Application of PLC in overhead crane frequency conversion control system
Xing Yuan
(Qingdao Rubber Tire Design Institute, Qingdao, Shandong 266042)
Abstract: Through research and analysis of the overhead crane currently used in the factory, the speed of the entire system is adjusted by the speed signal given to the PLC by the master controller. The motors of the main trolley, trolley, main hook and auxiliary hook of the bridge crane need to run independently. The main trolley is driven by two motors at the same time. Therefore, the entire system has 5 motors, 4 frequency converters, and is controlled by 1 PLC.
Keywords: overhead crane; PLC; frequency converter; motor
Intermediate Classification Number: TP9 Document Identification Code: B
Foreword
A bridge crane is a type of overhead crane that runs on elevated tracks. The bridge frame of the bridge crane runs longitudinally along tracks laid on both sides of the elevated structure, while the trolley runs laterally along tracks laid on the bridge frame, forming a rectangular working area. This allows for full utilization of the space beneath the bridge frame for lifting and transporting materials without obstruction from ground equipment. Bridge cranes are widely used in indoor and outdoor warehouses, factories, docks, and open-air storage yards. Bridge cranes can be classified into three types: ordinary bridge cranes, simple beam bridge cranes, and metallurgical-specific bridge cranes.
Crane traveling mechanisms can be broadly categorized into two types: centralized drive, where a single electric motor drives a long transmission shaft to power both drive wheels; and separate drive, where each drive wheel is driven by its own electric motor. Medium and small-sized bridge cranes often employ a "three-in-one" drive system, integrating the brake, reducer, and electric motor. For large-capacity ordinary bridge cranes, universal couplings are commonly used in the drive unit for ease of installation and adjustment. Crane traveling mechanisms typically use only four drive and driven wheels. If the lifting capacity is very large, additional wheels are often added to reduce wheel pressure. When there are more than four wheels, an articulated equalizing frame device must be used to ensure the crane's load is evenly distributed across all wheels.
A stripping crane is used to forcibly remove steel ingots from the ingot mold. The trolley has a specialized stripping device, and the stripping method depends on the shape of the ingot mold: some stripping cranes use a top bar to hold down the ingot and a large clamp to lift the mold; others use a large clamp to hold down the mold and a small clamp to lift the ingot. A charging crane is used to add furnace charge to the open-hearth furnace. The lower end of the main trolley's column is equipped with a boom to lift the charge box and send it into the furnace. The main column can rotate around a vertical axis, and the boom can swing up and down and rotate. The auxiliary trolley is used for auxiliary operations such as furnace repair. A forging crane is used in conjunction with a hydraulic press to forge large workpieces. A special tilting device is suspended on the hook of the main trolley to support and tilt the workpiece; the auxiliary trolley is used to lift the workpiece.
A grab-bucket bridge crane in the coal preparation workshop of a glass factory is used for loading coal and unloading glass, lifting it from the ground onto a train parked nearby. The bridge crane's electrical drive system consists of two trolley motors, one gantry motor, one grab bucket motor, and one grab bucket lifting motor, all wound-rotor AC motors. It uses a rotor resistance system for starting and speed regulation. Due to the harsh working environment, dust and harmful gases cause significant corrosion to the motor slip rings, carbon brushes, and 21 contactors. Combined with heavy workloads, unreliable operating procedures, high inrush currents, severe contact corrosion, carbon brush sparking, and frequent burnout and breakage of the resistors in the motor and rotor windings, averaging 2.5 major failures per month, significantly impacting production. The rotor resistance speed regulation system has a soft mechanical characteristic; the speed changes with load variations, resulting in poor speed regulation. The resistors also generate heat over time, leading to significant energy waste and low efficiency. Therefore, to fundamentally solve the high failure rate of the bridge crane, it is necessary to utilize a PLC as the control device and completely change the wound-rotor motor resistance speed regulation method.
120/5t overhead crane basic parameters
Overhead cranes have long been serialized and standardized. Their unified parameters are as follows:
1. Mass and load parameters: Lifting capacity G, effective lifting capacity Gp, rated lifting capacity Gn, total lifting capacity Gt, maximum lifting capacity Gmax, lifting moment M, overturning moment MA, total crane mass Go, wheel pressure P
2. Crane Dimensions: Radius L, Maximum Radius Lmax, Minimum Radius Lmin, Effective Cantilever Extension L, Lifting Height H, Lowering Depth h, Lifting Range D, Boom Length Lb, Crane Inclination Angle
3. Motion speed parameters: hoisting (lowering) speed Vn, micro-lowering speed Vm, slewing speed ω, crane (trolley) traveling speed Vk, trolley traveling speed Vt, luffing speed Vr
4. Parameters related to the crane's running path: Span S
5. General performance parameters: Working class, mechanism working class
6. Span refers to the horizontal distance between the center lines of the running tracks of a bridge crane, measured in meters (m). The distance between the center lines of the trolley running tracks of a bridge crane is called the trolley gauge. For ground-mounted jib cranes, the distance between the center lines of the running tracks is called the crane's gauge. Table 1 shows the technical parameters of a general bridge crane. Table 2 shows the technical parameters of a general bridge crane.
2. Selection of overhead crane motor
The speed ratio of the hoisting and traveling mechanisms of cranes is generally no greater than 1:20, and they operate on an intermittent duty system, typically with continuous power supply above 60%, and the loads are mostly high-inertia systems. Strictly speaking, variable frequency motors have smaller moments of inertia and faster response speeds, allowing them to operate at speeds much higher than their rated speeds. These characteristics are not specific to cranes. Ordinary motors and variable frequency motors have essentially the same characteristics under discontinuous operation; however, during continuous operation, cooling considerations limit the torque application value of ordinary motors. The only difference between ordinary motors and variable frequency motors during continuous operation is that the variable frequency drive characteristics of ordinary motors are slightly inferior to those of variable frequency motors.
A standard frequency converter ensures 150% overload torque for ordinary motors on a crane within a dispatch ratio of 1:20. Furthermore, crane motors are often used for high inertia and short-time duty, with typically idle time greater than or slightly less than working time. The motor can withstand 2.5 times its rated current during startup, therefore the 1.1 times current caused by high frequency can be disregarded. However, if the motor is required to operate continuously at a speed ratio greater than 1:4 throughout the entire duty cycle, an air-cooled motor must be used.
The motor used for the lifting mechanism is a variable frequency motor suitable for frequent starts, low moment of inertia, and high starting torque. Currently, four-pole motors are the preferred choice for variable frequency motors abroad. The motor power is:
In the formula, p represents power, in kW.
w—Rated lifting capacity (at minimum radius) + weight of the hook + weight of the wire rope, n;
v — lifting speed, m/s;
η – Mechanical efficiency.
When using a frequency converter to drive an asynchronous motor, the motor insulation deteriorates due to the commutation impulse voltage of the frequency converter and the impulse voltage (surge voltage) generated by the instantaneous opening and closing of the switching elements. For voltage-type PWM frequency converters, the wiring distance between the frequency converter and the motor should be shortened as much as possible, or a damping circuit (filter) should be added.
The selection of motor power must be based on production needs. Generally speaking, the power of motors used in cranes is about 10% greater than that used in general industrial production machinery.
The selection of the electric motor depends on the following two main conditions:
(1) Heat generation. When an electric motor is working, it converts electrical energy into mechanical energy to do work. On the other hand, due to the impedance of the motor windings themselves, some electrical energy is consumed and converted into heat energy, causing the motor temperature to rise. Due to limitations in size and structure, the internal insulation material of an electric motor has poor heat resistance and is prone to aging. When the temperature exceeds the allowable limit of the motor, the insulation is damaged, and the motor will burn out. The nameplate of an electric motor specifies the temperature rise, which refers to the difference between the allowable temperature rise of the stator after heating up under rated load and the ambient temperature.
(2) Overload capacity. All kinds of motors have a certain overload capacity. The overload capacity tm of AC motors is the ratio of the maximum torque m to the rated torque mc, that is, the overload capacity of AC asynchronous motors used in general cranes is 2.5 to 3.3.
3. Overhead Crane Electrical Principles
1) The electric drive system for the hook lifting and opening/closing mechanism each has one motor. Since the motors in both systems operate simultaneously, they cannot share a single frequency converter. The operator's cab control panel has separate master controllers Ks and Kf for the hook lifting and opening/closing mechanism. Control commands are issued by the master controllers Ks and Kf on the operator's cab control panel, and after PLC calculation, control frequency converter commands are issued: raise, lower, open, close, accelerate, and decelerate. The opening of the hook lifting and opening/closing mechanism brake is activated by a relay output from the frequency converter, which, after PLC logic calculation, drives the brake control contactors Cs and Cf, causing the brake to actuate.
The frequency converter has various protection functions such as short circuit, overvoltage, phase loss, undervoltage, overcurrent, overspeed, and grounding protection, as well as fault self-diagnosis and display alarm functions. When the frequency converter experiences faults such as short circuit or overcurrent, it sends a fault signal to the PLC and stops outputting. After receiving the fault signal, the PLC cuts off the power supply to the frequency converter, controls the brake to engage, and issues an alarm signal.
In addition to the internal protection functions of the frequency converter, the lifting and opening/closing mechanism of the hook is also equipped with circuit protection.
(1) Zero position protection, which is implemented by the zero position contact of the master controller;
(2) Limit protection is achieved by a height limiter;
(3) The line is equipped with a low-voltage circuit breaker for short-circuit protection.
2) Electrical drive system of trolley running mechanism: The trolley running mechanism is driven by a variable frequency motor and controlled by a frequency converter. The system control method is similar to that of the electrical drive system of the hoisting mechanism.
3) Electrical Drive System of the Trolley Traveling Mechanism: The trolley traveling mechanism is driven by two variable frequency motors and controlled by one frequency converter. The system control method is similar to that of the trolley traveling mechanism's electrical drive system. Figure 1 shows the on/off table of the trolley master controller.
4. PLC control
4.1 PLC Selection
This system uses an Omron CPM2AE-60CDR-A PLC from Japan to implement the logic control of the entire system, simplifying wiring and reducing power consumption. The inclusion of circuit fault diagnosis and effective electrical protection functions gives the system a degree of intelligence and higher reliability. The overhead crane PLC logic control diagram is shown in Figure 2.
Its main functions are as follows:
1) Inverter operation and stop control;
2) Control the brake to ensure that the motor can stop in a timely manner, neither too early nor too late;
3) Switching the control mode of the lifting frequency converter;
4) Electrical interlocking protection control;
5) When any one of the lifting or switching frequency converters alarms, both frequency converters can immediately stop output and brake simultaneously.
6) If power is lost at any time, the system will immediately stop operating and the brakes will engage.
4.2 PLC Control Allocation and Control Program
4.2.1 I/O Point Assignment Diagram
4.2.2 PLC Control Program
Setting up a common program can make full use of the PLC's I/O points and reduce external wiring. The program mainly realizes the forward and reverse rotation of the motor and its acceleration and deceleration, while using auxiliary relay outputs to prepare for subsequent program calls.
The program mainly uses comparison instructions to accelerate or decelerate the motor. When I0.1 or I0.2 is pressed, the numbers stored in memory VB100 change sequentially between 1 and 5, controlling the amount of rotor resistors connected in series to achieve speed regulation.
When designing the program, the design method in Scheme 1 can be continued, using the master controller program as a common program to save some input points. The common program is programmed according to the speed input numbers of different inverter models. Except for the power supply, all PLC outputs use small relays. The program controls the operation of these small relays to achieve forward and reverse rotation, lifting, forward and backward movement, and speed regulation of the motor. Since the program design is largely the same as Scheme 1, its speed output program is as follows.
LDNI0.1 // Stop button
AQ0.0 // Power-on
LPS
LDB=VB100,1 // Level 1
OB=VB100,2//2 levels
OB=VB100,3//3 gears
OB=VB100,4//4 gears
OB=VB100,5//5 gears
ALD
LPS
AI0.3
=Q0.1//Forward rotation
LPP
AI0.4
=Q0.2 // Reverse
LRD
LDB=VB100,1
OM10.0
OM10.1
ALD
=Q0.3//Variable frequency speed output selection 1
LRD
LDB=VB100,2
OM10.0
ALD=Q0.4 // Variable frequency speed output selection 2
LRD
LDB=VB100,4
OM10.1
OB=VB100,3
ALD
=Q0.5//Variable frequency speed output selection 3
=M10.0
LPP
AB=VB100,5
=M10.1
Caicai
