AC asynchronous motor has been widely used in China's coal, petroleum, chemical, steel, power generation, water supply, cement and building materials due to its simple structure, high operational reliability, strong environmental adaptability and excellent drag performance. . However, the fatal shortcoming of the AC asynchronous motor is that the starting impact is large and will adversely affect the system. The main impact is reflected in two aspects:
(1) Electrical issues:
The starting current can reach 5-7 times of rated current, causing the motor winding to overheat due to current, thereby accelerating its insulation aging, causing voltage fluctuation of the power supply network and affecting the normal operation of other equipment under the power grid.
(2) Mechanical problems:
Excessive starting torque produces mechanical shock, which causes a large impact on the driven equipment, shortens the service life and affects the accuracy. Such as damage to the coupling, tearing of the belt, etc. Causes abnormal wear and impact of mechanical transmission components, accelerates aging, shortens life and increases maintenance workload.
This problem was encountered in the air compressor of the nitrogen production line of Shougang Cold Rolling Plant. Because the motor power is relatively large (10KV/3730KW and 10KV/2250KW), the mechanical impact on the motor and compressor is relatively large in the starting process. The workload is increased for the maintenance of the system; secondly, the grid falls more seriously when the motor is directly started, and other equipment cannot work normally when the grid voltage drop reaches 85%. For the above reasons, it is decided to use the PowerEasy type thyristor type soft starter.
2. The structure and principle of thyristor series high voltage soft start
2.1 Main circuit composition
The thyristor series soft start device is a starter device designed with all-digital intelligence combined with power electronics technology, photoelectric technology control technology and micro-processing technology. The main circuit wiring diagram of the thyristor series soft start is shown in Figure 1. It is mainly composed of a high-voltage thyristor series valve group and a bypass contactor. The high-voltage thyristor series valve group is a power conversion execution unit, which is composed of multiple thyristors. It is composed of series and parallel, and is supplemented by absorption and voltage equalization clamp circuit to ensure its reliability in high pressure environment. After the input terminal is powered, the AC three-phase power supply is chopped by controlling the conduction angle of the thyristor to control the amplitude of the output voltage. And after the starting process is completed, the bypass contactor is closed, the soft start device is switched to the bypass state, and the thyristor is closed.
Figure 1 thyristor series soft start main circuit wiring diagram
2.2 Control circuit
As shown in FIG. 2, the control circuit of the thyristor series high voltage soft start device generally comprises a main control circuit, a trigger device, a signal acquisition circuit and a human-machine interface.
Figure 2 Soft Start Control Block Diagram
2.2.1 Main control circuit
The main control circuit is mainly composed of a digital circuit composed of a DSP microprocessor and a CPLD large-scale gate array. Its main function is to generate a pulse command for triggering the thyristor, and the trigger command of the thyristor is issued by a digital signal processor (DSP). The main control circuit can adjust the conduction angle of the thyristor according to the voltage and current signals collected by the signal acquisition circuit, thereby achieving the function of adjusting the motor current, and realizing the closed-loop control of the motor, that is, the current limiting start. While issuing the trigger pulse, it can monitor the running state of the motor and the soft starter itself through feedback signals such as voltage, current and temperature, and protect the entire system to ensure safe operation of the system.
2.2.2 trigger system
The trigger system is a key part of the system's stability and reliability. It must have the ability to resist noise and can turn on the thyristor in a timely and accurate manner. There are two main technical problems here:
1. The trigger pulse can trigger the thyristor in time and accurately
A thyristor is a current-controlled bipolar semiconductor component that requires a gate drive unit similar to a current source to provide a particularly steep spike current pulse to the thyristor to ensure accurate and reliable triggering at any time. Thyristor. Therefore, the gate trigger pulse characteristic of the thyristor has a very strong influence on the normal operation of the thyristor.
When the thyristors are used in series, we require that the thyristors connected in series should be turned on as much as possible, because the slower turn-on may be over-voltage and damage the components. At this time, the thyristor turn-on time difference of the same series is required to be as small as possible.
The turn-on speed of the thyristor is mainly related to the amplitude of the gate trigger pulse and the steepness of the rising edge of the pulse. That is to say, the larger the amplitude of the gate trigger pulse, the steeper the rising edge of the pulse, and the shorter the time for the thyristor to turn on. Therefore, in the field of motor soft start, most manufacturers use strong trigger mode to trigger the thyristor. That is, the current amplitude of the trigger pulse IG is greater than or equal to 10 IGT, and the pulse rising edge time tr ≤ 1 μs. In short. In order to ensure the reliability of the operation of the thyristor valve string, the IG is much larger than the IGT.
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