Keywords: soft starter; AC motor; motor startability; MCC; control cabinet
1 Introduction
The cold shear of the Guangzhou Iron and Steel Continuous Rolling Mill is driven by a 90kW motor. Before the transformation, the motor is started with a frequency-sensitive resistor. The starting current is above 380A, the starting time is above 3min, and the current of the motor during operation is 70A. Due to the large starting current and long time, the contacts of the motor rotor slip ring, carbon brush, terminal, frequency sensitive resistor and main contactor are often damaged, because of the large inrush current (generally Ie 5) ~ 8 times), at the same time, due to the greater starting stress, the service life of the load equipment is reduced.
2 SIEMENS 3RW22 soft starter performance and characteristics
The soft starter detects the motor current, controls the output voltage to be linearly added to the full voltage, limits the excitation starting current, and realizes the soft start of the motor. It has strong anti-interference ability and control ability, which can avoid high pressure during work. Disturbance of voltage and strong electrons. The soft starter is triggered by digital control. During the soft start, the constant current is smoothly accelerated to avoid the impact on the power grid. The starting current can be continuous within the range of 30% to 70% Ue (Ue is the rated voltage) according to the needs of the field load. Adjustable. The parameters of the soft starter can be adjusted to obtain the best torque with the minimum current. The mechanical advantage of the soft starter is that it can reduce mechanical stress and extend the service life of the motor and auxiliary machinery. The starting time can be set according to different loads to optimize the starting time. Within this time range, the motor speed rises slowly, with all-round protection of the motor such as phase loss, three-phase unbalance, overload, and overcurrent. High cost performance, simple operation, small size, light weight, easy installation and debugging, with thyristor overheating and overvoltage protection.
2. 1 Problems with the device before the soft starter is used
The cold-shearing machine of the continuous rolling mill adopts (star / delta) conversion start device. The heavy load and large diameter of the flywheel carried by the cold shear of the continuous rolling mill have large inertia during startup and shutdown, and high current spikes appear at the moment of switching. The inrush current of the motor at startup also has a great impact on the voltage fluctuation of the grid At the same time, it will cause great damage to the control equipment. The contactor is often burned and needs to be replaced frequently, which seriously affects production. The failure of the cold shear of the continuous rolling mill will cause the whole line to stop production. It is obviously not advisable to increase the power distribution capacity just to start the motor. The traditional buck start has not fundamentally solved the problem.
2. 2 The soft starter adopted after the start
Soft starter is a kind of motor control equipment integrating soft start, soft stop, light load energy saving and multi-function protection. It can realize the smooth start of the motor without shock during the whole start process, and can adjust various parameters in the start process according to the characteristics of the motor load, such as current limit value, start time, etc.
3 Working principle and operating characteristics
The starting torque Ma of the three-phase AC asynchronous motor is directly related to the quadratic power of the applied voltage, that is, as long as the voltage on the motor terminal is reduced, these values ​​will be affected (see Figure 1). The working principle of soft start is to increase the terminal voltage of the motor from the preset value to the rated voltage by controlling the conduction angle of the three opposite parallel thyristors connected in series between the power supply and the controlled motor (see Figure 2).
(1) Voltage double-slope start (see Figure 3), during the start-up process, the output torque of the motor increases with the increase of voltage, and an initial start voltage Us is provided at the start. Us is adjustable according to the size of the load. Us is adjusted to greater than the static friction torque of the load to produce the best starting characteristics. At this time, the output voltage starts to rise at a certain slope from Us, and the motor continuously accelerates. When the output voltage reaches the up-speed voltage Ur, the motor basically reaches the rated speed. The soft starter automatically detects the up-speed voltage during the starting process, and when the motor reaches the rated speed, the output voltage reaches the rated voltage.
(3) Sudden start (see Figure 4): In the start-up phase, the thyristor is quickly turned back after being fully turned on in a very short time. This time can be adjusted artificially and then linearly increased according to the original value to enter Flow start, this start method is suitable for heavy load and need to overcome friction start occasions.
The asynchronous motor can be used in various applications that do not require speed regulation. It is suitable for various fan loads and requires soft start and soft stop. For changing load conditions, the motor is in light load operation for a long time, and only in short-term or instantaneous operation at full load, the application of a soft starter (without bypass contactor) has the effect of light load energy saving.
5 Conclusion
By understanding the characteristics and performance of Siemens soft starter 3RW22, the use in the continuous rolling mill is compared with the traditional step-down starting equipment. The traditional control method has been unable to meet the needs of modern equipment due to the inherent weakness in some special occasions. Need, so choosing a soft starter for intelligent motor starting equipment will be an inevitable development direction.
references
[1] Operating Instructions for Siemens Soft Starter
[2] GB12497-1995 "Economic operation of three-phase asynchronous motor"
[3] SMC Dialog Plus TM Controller User Manual 1997.
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