Research on Vector Control of Variable Frequency Speed ​​Regulation for Cranes

Foreword

Bridge cranes are widely used in metallurgical enterprises and other industries, primarily for lifting and transferring objects. They operate in harsh environments and handle heavy workloads. Their proper functioning directly impacts production efficiency, task completion, and even the safety of personnel and equipment. Over decades of development, Chinese bridge crane manufacturers and users have accumulated experience and continuously improved their designs, manufacturing processes, equipment operation, maintenance, and management, driving technological advancements in bridge cranes. However, structural cracking still occurs frequently in actual use. This is primarily due to mechanical fatigue caused by frequent overloading and excessive mechanical vibration and impact. Therefore, in addition to mechanical design improvements, improving AC electrical drives and reducing starting and braking impacts are also crucial aspects.

Traditional crane drive schemes generally employ: (1) direct starting of the motor; (2) speed regulation by changing the number of motor pole pairs; (3) rotor series resistance speed regulation; (4) eddy current brake speed regulation; (5) thyristor cascade speed regulation; and (6) DC speed regulation. The first four schemes are all stepped speed regulation, with a small speed range, unable to operate at high speeds, and can only regulate speed below the rated speed; the starting current is large, causing a large impact on the power grid; mechanical braking is often performed at the rated speed, causing a large impact on the crane mechanism and severe wear of the brake shoes; the power factor is low, below 0.2-0.4 under no-load or light-load conditions, and below 0.75 even under full load, resulting in large line losses. Currently, the control technology of cascade speed regulation products is still in the simulation stage, and the control system has not yet achieved good speed regulation performance and starting and braking performance, good protection function and system monitoring function. Therefore, DC motors are sometimes used. However, DC motors have complex manufacturing processes, high usage and maintenance requirements, and a high failure rate.

This paper studies an electric double-girder bridge crane, which consists of four parts: the hoisting trolley, the bridge metal structure, the bridge traveling mechanism, and the electrical control equipment. The mechanism mainly refers to the main hoisting mechanism, the auxiliary hoisting mechanism, the trolley traveling mechanism, and the crane traveling mechanism. At the request of on-site personnel and to accommodate operating habits, the crane's speed control still uses the original master controller and cam controller. The five pairs of contacts of the master controller are used to obtain six speeds output by the frequency converter. The acceleration and deceleration times of the motor can be changed through the frequency converter settings. In the electrical control system, power is generally delivered to the central electrical unit via a cable reel. The crane uses a low-voltage power supply system, and the electrical control components are concentrated in the operator's cab and electrical room. Safety protection devices are installed in appropriate locations.

1. Basic Principles of Variable Frequency Speed ​​Control

Figure 1 Constant voltage frequency ratio control characteristics

2. Control circuit composition of the frequency converter

The control circuit, comprising the main control circuit, signal detection circuit, gate drive circuit, external interface circuit, and protection circuit, is the core component of the frequency converter. The quality of the control circuit determines the performance of the frequency converter. Its main functions are to control the inverter's switching, control the rectifier's voltage, and perform various protection functions.

With the rapid development of power semiconductor devices and microcomputer control technology, new breakthroughs in power frequency conversion technology have been promoted. Pulse Width Modulation (PWM) technology, which developed in the late 1970s, has become the most commonly used control strategy for power switching devices in frequency converters. SPWM (Sinusoidal PWM) is a commonly used technique. It usually uses a modulation method, that is, using a sine wave as the modulating signal and the signal to be modulated as the carrier. By modulating the carrier, a SAM waveform can be obtained. An isosceles triangular wave is usually used as the carrier because the width and height of the isosceles triangular wave are linearly related and symmetrical. When it intersects with the sine wave modulating signal, if the switching device in the circuit is turned on or off at the intersection point, a pulse with a width proportional to the amplitude of the sine wave can be obtained, which meets the requirements of SPWM control. The ratio of the frequency fc of the triangular carrier wave to the frequency fr of the sine modulating wave, i.e., fc/fr=Nc, is called the carrier ratio. Using the generated SPWM wave to control the switching device of the inverter, a rectangular pulse train output voltage with equal amplitude and pulse width varying according to a sine law can be obtained. The frequency fr of the sinusoidal modulation wave is the output frequency f1 of the inverter. Changing fr changes the amplitude of the triangular carrier wave f1. Since the amplitude of the triangular carrier wave is constant, changing the amplitude of the sinusoidal modulation wave changes the area of ​​the rectangular pulse, thereby changing the output voltage amplitude.

3. Variable frequency speed regulation vector control

The basic idea of ​​vector control is to transform the physical model of the asynchronous motor into an equivalent model similar to that of a DC motor, and then control it in the same way as a DC motor. The principle of equivalence is that the magnetomotive force generated in different coordinates is the same.

As the principle of electric motors states, the current in the three-phase stator windings of an asynchronous motor generates a rotating magnetic field with an angular velocity of ω1 in space. If two mutually perpendicular windings, M and T, rotate synchronously with this rotating magnetic field, and DC currents iM and iT flow through these windings respectively, the resulting magnetomotive force (MOMF) is equivalent to the combined three-phase MOMF, and each MOMF has the same amplitude, speed, and direction. Furthermore, if the axis of the M winding is parallel to the direction of the combined three-phase rotating magnetic field, then iM is equivalent to the excitation current component iT of the motor, and the torque current component of the motor. Adjusting the magnitude of iM can change the torque when the magnetic field is constant. The control principle of a motor composed of such windings is the same as that of a DC motor.

In the actual equivalent transformation, the stator currents iA, iB, and ic of the asynchronous motor in the three-phase stationary coordinate system are first transformed into equivalent AC currents iα and iβ in the two-phase stationary coordinate system through a three-phase/two-phase transformation. Then, through a field-oriented rotational transformation, they are equivalent to DC currents iM and iT in the synchronous rotating coordinate system. The equivalent motor winding model is shown in Figure 2.

4. Selection of frequency converter

This system uses Siemens frequency converters, which are reasonably priced, come with complete theoretical calculation sheets and recommended values ​​for auxiliary components, making it easier for users to choose the right one.

The average starting torque of the hoisting mechanism is generally 1.3-1.6 times the rated torque. Considering factors such as power supply voltage fluctuations and the requirement to pass a 125% overload test, its maximum torque must be 1.8-2 times the load torque to ensure safe operation. An equivalent frequency converter can only provide less than 150% overload torque; therefore, a 200% torque can be obtained by increasing the converter capacity (Yz type motor) or simultaneously increasing the converter and motor capacity (Y type motor). At this point, the converter capacity is...

Table 1. Inverter parameters for each organization

5. Conclusion

As required, this system must allow the operator to monitor the on-site operating status and data from the control room. Users in the control room can set the inverter's operating frequency, start and stop the motor via a human-machine interface (HMI). Furthermore, inverter fault information should be displayed on the HMI to alert the user. By using an inverter to achieve speed regulation of the crane motor, combined with the powerful functions and reliability of a PLC, and the excellent HMI and communication capabilities developed based on configuration software, remote control of the motor's operating parameters can be adjusted from the operator's control room.

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