Analysis of Application of AC-DC Converter in Control System of AC Hoist
1 Introduction
AC winding motors used in mine hoists are usually speed-controlled by switching their rotor resistance. However, the electric motor relies on the low speed obtained by the rotor resistance and its operating characteristics are soft. When the lifting container passes a given deceleration point, different decelerations will be obtained due to different loads, and stable low-speed crawling cannot be achieved. Eventually, the parking position is inaccurate and loading and unloading cannot be performed normally. Through the simultaneous application of a mechanical brake by the operator, the combined characteristics of the brake brake and the motor drag are used to obtain the required deceleration and low-speed crawling. In doing so, not only the power consumption is large, the brake shoe wears too much, but the operator's work is very tense and the safety and reliability are poor.
Thyristor cascade speed control automation hoist can get better control characteristics. However, there are many electronic control devices and large capacity. In order to obtain the braking torque in the deceleration phase, a dynamic braking device is also required, thus making the system complicated and increasing investment. Especially for winding motors of 500 kW or more, the rotor voltage is about 700V, which makes the selection of thyristor devices difficult.
When the AC hoist only adopts dynamic braking, the brake-electric and electric-braking must be converted several times in the deceleration and crawling phase to obtain an average, rather than a smooth, crawling speed, which can meet the long crawl distance. Hoist. This method requires the main reducer to have two spindles and increase the airbag clutch, increasing the mechanical structure and the complexity of the manufacturing process. The biggest weakness of dynamic braking is that it cannot provide positive torque. When the system needs to crawl slowly at a low speed, it is necessary to switch from dynamic braking to high-pressure operation, and to carry out the second-time pulse feed crawling in the crawling phase. This method is relatively soft and difficult to control.
Low-frequency braking is used to disconnect the motor stator windings from the three-phase power grid (6kV, 50Hz) and connect them to the same low-frequency power supply in the phase sequence of the voltage. The low frequency drag of the hoist makes the motor run in the regenerative power braking zone during the deceleration phase and operates in the electric zone during the crawling phase. And, lifting the motor from the braking state to the electric state is a natural transition. AC frequency converter is a kind of AC speed regulation program which is well applied in high power and low speed range. Its frequency range is easy to adjust, and it is suitable as a low frequency power supply for AC hoists of various operations. Digitalization is the development trend of modern transmission technology. The realization of full digital control is a new topic for the automation of AC hoists.
2, cross-converter
AC frequency conversion speed control system is a frequency conversion speed control system that directly converts higher fixed frequency voltage into lower frequency and variable output voltage without intermediate DC link. Each phase consists of two sets (positive and negative sets) of three-phase full-wave converters in anti-parallel configuration. The output rectified voltage is:
Ud=Ud0cosαp=-Ud0cosαN(1)
Where: αP positive group rectifier control angle;
αN negative group rectifier control angle;
Ud0α = 0° average output voltage.
The fundamental wave delivered to the inverter output voltage is a sine wave, that is:
Ud=Udmsinω1t(2)
then
cosαP=(Udm/Ud0)sinω1t=ksinω1t(3)
Where: k output voltage ratio, k = Udm / Ud0;
The angular frequency of the fundamental wave of the output voltage ω.
The frequency of the output voltage can be changed by changing the positive and negative frequencies of the two sets of rectifier firing angles; changing the output voltage ratio k can change the output voltage value.
The cross-converter uses two anti-parallel thyristors to work alternately to generate a low-frequency AC voltage supply load. There is a circulation problem. The working modes adopted in the reversible DC drive (such as logic no-loop, misaligned no-loop current, and controllable circulating current) are generally applicable to the cross-converter. The main circuit and the basic control part of the AC-DC converter can use the same components and technologies of the DC drive.
3, the main circuit wiring and its characteristics
SIMOREGK6RA24 is a compact three-phase AC direct-powered all-digital DC speed controller produced by SIEMENS. The design current range is 15A to 120A. It is based on a high-performance 16-bit microprocessor and uses software to implement speed control using parametric configuration. The various control functions of the transmission control system have a high level of technology. The all-digital cross-converter system consists of three 6RA24s, the main circuit is a three-phase bridge, speed-current dual closed-loop control, logic, no circulation work.
The outer ring is a speed loop, which realizes accurate speed control. The inner loop is composed of three current loops to meet the balance and coordination of the three-phase currents. The three-phase currents are ac-modulated so that the output current waveform is a sine wave. The main circuit wiring is shown in Figure 1.
As can be seen from Figure 1, the system is constructed as a three-phase bridge type 6-pulse wiring cross-converter. The phase voltages are UOR, UOS, and UOT, respectively, which are 120 degrees out of phase with each other and output as a three-phase voltage. 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 DC voltage is not included in the line voltage, and no DC component is present in the voltage wave output from the inverter to the load. This improves the inverter's input power factor. If the 3 phase voltages contain 3rd, 6th, and 9th harmonics, these harmonics are in phase with each other and they cancel each other out in the wiring (Y-connected output) and do not reflect the load and line voltage, ie, the output phase. The 3rd harmonic in the voltage will not be transmitted to the motor. Therefore, the system has large output power, less high harmonics, good output waveform, and reliable operation.
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