1. Introduction
Currently, the vast majority (over 70%) of mine hoists in my country use traditional AC hoist electrical control systems (represented by TKD-A). The TKD control system is a contact-based control system composed of relay logic circuits, large air contactors, and tachogenerators. While the TKD-A series hoist electrical control system has developed its own characteristics over many years, its shortcomings are also obvious. Its electrical circuitry is overly complex, with numerous intermediate relays, electrical contacts, and electrical connections, leading to frequent hoist shutdowns due to electrical faults. New electrical control systems using PLC technology have been successfully applied in mine hoisting practices, achieving good operational experience and overcoming the shortcomings of traditional electrical control systems, representing the trend in AC mine hoist electrical control technology development.
2. Overall Design Scheme
The control circuit structure of the mine AC hoist electrical control system based on PLC technology consists of the following five parts: high-voltage main circuit (including high-voltage commutator, motor, starter cabinet, and power braking power supply), main control PLC circuit, hoisting stroke detection and display circuit, hoisting speed detection, and hoisting signal circuit. The high-voltage main circuit still uses the traditional relay control circuit.
Operating Process: Once a start-up signal is sent from the wellhead or bottom via the signal communication circuit, the start-up conditions are met. The operator pushes the brake handle forward away from the engaged position, releasing the main motor brake. The operator then pushes the operating handle of the master controller to the forward (or reverse) extreme position. The main control PLC, through its program, controls the high-voltage commutator to be energized first, sending a high-voltage signal to the stator winding of the main motor. The main motor starts with all rotor resistance connected, then sequentially disconnects eight resistor segments to achieve automatic acceleration, finally operating on its natural mechanical characteristics. During AC hoist operation, the rotary encoder rotates with the main motor, outputting two A/B phase pulses, which are connected to the A/B phase pulse input terminals of the high-speed counter HSC0 on the main control PLC. The main control PLC automatically determines the increment/decrement counting mode of HSC0 based on the phase relationship of the A/B pulses. The hoisting stroke can be calculated and displayed based on the count value of HSC0. Simultaneously, the main control PLC increments the count based only on the A phase pulses output by the rotary encoder. The hoisting speed can be calculated based on the count value of HSC1 within a constant interval.
3. Hardware Design
3.1 Design of the main circuit of the hoist
The main circuit supplies power to the hoisting motor, provides undervoltage and overcurrent protection, controls the motor's direction of rotation, and regulates its speed. The main circuit consists of a high-voltage switchgear, normally open contacts of the high-voltage commutator, normally open main contacts of the dynamic braking contactor, a dynamic braking power supply unit, the hoisting motor, the motor rotor resistor, normally open main contacts of the acceleration contactor (1jc～8jc), and indicating ammeters and voltmeters mounted on the driver's control panel.
Main drive motor selection: Although squirrel-cage induction motors are simple in structure, inexpensive, and easy to maintain, they are difficult to meet the starting and speed regulation requirements of hoists. Therefore, wound-rotor induction motors are selected as the main drive motors for AC drive systems of mine hoists. The rotor resistance of a wound-rotor induction motor limits the starting current and increases the starting torque, and allows for speed regulation within a certain range. Two 6kV power supplies from the ground substation, one for operation and one for standby, pass through the isolating switch GLK1, oil switch gyd, main contacts of the high-voltage commutator line contactor XLC, and forward (or reverse) contactor ZC (or FC) of the TGG-6 type high-voltage switchgear before reaching the stator of the main motor. The high-voltage switchgear also includes a voltage transformer YH, an undervoltage release coil SYQ, a current transformer LH, and an overcurrent release coil GLQ for undervoltage or overcurrent protection. An emergency stop switch JTK1 and a commutator compartment gate interlock switch LSK are also connected in series in the SYQ coil circuit.
3.2 Braking Circuit Design
Most mine hoists are driven by wound-rotor asynchronous motors, and in most cases, speed regulation is achieved by step-switching rotor circuit resistance. Their braking systems typically employ thyristor-controlled dynamic braking and adjustable brake systems. The former is an electrical brake, and the latter a mechanical brake. During deceleration, when the speed is within the 0-5% range, the electrical brake is activated, and the adjustable brake is inactive. When the overspeed is within the 5-10% range, the electrical brake limits the speed and maintains maximum braking power, while the adjustable brake activates, increasing the total braking torque. When the overspeed exceeds 10%, the overspeed relay GSJ1 activates the safety circuit, and the adjustable brake stops the hoist drum.
The thyristor-based power supply unit mainly consists of two parts: a main circuit and a trigger circuit. This design employs a KZG-type three-phase thyristor-based power braking system. This system is a single-loop power braking system. Speed deviation control and foot pedal control are ORed; whichever signal is larger allows the corresponding control method to function. Therefore, in single-loop control, the driver can use the foot brake for control. However, in foot-operated control, if the hoist overspeeds, the closed-loop system can also provide monitoring and protection.
3.3 Speed Setpoint Loop
The speed setting method generates a speed setting signal based on the stroke principle. In the electrical control system of mine hoists, the cam plate setting method is usually adopted, that is, the cam plate controls the output voltage of the synchro. Since the synchro has no sliding contacts, the voltage change is relatively stable, the operation is more reliable, and the maintenance is less.
When the synchro is used as a setpoint device, the excitation winding is energized with a single-phase 110V AC current, and the output of any two phases from the three-phase synchronous winding is used as the setpoint voltage. Its output voltage is AC; if DC is required, it should be output through a bridge rectifier.
3.4 Power Braking Circuit
The thyristor rectifier and its triggering device are installed as a set in the power supply cabinet. The output voltage of the power braking power supply is related to the level of the control signal voltage input to the triggering device.
The control signal voltage consists of an OR gate circuit composed of two loops. When either loop meets the trigger requirement, the thyristor is activated to initiate braking. One loop automatically controls the output of the CF3 magnetic amplifier and the power brake output based on the speed deviation between the actual speed and the given speed. The other loop allows the driver to manually adjust the output of the synchro (CD2).
When the power braking system is manually controlled, the driver controls the foot pedal, which drives the synchro CD2 to generate a control voltage. During adjustment, this voltage should be matched with the output of the magnetic amplifier CF3. When the heel is just pressed down, before the toes are fully depressed, it's equivalent to the control switch closing, energizing the DZC and engaging the thyristor-driven power braking. However, at this time, the output of the synchro CD2 is very small, and the power braking current is minimal. When the driver's toes are fully depressed, the output of the synchro CD2 is at its maximum.
In the foot pedal power brake and cf3 output circuit, two diodes, z1 and z2, respectively form an OR gate circuit. These two control signals are connected in parallel and do not affect each other.
3.5 Stroke Detection and Display
A rotary encoder converts the hoist's operating position into pulses. The PLC counts these pulses at high speed and automatically generates relevant data about the hoist's position through corresponding calculations, which is then transmitted to the storage unit of the PLC's internal high-speed counter. To improve the pulse accuracy of the counter, an E6C-CWSC reversible rotary encoder from Omron Corporation of Japan is selected, as it offers high pulse accuracy and does not lose pulses at low speeds.
To facilitate operation by the hoist operator, the hoist's electrical control system needs to be equipped with a reliable travel display device (also known as a depth indicator) to show the position of the hoisting container in the shaft. This paper designs a hoist position display using three LED seven-segment displays based on the travel distance (0-570m) measured by the encoder.
3.6 Auxiliary Circuit Design
The auxiliary circuit is used to supply power and control auxiliary equipment. The auxiliary circuit operates on AC 380V and is powered by two circuits. Loads supplied by the auxiliary circuit include: thyristor-driven dynamic braking power supply unit, brake oil pump motor, and lubricating oil pump motor, etc.
4. Calculation of rotor resistance of the hoist main motor
The calculation of the motor rotor resistance plays a crucial role in the normal operation of lifting equipment. When calculating the starting resistance, the number of preparatory and acceleration stages should be determined first. This is because the selected number of stages directly affects the increase or decrease of the maximum switching torque and the increase or decrease of the average starting acceleration, and may even necessitate increasing the motor capacity due to insufficient overload capacity. Therefore, a comprehensive consideration should be given to select an economically reasonable number of stages. Generally, the selection of the preparatory and acceleration stages is shown in the attached table.
There are many methods for calculating the three-phase balanced starting resistance, but they can be basically divided into two types: one is to calculate the starting resistance based on a given acceleration, and the other is to calculate it based on making full use of the motor's overload capacity. Because the first method is simple and accurate, this method is used in this paper.
5. PLC Control System Design
5.1 Main PLC Control Circuit Design
Based on the operating mode of hoists and the inherent characteristics of coal mining enterprises, the application of PLCs in the electrical control systems of domestically produced mine hoists has developed rapidly. However, from the perspective of field use, currently, in the control systems of domestically produced coal mine hoists, PLCs are mainly used to process switching quantities, replacing the numerous relays, contactors, complex wiring, and signal display systems in older hoist control systems. The analog quantities and automatic adjustment processes in the braking system, which are crucial for the safe operation of the hoist, are mostly handled using ordinary electronic modes such as adjustable brakes and thyristor-controlled braking with semiconductor devices and operational amplifiers. During use, zero-point drift and electronic component failure frequently occur, and there are drawbacks such as difficulty in maintenance and re-adjustment, and poor reliability, thus reducing the reliability of the hoist electrical control system. Therefore, in-depth research on using PLCs to control coal mine hoist control systems is essential to address these issues.
The main control unit, a programmable logic controller (PLC), is designed in this paper. It consists of a CPU226 main unit and two I/O expansion modules, EM223 and EM222, and has 40 input points and 40 output points.
5.2 PLC Control Software Design
(1) The initialization subroutine is used to perform the following operations on the high-speed counters hsc0 and hsc1: write control word, define working mode, clear to zero, write set value, set timer interrupt, connect interrupt, and start counting.
(2) The control of brake oil pump, lubricating oil pump, power brake power supply, five-way valve solenoid, four-way valve solenoid and safety valve solenoid, etc., belongs to the control of auxiliary equipment required for the safe operation of AC hoist.
(3) The display and control of brake oil overpressure signal, brake oil overheat signal and lubricating oil overpressure signal are used to display and control the working status of AC hoist.
(4) The rope adjustment interlocking circuit plays a safety protection role during the rope adjustment process. When the double drum hoist changes to horizontal rope adjustment, the rope adjustment changeover switch 1hk-3 is disconnected, causing the rope adjustment interlocking link to be connected in series with the safety circuit. During normal operation, 1hk-3 is connected, and the rope adjustment interlocking does not work.
(5) The lifting signal circuit is used to prepare for the start-up or deceleration of the AC lifting motor.
(6) The position measurement subroutine is used to measure the position of the hoist in the mine.
(7) The stroke display subroutine displays the current stroke position based on the number of pulses from the rotary encoder.
(8) The deceleration signal circuit and deceleration signal bell are used for deceleration control and to issue a bell prompt signal.
(9) The automatic reversing circuit and the manual forward and reverse reversing circuit are used to control the forward and reverse rotation of the AC hoisting motor in automatic and manual modes, respectively.
(10) Safety circuits are used to prevent and avoid accidents in AC hoists.
(11) The timer control circuit and rotor resistance switching control are used for rotor resistance switching control when the AC boost motor starts or decelerates.
(12) The power braking circuit is used for the control of power braking power supply input and output.
(13) Foot brake interlock and working brake relay are used for braking control of AC hoist motor.
6. Conclusion
The hoist control system combines PLC control with a TKD-A control system, offering advantages such as reliability, safety, and ease of implementation. The PLC is used to implement the main control logic of the hoist, increasing control functions and achieving efficient automated production. The key is to fully leverage the advantages of the PLC, utilizing its comprehensive measurement and control mechanism to solve problems related to speed measurement and protection, achieving seamless integration with the original system, improving the overall system performance, and achieving high output with low investment. However, some issues still require further improvement, such as network communication functionality and advanced control technologies and strategies like intelligent control. Further functional expansion based on existing PLC technology will further enhance the modernization level of mine hoisting electrical control systems in my country.
About the Author
Xu Chengyi, male, graduated from Liaoning Institute of Technology with a major in Electrical Engineering and Automation. He currently works at the Technology R&D Department of Dalian Sanyo Refrigeration Co., Ltd.
References
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