Control scheme for soft start of main motor of the

2022-08-02
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Control scheme for soft start of main motor of vertical plate shears

1 introduction

25MN vertical plate shears, the main production equipment used for blanking narrow steel plates by China Railway Zhuzhou Bridge Co., Ltd., is an old equipment produced in the early 1990s. Its electric control system is controlled by traditional relay logic. The main motor is started by wound rotor asynchronous motor in series with frequency sensitive rheostat. Due to the aging of the electric control system, In addition, the wound rotor asynchronous motor has mechanical commutator and brush, the electrical fault is increasing day by day, and the maintenance cost is rising. The electric control system of this vertical plate shears needs to be reformed urgently

in recent years, the development of electric drive and control technology has been changing with each passing day. PLC has been widely used in the automatic control of mechanical equipment. The new motor electronic controller (i.e. soft starter) can be adopted in all kinds of walls and has also been widely used in the current limiting starting of high-power three-phase asynchronous motors. Since the power of the main driving motor of the vertical plate shears is 37KW, it is necessary to adopt the starting current limiting measures to reduce the impact of the starting current on the electricity. Considering that wound rotor asynchronous motor has mechanical commutator and brush, and has the disadvantage of high failure rate, we choose YH type high slip three-phase asynchronous motor to replace the original YZR type wound rotor three-phase asynchronous motor, and select soft starter to realize current limiting starting of main motor. The logic control of the electric control system is completed by Mitsubishi FX2N PLC

2 necessity of applying soft starter in vertical plate shears

since the power of the main driving motor of the vertical plate shears is 37KW, starting current limiting measures should be taken to reduce the impact of starting current on electricity. As we all know, when the squirrel cage asynchronous motor starts directly with load, it will produce a starting current of 4 ~ 8 times the rated current, which will have an adverse impact on electricity. The motor with large capacity is generally not allowed to start directly with load, and the starting current limiting measures shall be taken to reduce the impact on the electricity. To avoid the impact of large current when the squirrel cage asynchronous motor is started directly, the common method is to start by reducing the voltage. There are various new trends in the traditional step-down starting of squirrel cage asynchronous motors, and the cool methods of black technology are: star angle starting, stator series resistance starting and autotransformer starting. The attached table lists the starting torque and starting current values (Mn: Rated torque; in: rated current) of common step-down starting methods for squirrel cage asynchronous motors

Figure 1 depicts in detail the speed current and speed torque characteristic curves of three traditional step-down starting methods for squirrel cage asynchronous motors. It can be seen from Figure 1 that the three traditional methods for step-down starting of squirrel cage asynchronous motor have a common disadvantage, that is, high current torque peak will still appear during the switching of motor starting process. Except that the star angle starting mode requires less starting equipment, all other modes require bulky equipment, which is not only large in size, but also has a large loss of starting energy consumption. Another common disadvantage of all traditional step-down starting methods is that they can not be well adjusted and have received strong support from national quality inspection departments and enterprises at all levels to help the motor start characteristics to meet the requirements of load soft start. That is to say, although the starting current and starting torque can be reduced, higher technical index requirements, such as the starting current is limited to a specified value, or the acceleration torque of the motor should change according to a certain law, cannot be met

Fig. 1 the curve of three traditional methods of step-down starting of squirrel cage asynchronous motor

these problems can be solved by using a new type of motor electronic controller (i.e. soft starter). The soft starter adopts microprocessor control technology to realize stepless step-down starting of motor by adjusting the phase angle of three-phase thyristor, so as to avoid large current peak during motor starting. The soft starters sold in the market can meet the following technical indicators: the starting torque can be 0.15 ~ 1.5mn, and the starting current can be flexibly adjusted between 1.5 ~ 5in. As the soft starter is a new power electronic device using microprocessor control technology, in addition to the above advantages, it also has the following characteristics different from the traditional step-down starting mode:

(1) compact structure, small space, easy to install in the electric control cabinet

(2) it is equipped with a variety of optional setting parameters for starting and stopping processes, and the parameter setting is flexible

(3) it has a variety of motor protection and monitoring functions

(4) simple installation and commissioning

(5) protect the transmission machinery and avoid the damage of surge torque

in view of the above advantages, the main motor of the vertical plate shears adopts a soft starter to control the soft start of the main machine

3 selection of soft starter and its peripheral circuit design

the starting process of vertical plate shears belongs to the conventional starting with standard load, so the selection of soft starter can be directly found out from its product selection manual. Main motor parameters: pn=37kw, in=72a. According to the product selection manual of ABB soft starter, the model of soft starter meeting the technical conditions of voltage class 400V and rated current ≥ 72A is pss85/l, and its technical parameters are in=85a

Figure 2 peripheral circuit schematic diagram of soft starter

peripheral control circuit of soft starter of vertical plate shears is shown in Figure 2. As the soft starter uses common thyristor as power element, it still has voltage drop loss and heat generation after it is fully opened, so a bypass contactor is used to connect to the main circuit after the soft starter completes the soft start task of the host, so that the soft starter can exit from operation. This can bring the following benefits:

(1) reduce the on state loss of thyristor components

(2) since the thyristor element has cooled down before the next start, it is more favorable for the operation of the soft starter

according to the approximate constant torque load characteristics of the vertical plate shears, the soft starting mode with starting current limitation is adopted. When the soft starter pss85/l is used for the soft starting mode of starting current limitation, its terminals 11 and 12 shall be connected to the external current transformer for current detection. The current transformer shall be the product of ankery company with the order number of akh-0 I-400/5。

terminals 4, 5 and 6 of pss85/l soft starter are start signal input terminals. Terminals 4 and 5 are short circuited with wires, and terminals 5 and 6 are connected to the normally open contact of starting relay ka10. When the soft starter works, first the incoming contactor KM6 is closed, then the starting relay ka10 sends a starting signal, and the soft starter starts to work with the preset starting curve. Terminals 7 and 8 of the soft starter are output normally open contacts of the fault signal relay. When the soft starter fails, the normally open contact of the fault signal relay is closed. After PLC detects the fault information, it immediately cuts off the incoming contactor KM6 to avoid further expansion of the fault. The specific fault information can be carried out according to the fault tracing steps in the product manual of ABB soft starter. Terminals 9 and 10 of the soft starter are start-up completion signal indicating relays. When the starting process is completed, the built-in contact will be closed; When the stop signal is sent, the built-in contact will open. After the PLC detects the start completion signal, the contactor KM7 is closed, KM6 is disconnected, and the soft starter is out of operation

4 opening and commissioning of soft starter

the vertical plate shears have approximate constant torque load characteristics, so the soft starter adopts the soft starting mode of starting current limitation. Its opening and commissioning shall be carried out according to the following steps:

(1) carefully check the main circuit and control circuit of the soft starter to see if there is any wiring error

(2) turn 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) set the starting ramp time knob on the front panel of the soft starter to 10s and the starting current limit knob to 3 times according to the standard parameters in Section 4 - basic settings for different applications of ABB soft starter product manual

(4) after the main circuit is powered on, the control system gives a soft start signal and the main motor starts. In order to obtain the best starting parameters, the test can be repeated. Finally, the starting ramp time of the soft starter of the vertical shear is set to 6S and the starting current limit is set to 2.8 times

in the opening and commissioning stage 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 problems must be paid attention to:

● the cooling fan of the electric control cabinet of the soft starter should always operate, so as to facilitate the smooth heat dissipation of the power thyristor in the soft starter and reduce the junction temperature

● an interval of 10 ~ 15min shall be reserved between each start to allow the power thyristor in the soft starter to cool down, so as to avoid the alarm of the soft starter due to overheating

5 conclusion

with the further development of computer technology and power electronics technology in the field of electrical control, the performance of soft starter is also continuously improved, and its function is becoming more and more powerful. It is also increasingly widely used in the soft starting of high-power motors in industrial sites. This paper introduces the use of ABB soft starter to realize the soft start of the main motor of vertical plate shears. After more than one year's operation observation, the electric control system of the whole machine has reached the design requirements, with stable operation and flexible operation. The electric control system of the vertical plate shears also involves the automatic control of devices such as PLC, which is not discussed due to space limitations

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