improved. When the differential time is selected appropriately, overshoot can be reduced and the regulation time can be reduced. The sampling time is the period of the system detecting the motor temperature. The regulation dead zone is when the difference between the measured value and the set value is less than the value of the regulation dead zone, the system does not play a regulatory role, and the stability of the system is improved.
[page]The temperature rise of the motor is the main factor that determines the rated power and rated current of the motor. The degree of heat generation of the motor depends not only on the heat generated per unit time inside the motor, but also on the heat dissipation conditions of the motor. Therefore, ventilation calculation must be performed to set reasonable parameters. When calculating the ventilation of the motor, it is necessary to determine the wind consumption required to remove the heat generated inside the motor. When the motor is working under continuous industrial control, the amount of air blowing through the motor should be able to remove the heat loss generated inside the motor, and the motor works under a stable temperature rise that meets the standard. The formula for calculating the motor ventilation volume is ∑△P∞=CBγB△θBQ, where ∑△P∞ is the loss of the motor under continuous industrial control; CB=1.1 is the density of air (Kg/m3) at a pressure of 760 mm Hg and a temperature of 50 ℃; γB=1000 is the specific heat of air (W·s／Kg·℃)△θB is the temperature rise of the air after passing through the motor, and the required wind consumption is calculated from this .

2 Hardware solution design
Hardware solution design is a key link in auxiliary motor control. The design of the entire system should first meet the principle of fault-oriented safety, and secondly require high reliability, rich functions, high degree of automation, good economy and advanced technology.
The main hardware modules of this system include programmable controller (PLC), temperature sensor, wind speed sensor, speed sensor, current sensor, human-machine interface (HMI) and power module. PLC is the core control device of this system, which realizes multi-channel detection of ambient temperature and motor temperature, detection of fan motor speed and traction motor air flow, and detection of each phase current of auxiliary motor. These detected electrical signals are input from the analog AIO port of PLC, and after being processed by PLC, the control and protection of auxiliary motor are realized. In addition, the digital input DI port and digital output DO port of PLC serve as the input and output of the logic control link of auxiliary motor. Temperature sensor is used to detect ambient temperature and traction motor temperature. Ambient temperature is the reference point of motor temperature rise. For reliability consideration, the ambient temperature detection should detect at least 3 different points, so there should be 3 sensors for detecting ambient temperature. The motor temperature should also be detected by temperature sensor. Each traction motor temperature detection should also be 3 channels to detect the temperature of different parts of traction motor. Since the temperature of traction motor cannot be detected by direct contact, infrared temperature sensor should be used. The function of the wind speed sensor is to measure the wind speed of the traction motor air duct, so as to calculate the ventilation volume of the traction motor, which is used as important data for setting PID parameters. The speed sensor monitors the speed of the fan motor to determine whether the fan is actually started and whether it is working normally. The current sensor detects the current of the fan motor to realize overcurrent protection of the fan motor. The human-machine interface (HMI) realizes the setting of the corresponding working parameters of the system, the display of relevant data and system working status, the recording and storage of data, and the fault detection and diagnosis information. The communication between PLC and HMI is connected by Modbus communication bus, and the communication protocol follows the universal standard Modbus communication protocol to realize information exchange between the two. The design of the hardware solution is shown in Figure 1.

[page]3 Software Design
Software design includes HMI software design and PLC software design. HMI software design is to design the main interface, parameter setting interface, record query interface, curve display interface and system management interface with the help of a special software development tool for text display. The main interface is the interface displayed when the system is working normally, and it is also the default interface entered after the system is turned on. It is mainly divided into traction motor temperature, fan parameters, ambient temperature and locomotive model, operation buttons, system date and time display area. The traction motor temperature display area displays the real-time temperature of each traction motor in turn. The fan parameter display area displays the current speed and air volume parameters of each fan. The bottom of the main interface displays the ambient temperature and locomotive model. The ambient temperature is also real-time. The locomotive model is pre-set in the system management interface. In order to realize the switching of other interfaces in the main interface, four buttons, parameters, records, curves and systems, are designed in the main interface. You can enter the corresponding interface for operation by touching it. In addition, the main interface also displays the current date and time of the system. The operation result of the main interface is shown in Figure 2.

The parameter setting interface is the interface for setting the automatic control mode, action control mode, set value, proportional coefficient, integral time, differential time, sampling time, adjustment dead zone, input upper limit, input lower limit, output upper limit and other system parameters of the PID controller. Different locomotive models have different motor models, and the corresponding PID control parameters are also different. The locomotive model is selected using the drop-down menu in this interface. Each parameter of the PID controller has an upper limit and a lower limit. If the input parameter exceeds the upper and lower limits, the setting is invalid.
The record query interface is the interface for querying historical records. During the operation of the system, if the traction motor temperature rise is abnormal, the temperature is too high, the fan motor speed is abnormal or even stops, the sensor is open, and the parameters are changed, the system immediately starts the event response, records the date and time of the abnormal event, the event name and the recovery time, and saves it in the memory. These historical records can be viewed in this interface or stored in the U disk as a read-only file, and processed on the PC with dedicated software to generate EXCEL format files. The curve display interface displays the temperature of the traction motor that changes continuously over time in real time. One motor corresponds to one curve. All curves are displayed on one screen and distinguished from each other by different colors. The system management interface is used to set the system date and time, and to set and change the operation password.
The PLC software design is implemented by ladder diagram programming in the PLC special software design tool. The main contents include the selection of PLC model, the filter time of switch input, the upper and lower limits of analog input and output, data collection and processing, and the main program design with the automatic adjustment of PID controller parameters as the main function. The communication between PLC and HMI is implemented by the widely used Modbus communication protocol. The communication parameters are set as: baud rate 19200, no check bit, 8-bit data, 2-bit stop bit. In order to meet the fault-oriented safety principle and ensure the normal operation of the locomotive in the case of system failure, a fault diagnosis function is designed in the PLC software. Once a system failure occurs, the control system will stop working immediately. At the same time, the digital output of the PLC is connected to the corresponding logic control circuit and switched to manual control determined by the locomotive working condition.

4 Conclusion
By analyzing the characteristics and requirements of the control object, and under the premise of meeting the fault-oriented safety principle, this paper proposes to use PID control as a control strategy, designs a hardware implementation scheme, and compiles the software of PLC and HMI respectively. Through simulation operation, the control function proposed in this paper is realized.

Reference address：Research on PID Control of Auxiliary Motors in Electric Locomotives

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