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Application of PLC in the transformation of driving electric control circuit

Authors: Li, Hu Jiejia

Keywords: electric drive variable frequency speed regulation programmable controller

1 Introduction

The rotor of wound asynchronous motor is connected with frequency-sensitive resistor in series, which is used for starting and speed regulation of mine grab crane in factory. This relay-contactor control mode has the following problems in actual operation:

(1) The driving work environment is harsh and the work tasks are heavy. The series frequency sensitive resistor of the motor burns out from time to time, which leads to frequent burning of the motor;

(2) Due to the vibration of the machine body and the conductive dust environment, the relay-contactor control system has poor reliability, high failure rate, difficult maintenance and high maintenance cost, which makes the maintenance workers tired;

(3) The rotor is connected in series with frequency-sensitive resistor for speed regulation, with soft mechanical characteristics. When the load changes, the operation is unstable, and the intermediate frequency sensitive resistor runs for a long time, which leads to a serious waste of electric energy;

(4) Contactors frequently open and close under high current, which causes serious harmonic pollution and low power factor.

Therefore, the factory adopts PLC instead of relay-contactor control, and frequency converter instead of motor rotor series frequency sensitive resistor. After the transformation, the operation effect is remarkable, which solves the above problems.

2 PLC-controlled variable frequency drive system for driving vehicles

2. 1 system composition

Walking trolley, trolley, grab lifting and grab opening and closing motor all need to run independently. The trolley is driven by two motors at the same time, and the trolley, grab lifting and grab opening and closing are driven by one motor respectively. The whole system has five motors. In order to ensure the safe operation of each part and not affect each other, four frequency converters are used to drive and four PLC control each other. The system composition is shown in figure 1:

PLC receives the speed control signal of the main controller, which is a digital control signal with the signal level of AC220V V V. These signals include: positive and negative signals sent by the main controller, motor overheating protection signals, safety limit signals, start-up, emergency stop, reset, zero lock and other signals, which are all input by sink. According to these signals, PLC completes the logic control function of the system, and sends control signals such as start, stop, forward rotation, reverse rotation and speed regulation to the frequency converter, so that the motor is in the required working state.

The frequency converter receives the control signal provided by PLC, and outputs the variable frequency and variable voltage power supply to the motor according to the setting, so as to realize the speed regulation of the motor. Operators send various control signals to PLC through the main controller according to actual needs.

When the lifting motor lowers the weight, the motor rotates reversely. Due to the acceleration of gravity, the motor is in regenerative braking state, and the mechanical energy of the driving system is converted into electrical energy, which is stored at both ends of the filter capacitor of the voltage source inverter, so that the DC voltage keeps rising and even breaks down the electrical insulation. When the voltage rises to the set value, the energy drain resistor is connected to consume this part of the energy of the DC circuit to ensure the safe operation of the inverter.

2.2 communication between frequency converter and PLC

The system adopts the field bus mode to control the frequency converter, instead of the traditional analog or switching mode. In this system, the trolley and the lifting frequency converter are connected to the Profibus-DP bus through optional modules. Considering the real-time and stability of data transmission, the system chooses PPC-3 as the data transmission format, and the baud rate is 387.5kbps. After adopting the bus structure, the system is further optimized, as shown in the following figure:

(1) simple wiring

Only 1 two-core shielded twisted pair is needed, and other methods require at least 4 cables, which reduces the maintenance workload.

(2) given stability

The jitter of analog reference caused by signal drift, electromagnetic interference and many other factors is avoided, and the system speed reference is more reliable.

(3) continuous speed

Compared with the system that uses the switch value as the speed reference value, the speed reference value is changed from discrete value to continuous value, so that the frequency converter can accept the speed fine-tuning instruction of PLC and realize the balance of lifting operation.

2.3 standby emergency system

When the bus trunk cable or a point on the bus is damaged, the system may not work normally. So there is a backup system in the system to prevent the bus from being used normally in an emergency, but it can't stop working. The frequency converter has two control modes, one of which adopts bus communication for normal control; One set adopts switch control in case of emergency. The two groups of parameters are switched by PLC, and the speed setting of the handle gear is completely consistent, so the switching of the two groups of parameters is imperceptible from the use point of view.

2.4 Synchronization and Rectification

When the crane lifts the grab bucket, the system will enter the automatic deviation correction mode to ensure that the position of the steel wire is synchronized when the hook is lifted. Because the adjustment of friction torque and mechanical braking during mechanical installation is not completely consistent, the system does not adopt dynamic real-time rectification, but adopts a compromise scheme. Its working principle is as follows: firstly, the system sets two thresholds in PLC, the threshold 1 is used to start the hook automatic rectification, and the threshold 2 is used to end the automatic rectification; Secondly, PLC reads the data of the encoder installed on the lifting drum and calculates the lifting height in real time; Thirdly, PLC compares the two read-in lifting heights. When the difference between the two heights is greater than the threshold 1, the PLC will superimpose a slight speed deviation on the reference speed determined by the handle. When the difference between the two heights is less than the threshold 2, it cancels the deviation and further reduces the lift difference through inertia. Finally, when the speed of grab crane motor is given, PLC will download the calculated and synthesized speed value to the frequency converter through Profibus-DP.

3 PLC software and hardware design and application

3. Hardware design of1PLC

The crane cart, trolley, grab lifting and grab opening and closing motors are controlled by different PLC. The crane cart, trolley, lifting and opening/closing motor all run in electric working state, and the control structure, software and hardware realization of frequency converter and PLC are basically the same. The running state of hoist motor includes electric, reverse braking and regenerative braking, and the control structure between inverter and PLC is more complicated than that of cart and trolley. Take the hoisting motor as an example, the I/O wiring of its PLC is shown in Figure 2, and the wiring diagram of frequency converter is shown in Figure 3.

3.2 Working Process of Automobile

When the cab door and the crossbeam track door are closed, the normally closed contacts of safety switches 1# and 2# are opened, the emergency stop switch is closed, and the main controller is set to zero. Only in this way can you press the start button and turn on the power. When the main controller is placed in the upshift gear, the motor rotates forward. By adjusting the speed gear, the inverter is controlled to output different voltages, so as to adjust the speed of the grab hoist motor. When the main controller is placed in the third gear with full load, the motor rotates forward and the motor is in the reverse braking state. When the main controller is placed in the 2nd gear down gear and the load is heavy, it is a forced down stage, and the motor reverses direction. Under the action of gravity acceleration, the motor enters a regenerative braking state. In addition, when the motor quickly switches from a stable high speed to a low speed, the motor will also enter a regenerative braking state. When the main controller is set to the downshift of 1, the motor reverses and is in an electric state. During operation, the motor stops running for whatever reason, and the original three-phase hydraulic brake is retained to prevent the heavy objects from falling rapidly.

In case of emergency, you can press the emergency stop button, on the one hand, the mechanical brake will act, on the other hand, the emergency stop control terminal EMS of the inverter will be connected, and the inverter will stop working. When the motor of grab crane trips due to fault, the thermal relay acts and the motor is overloaded. After troubleshooting, press the reset button to open the reset control terminal RST of the inverter, so that the inverter can be restored to the running state.

3.3 PLC software design

FXON series PLC is selected and modular programming is adopted. The specific modules are as follows:

(1) height conversion function block. Used to convert Gray code into binary code, and convert binary code into lifting height and deviation adjustment of lifting height;

(2) Inverter switch control function block. Used for starting, stopping and speed control of frequency converters of cranes, trolleys and grab cranes;

(3) Communication control function block of frequency converter. Used for starting, stopping and speed setting of frequency converters for carts, carts and lifting motors. It is also used to read the control word and status word of the frequency converter. Fig. 4 is the software control flow chart of the trolley, and the software flow charts of the trolley, the lifting motor and the opening and closing motor are similar to those of the trolley.

3.4 Safety protection measures

(1) Power distribution part: In addition to phase-missing, overcurrent and short-circuit protection, two safety switches are set at the end beams and platforms on both sides of the vehicle. Only when the switch is closed can the vehicle run. There is also a boarding request and answer button on the train for other staff to board the plane safely during driving.

(2) Inverter: The ACS600 series inverter has motor overload, phase loss, grounding, overcurrent and DC bus overvoltage protection, and the grab hoist motor and trolley inverter have communication fault monitoring function when switching to bus control mode.

(3) Travel switch protection: All mechanisms have travel limit protection. In the single-action state, the trolley and the grab bucket lifting limit switch are independent; Under the linkage working condition, the rear limit of trolley 1 and the front limit of trolley 2 are the allowable conditions of the linkage working condition, and the front limit of trolley 1 and the rear limit of trolley 2 are the trolley limits. As long as there is a limit action of lifting 1 and lifting 2, it is regarded as the lifting limit.

(4) Other protection: All institutions have zero connection protection and overcurrent protection. The grab hoisting mechanism also has overload protection and overspeed protection. When the overspeed switch acts, disconnect the power supply of the main contactor of the frequency converter.

4 conclusion

Application of frequency conversion drive system controlled by PLC in driving. The speed, acceleration time and braking time of each motor can be set according to the actual working conditions, which is very convenient. From the operation results, the motor speed runs smoothly when the load changes. The equipment failure rate is greatly reduced, the motor burning loss is obviously reduced, and the influence of higher harmonics on the power grid is reduced. When the equipment is overhauled, the speed of troubleshooting is obviously accelerated, and the amount of equipment maintenance is greatly reduced.

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