1 Introduction
China Railway Zhuzhou Bridge Co., Ltd. is the main production equipment for narrow steel sheet blanking. The 25MN vertical shear is an old equipment produced in the early 1990s. Its electronic control system is the traditional relay logic control, and the main motor adopts a wound rotor. Asynchronous motor series frequency sensitive rheostat starting method, due to the aging of the electronic control system, coupled with the wound rotor asynchronous motor mechanical commutator and brushes, electrical faults are increasing, maintenance costs continue to rise. The electric control system of this vertical plate shear is urgently needed to be transformed.
In recent years, the development of electric drive and control technology is changing with each passing day. PLC is widely used in automatic control of mechanical equipment. New types of electric motor electronic controllers (ie, soft starters) have also begun to be used in large-capacity three-phase asynchronous motors for current limiting startup. . Since the power of the main drag motor of the vertical shears is 37 kW, the start current limiting measures must be adopted to reduce the impact of the starting current on the power grid. Considering that the wound rotor asynchronous motor has mechanical commutator and brushes, it has the disadvantage of high failure rate. Therefore, we chose YH type high slip three-phase asynchronous motor to replace the original YZR wound rotor three-phase asynchronous motor. At the same time, a soft starter is used to achieve the current limiting start of the main motor. The logic control of the electronic control system was accomplished using Mitsubishi's FX2N PLC.
2 The necessity of application of soft starter in vertical shears
Since the power of the main drag motor of the vertical shears is 37 kW, the start current limiting measures must be adopted to reduce the impact of the starting current on the power grid. As we all know, the squirrel cage asynchronous motor will have a starting current of 4 to 8 times the rated current when it is directly started with the load, which will have an adverse effect on the power grid. Motors with large capacity are generally not allowed to start directly with load, and start-up current limiting measures are required to reduce the impact on the power grid. To avoid high current surge when the squirrel cage induction motor is directly started, the usual method is to use a reduced voltage starter. The traditional squirrel-cage induction motor step-down starting methods are: star-angle start, stator string resistance start and autotransformer start. The attached table shows the starting torque and starting current value of the step-down starting method of a common squirrel cage asynchronous motor (Mn: rated torque; In: rated current).
The speed-current, speed-torque characteristic curves of the three kinds of deceleration starting methods of the conventional squirrel-cage asynchronous motor are described in detail in FIG. 1 . As can be seen from FIG. 1, the three types of conventional squirrel-cage induction motor step-down starting methods have a common disadvantage that high current torque peaks still occur during switching of the motor starting process. In addition to the fewer starting equipment required for the star-corner starting method, other methods require bulky equipment, which is not only bulky, but also has a large start-up energy loss. Another common disadvantage of all conventional step-down starting methods is that they do not adjust the starting characteristics of the motor very well to meet the soft start requirements of the load. In other words, although the starting current and starting torque can be reduced, higher specification requirements, such as the starting current requirements are limited to a certain value, or the motor's acceleration torque changes according to a certain law, but Can not reach.
Fig. 1 Curve of surname of three kinds of traditional squirrel-cage induction motor step-down starting method
These problems can be solved by using a new type of motor electronic controller (soft starter). The soft starter adopts the microprocessor control technology and realizes stepless step-down starting of the motor by adjusting the phase angle of the three-phase thyristor so as to avoid the large current peak in the motor starting process. The soft starter currently on the market can achieve the following technical indicators: starting torque can be 0.15 ~ 1.5Mn, starting current can be flexibly adjusted between 1.5 ~ 5In. As the soft starter is a new type of power electronic device adopting microprocessor control technology, in addition to the above advantages, it also has the following features different from the traditional step-down starting mode:
(1) Compact structure, small footprint, easy installation in electrical control cabinets;
(2) There are multiple settable parameters for starting and stopping process, and the parameters can be set flexibly;
(3) Has a variety of motor protection and monitoring functions;
(4) Simple installation and commissioning;
(5) Protect the transmission machinery from damage caused by surge torque.
In view of the above-mentioned various advantages, the main motor of the vertical shears adopts a soft starter to perform soft start control on the host.
3 selection of soft starter and its peripheral circuit design
The starting process of the vertical shears belongs to the normal start with standard load, so the selection of the soft starter can be found directly from the product selection manual. The main motor parameters are: Pn = 37kW, In = 72A. Check ABB's soft starter product selection manual, meet the technical requirements of voltage level 400V, rated current ≥ 72A soft starter model: PSS85/147-500L, its technical parameters: In = 85A.
Fig. 2 Schematic of peripheral circuit of soft starter
Vertical shearing machine soft starter peripheral control circuit shown in Figure 2. Since a soft starter uses a normal thyristor as a power element, it still has a pressure drop after complete opening, and is accompanied by heat generation. Therefore, after the soft starter completes the host soft start task, a bypass contactor is used to access the main circuit. Let the soft starter exit the operation. This can bring the following benefits:
(1) Reduced on-state losses of thyristor components;
(2) Since the thyristor element has cooled down before the next start, this is more advantageous for the operation of the soft starter.
In view of the approximately constant torque load characteristics of the vertical shears, a soft start method using a starting current limit is adopted. When the soft starter PSS85/147-500L is used in the soft-start mode for starting current limitation, its terminals 11 and 12 should be connected to the external current transformer for current detection. The current transformer is selected by the Ankerui company and ordered. The number is AKH-0.66-40I-400/5.
Terminals 4, 5, and 6 of the soft starter PSS85/147-500L are start signal input terminals. The terminals 4, 5 are short-circuited with wires, and the normally open contacts of the starter relay KA10 are connected between the terminals 5, 6. When the soft starter is working, first the line contactor KM6 is closed, and then the starter relay KA10 sends a start signal, and the soft starter starts working with a preset starting curve. Terminals 7 and 8 of the soft starter are output NO contacts of the fault signal relay. When the soft starter fails, the normally open contact of the fault signal relay is closed. Immediately after the PLC detects the fault information, the line contactor KM6 is cut off to avoid further expansion of the fault. The specific fault information can be performed in accordance with the fault tracing steps in the ABB soft starter data sheet. The terminals 9 and 10 of the soft starter are start signal indicating relays. When the start-up process is complete, the built-in contact will close; when the stop signal is issued, the built-in contact will open. After the PLC detects the start completion signal, the contactor KM7 pulls in, KM6 opens, and the soft starter exits operation.
4 Soft starter commissioning
Vertical shears are approximately constant torque load characteristics, so the soft starter uses a soft start method with a starting current limit. Its commissioning is carried out according to the following steps:
(1) Carefully check the main circuit and control circuit of the soft starter to see if there are any wiring errors;
(2) Set the switch S1 on the front panel of the soft starter for selecting the motor connection mode to the “external†position of the motor;
(3) According to the standard parameters in the basic settings of Section 4 of the ABB soft starter product specification for different applications, set the start ramp time knob on the front panel of the soft starter to 10s and the start current limit knob to 3 times;
(4) After the main circuit transmits power, the control system gives a soft start signal and the main motor starts. To obtain the best starting parameters, repeat the test. Finally, the starting ramp time of the vertical shears soft starter is set to 6s and the starting current limit is set to 2.8.
During the commissioning phase of the soft starter, in order to obtain the best starting parameters, the number of starts is greatly increased due to repeated tests. Therefore, the following issues must be noted:
â— The cooling fan of the soft starter control cabinet must be running to facilitate the smooth heat dissipation of the power thyristor in the soft starter and reduce the junction temperature.
• There should be an interval of 10 to 15 minutes between each start so that the power thyristors in the soft starter can be cooled down to prevent the soft starter from alarming due to overheating.
5 Concluding remarks
With the further development of computer technology and power electronic technology in the field of electrical control, the performance of soft starters has also been continuously improved, and their functions have become increasingly powerful. The application of soft starters in industrial field high power motors has also become increasingly widespread. This article describes the use of ABB soft starter to achieve the soft start of the main motor of the vertical shears. After more than one year of operational observation, the entire electrical and electronic control system has achieved design requirements, stable operation and flexible operation. The electric control system of the vertical shears also involves the automatic control of PLCs and other devices. Due to space limitations, none have been discussed.