Simulation Research of Sliding Mode Soft Starter Based on Matlab/Simulink

Abstract: With the widespread use of electric motors, the requirements for motor starting are getting higher and higher, and soft start devices are also used more and more. The changes in several soft start modes of sliding mode soft starters are mainly achieved through different calculation methods of the trigger pulse conduction angle (alpha). There is no need to make major changes to the hardware of the soft start device, and it will not increase the hardware cost. A sliding mode soft start device is proposed, and four soft start models are constructed, including ramp starting, current limiting starting, torque control starting and graded frequency conversion starting. Through sliding mode control, the soft start device can meet the starting requirements of the motor in different working occasions, thereby expanding the scope of use of the soft start device. The soft start control device is modeled and simulated in the Matlab/Simulink environment.

Keywords: sliding mode; soft starter; Simulink; ramp start; current limiting start; torque control start; graded variable frequency start

0 Introduction

Different motor soft starting methods have different characteristics. Ramp starting and current limiting starting solve the problem of excessive impact current generated during the motor starting process. This starting method is suitable for light-load motor starting; torque control starting and graded frequency conversion starting can solve the problem of too small starting torque caused by using step-down starting, and are suitable for heavy-load motor starting.

The sliding mode soft start controller has engineering application value. The sliding mode soft start device integrates ramp starting, current limiting starting, torque control starting, graded frequency conversion starting and other methods. By changing the thyristor conduction pulse input to the AC module to select the soft start mode of the motor, the application scope of the soft start device is expanded. This paper conducts modeling and simulation research on the sliding mode soft start in the Matlab/Simulink environment.

1 Soft starter control system structure

1.1 Simulation system modeling

In the Mafiab/Simulink environment, a soft starter model is established, which integrates several soft start modes, including ramp soft start, current limiting soft start, torque control soft start and graded frequency conversion soft start. And the soft starter can enable users to choose a suitable method for motor soft start to start according to their needs. Figure 1 is a system simulation block diagram based on Simulink. In the simulation system, the main units include sliding mode soft start control, AC voltage regulation, motor and electrical measurement modules.

The sliding mode soft start control module is composed of four soft start submodules (four soft start) and a soft start mode sliding mode selection module (switch subsystem). The input quantities include the single-phase effective value RMS of the motor stator current, the voltage value Va and current value Ia of the motor A phase input, and the synchronous voltage.

1.2 Trigger Pulse Submodule

The trigger pulses of the thyristors in the whole system are mainly generated by the 6-pulse generator module, which can delay the 6 pulses to generate a set of controllable 6-phase pulse waves. The generated 6-phase pulses are used in the 4 soft start modes. Figure 4 shows the subsystem of the 6-pulse generator.

In the 6-pulse generator module, there are two constants "Pwidth" and "freq". The value of "Pwidth" is the width of the 6-phase trigger pulse; "freq" is the synchronous voltage frequency; the value of the trigger angle is input from the "alpha deg" input terminal, which can be a set of continuous variables. A set of controllable thyristor conduction 6-phase pulses is generated by the value of "alpha_deg"; "block" is the latch end of the 6-pulse generator. When the input end is zero, the module works normally to generate a set of 6-phase pulses; when the input end is high, the cut-off protection is started, and the pulse will no longer be generated, and the thyristor will not be able to conduct.

However, in the 6-pulse generator module, the pulse generation is calculated based on the synchronous voltage of the power supply. The frequency setting in the subsystem is consistent with the frequency of the synchronous voltage, and the generated 6-phase pulse frequency cannot be changed. In the graded frequency conversion soft start module, the 6-phase conduction of the thyristor is generated by a group of base frequency pulse waves generated by the 6-phase pulse generator and a group of low-frequency pulses. This group of pulses uses the frequency of the low-frequency pulse as the frequency and the base wave pulse width as the pulse width, and the pulse can be delayed by "alpha deg". In this way, a group of low-frequency pulses with controllable conduction angles are generated to meet the pulse requirements of graded frequency conversion soft start. In the graded frequency conversion pulse synthesis module of Figure 5, square wave pulses with frequencies of F/2, F/4, F/5, and F/7 are ANDed with the pulses generated by the 6-pulse generator to generate F/2, F/4, F/5, and F/7 frequency square waves, and the operation time of each graded frequency is selected according to the input time to complete the graded frequency conversion soft start control.

2 Soft start control method

In the soft start simulation system, there are four soft start submodules in the soft start module, namely ramp soft start, current limiting soft start, graded frequency conversion soft start and torque control soft start submodule. The four soft start modes use different calculation methods for the conduction angle (alpha) of the thyristor to generate six pulses of the four soft start modes to achieve their respective soft start purposes.

2.1 Ramp soft start

Ramp soft start is a soft start method that increases the voltage on the motor in a ramp manner from the initial angle to the rated voltage. The initial angle of the ramp soft start can be set according to the situation, and the starting time can also be adjusted by the user. Figure 6 is a submodule for the calculation of the alpha angle of the ramp soft start. "uini" is the initial angle of the soft start, and "uincr" is the slope control of the ramp growth of the soft start.

Ramp voltage soft starting is a common soft starting method. The starting time of this starting method is longer than that of direct starting, the starting current is smaller than that of direct starting, the starting torque is not high, and it is easy to implement.

2.2 Current Limiting Start

Current-limited starting is to limit the starting current of the motor to a certain set value during the starting process. This starting method is mainly used for step-down starting of light-load starting. Its output voltage increases rapidly from zero until its output current reaches the preset current limit, and then gradually increases the voltage to the rated voltage while keeping the output current less than the current setting value, so that the motor speed gradually increases until the rated speed. Figure 7 is a current-limited soft start alpha calculation submodule built based on Matlab/Simulink. Alpha calculation uses a fuzzy controller to calculate the alpha angle in the closed-loop control of the current and adjust the voltage across the motor.

The current-limited soft start uses closed-loop fuzzy control with current feedback, which can trigger the thyristor quickly and accurately, avoid the impact of large current, and effectively limit the starting current.

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Torque control starting is to control the starting torque of the motor to increase linearly from small to large, and calculate the conduction angle of the thyristor according to the starting torque change to control the input voltage of the motor so that the motor torque meets the motor use requirements. Figure 8 is the torque control soft start alpha angle calculation submodule.

The advantages of torque-controlled soft starting are smooth starting and good flexibility, which is beneficial to the drive system and reduces the impact on the power grid. It is the optimal load starting method.

2.4 Gradual frequency conversion soft start

Gradual frequency conversion (discrete frequency method) is to make the frequency of the output voltage of the traditional soft starter start from a lower value, rise in stages, and finally reach 50Hz. Although graded frequency conversion can achieve frequency conversion, it cannot make the frequency change continuously, but can only make the frequency change in stages, and the frequency of each stage is one nth of 50Hz (that is, the frequency division of 50Hz), so as to achieve a higher starting torque. Figure 9 is the calculation submodule of the conduction angle alpha of graded frequency conversion soft start. During the low-frequency starting process, in order to keep the main magnetic flux of the motor unchanged, the stator voltage should be reduced accordingly. The conduction angle alpha of the motor at F/2, F/4, F/5, and F/7 is obtained by calculation. After the frequency reaches 50Hz, the conduction angle slowly decreases to 0, so that the voltage on the motor reaches the rated voltage, and the soft start is completed.

Gradual frequency conversion starting can effectively improve the starting torque of the motor and enable the motor to start smoothly under heavy load.

3 Simulation Results Analysis

In the matlab/simulink simulation environment, the ramp control, current limiting control, torque control and graded frequency conversion control are simulated by selecting the soft start mode in Figure 1. The waveforms of the stator effective current, speed and torque of the four soft start modes are obtained respectively.

The motor parameters are 10kW, 220V, 50Hz, the number of pole pairs is 2, and the per-unit parameters of the stator and rotor ends are: Rs=0.0401Ω; Rr=0.0377Ω; Ls=Lr=1.0349Ω.

Through the analysis of the simulation results, we can draw the following conclusions: ramp starting is an open-loop starting control method, which can reduce the starting current of the motor, has a low starting torque and is easy to operate; current limiting soft starting uses closed-loop current control, which can effectively limit the starting current of the motor, but the starting time is long and the starting torque is not high; the torque of the torque-controlled soft starting torque can rise smoothly, reducing the impact of torque mutations on the traction system, and is a good load start; graded frequency conversion starting can limit the starting current of the motor, has a large starting torque, and is an effective high-torque starting method suitable for starting motors with large loads.

4 Conclusion

The sliding mode soft starter is a soft start device that integrates multiple soft start modes. It selects the soft start mode through sliding mode control to achieve multiple soft starts, expands the application range of the soft start device, improves the practicality of the soft start device, and has great practical value.