Mar 26, 2021
Research on SAFESAVE frequency converter in AC hoist control system
The AC wound motor used in mine hoists usually adjusts its speed by switching its rotor resistance. But the motor relies on the rotor resistance to obtain the low speed, and its operating characteristics are relatively soft. When the lifting container passes a given deceleration point, due to different loads, different decelerations will be obtained, and stable low-speed crawling cannot be achieved, resulting in an inaccurate parking position and failure to load and unload normally. The operator applies mechanical brakes at the same time, and uses the combined characteristics of brake braking and motor driving to obtain the required deceleration and low-speed crawling. In doing so, not only the power consumption is large, the brake shoe wears a lot, but the operator is very stressful, and the safety and reliability are poor.
The thyristor cascade speed control automatic hoist can obtain better control characteristics. But there are many electronic control equipment and large capacity. In order to obtain the braking torque in the deceleration phase, a set of dynamic braking device is needed, which makes the system complicated and increases the investment. Especially for wound motors above 500kW, the rotor voltage is about 700V, which makes the selection of thyristor devices difficult.
When the AC hoist only uses dynamic braking, there will be multiple conversions of braking-electrical and electric-brake during the deceleration crawling stage, in order to obtain an average, rather than stable crawling speed, which can meet the requirements of longer crawling distances. Lifting machine. This method requires the final drive to have two main shafts, and an air bag clutch is added, which increases the complexity of the mechanical structure and manufacturing process. The biggest weakness of dynamic braking is that it cannot provide positive torque. When the system needs low-speed positive force crawling, it is necessary to switch from dynamic braking to high-voltage operation, and implement the pulse crawling of the secondary power supply in the crawling stage. This method has relatively soft mechanical properties and is difficult to control.
Low-frequency braking is used, that is, the stator winding of the motor is disconnected from the three-phase power grid (6kV, 50Hz), and then connected to a low-frequency power supply with the same voltage and phase sequence. The low-frequency drive of the hoist makes the motor run in the regenerative braking zone during the deceleration phase, and runs in the electric zone during the creeping phase. In addition, the lifting motor has a natural transition from the braking state to the electric state. As a kind of AC speed regulation scheme that is well applied in the range of high power and low speed, the AC frequency converter is easy to adjust the frequency range. As a low frequency power supply, it is suitable for AC hoists for various operations. Digitization is the development trend of modern transmission technology, and the realization of full digital control is a new topic in the automation of AC hoists.
2 AC-AC inverter
The AC/AC variable frequency speed control system is a variable frequency speed control system that directly converts a higher fixed frequency voltage into a lower frequency and variable output voltage without passing through an intermediate DC link. Each phase is composed of two groups (positive and negative groups) of three-phase full-wave converters in anti-parallel connection. The output rectified voltage is:
In the formula: αP-positive group rectifier control angle;
αN-negative group rectifier control angle;
Ud0——α=0°Average value of output voltage.
The fundamental wave of the output voltage of the AC frequency converter is a sine wave, namely:
In the formula: k——output voltage ratio, k=Udm/Ud0;
ω——The angular frequency of the fundamental wave of the output voltage.
By changing the frequency of the firing angle of the positive and negative rectifiers, the frequency of the output voltage can be changed; by changing the value of the output voltage ratio k, the output voltage value can be changed.
The AC-AC frequency converter uses two sets of anti-parallel thyristors to work alternately to generate a phase of low-frequency AC voltage to supply the load, and there is a circulation problem. The working methods adopted in the reversible DC drive (such as logic without circulation, dislocation without circulation, controllable circulation) are generally applicable in AC frequency converters. The main circuit and basic control part of the AC-AC converter can use the same components and technology of the DC drive.
3 Main circuit wiring and its characteristics
SAFESAVE frequency converter is based on a high-performance 16-bit microprocessor, adopts parameter configuration mode to realize various control functions of the speed control transmission control system with software, and has a high technical level. This all-digital AC-AC frequency conversion system is composed of three 6RA24s, the main circuit is a three-phase bridge, speed-current double closed-loop control, and logic non-circulating current working mode. The outer loop is a speed loop to achieve precise speed control, and the inner loop is 3 current loops to meet the balance and coordination of the three-phase current, and perform AC modulation on the three-phase current to make the output current waveform a sine wave.
The system is composed of a three-phase bridge type 6-pulse wiring AC-AC converter. The phase voltages are UOR, UOS, UOT, which are 120° apart from each other, and are output as three-phase voltages. This connection can increase the output voltage of the device when the selected thyristor withstands a lower voltage. If the three phase voltages contain the same DC component, due to the star connection, the line voltage does not contain the DC component, and there will be no DC component in the voltage wave output by the inverter to the load. Thereby improving the input power factor of the inverter. If the three phase voltages contain 3, 6, and 9 harmonics, because these harmonics are in phase with each other, they also cancel each other in the wiring (Y connection output), and they are not reflected in the load and line voltage, that is, the output phase The 3 times harmonics in the voltage will not be transmitted to the motor terminal. Therefore, the system has high output power, few high-order harmonics, good output waveform, and reliable operation.
4 Control system composition
The low-frequency braking mode enables the hoist to convert part of the mechanical energy into electric energy in the deceleration section and feed it back to the power grid, and naturally transition to the crawling stage to achieve stable low-speed crawling. Through the use of digital control technology, its control performance has been greatly improved. This system is double closed-loop control of speed and current, making full use of the basic control functions of SAFESAVE inverter. It is mainly composed of the following parts:
(1) Motherboard. The core is a 16-bit single-chip microcomputer, which is used to complete the system's automatic adjustment, logic operation, fault diagnosis, automatic optimization and running status and fault display;
(2) Signal board. Complete control power distribution, current feedback signal transmission, pulse signal isolation and amplification, operation interlocking, etc.
(3) Photoelectric isolation switch input and output board;
(4) Intelligent A/D, D/A board;
(5) Bus board.
The control of each function constitutes a control system through an external bus, which jointly completes various control tasks.
The three current feedback signals are detected by current transformers and sent to the single-chip microcomputer after being shaped by two pairs of sampling switches; the speed setting and speed feedback signals are transformed by the filter circuit and the absolute value circuit and then sent to each group of single-chip microcomputers. The current and speed adjustment are done by computer software. The digital trigger pulse signal is output by the 6 high-speed channels of the single-chip microcomputer, and the high-frequency modulation signal is sent to the logic gate array circuit together, and converted into a double pulse train with a mutual difference of 60°, and then amplified and isolated to trigger the power of each phase. Components. All adjustments and controls are fully digital to ensure the accuracy of the system's adjustments.
Research on AC-AC frequency converter used in AC hoist control system
The working state of the transmission device is selected by the switch. In the case of "internal control", use the panel keys of the host to perform parameter setting and device debugging; in the case of "external control", the operating console is connected to the transmission device, and the main set is applied through the host serial interface RS232 (485) to make the AC hoist low frequency braking The operation is automated. At the same time, the status word of 6RA24 can be used to observe the feedback signal of the thyristor working status, read the actual value and write and store the parameter group, and complete the communication between each data and the PC.
5 application examples
The AC-AC frequency conversion all-digital drive control system is used in the main shaft of a mine. The hoist model is JKMD-2.25×4E, AC6kV, 800kW. Its stator and rotor circuit adopts vacuum contactor to commutate, and the whole operation process is controlled by PLC with CRT monitoring. Manufactured and installed by CITIC Heavy Machinery Automation Engineering Company, the technical performance fully meets the design requirements, and the operation effect is good, which ensures the safe production of the lifting equipment.
(1) Adopting a fully digital controlled AC/AC frequency conversion device as a low-frequency power supply for the drag control of the AC hoist, which improves the safety of hoist operation and has a significant energy-saving effect. It is helpful to improve the control performance of the hoist during braking and crawling, reduce the crawling distance, shorten the lifting cycle, and increase the production capacity.
(2) According to the operating results of the system, this device can be applied to various transportation modes of mine hoists. Because the controller hardware adopts SIEMENS's technologically advanced standard products, it has high reliability, combined with self-developed software and main circuit control system, the whole set of devices has a higher cost-performance ratio.