Foreword
my country's soft starter technology began in the early 1980s. Currently, many manufacturers produce motor starters, and various brands of soft starters have been launched. However, due to relatively weak domestic independent development and production capabilities, there is still a heavy reliance on foreign products. There is still a certain gap in technology and reliability compared to similar foreign products. Therefore, foreign products still dominate the soft starter market, while domestic products have a very low market share. Currently, the soft starters produced on the market are mainly mechanical and three-phase anti-parallel thyristor types. Mechanical starters are currently the most widely used starting method, but they are stepped starters, which generate a secondary inrush current. The starting current is still 3-4 times the nominal current, and they have many drawbacks such as large size, high noise, high maintenance costs, and inability to adapt to harsh environments.
Currently, in developed countries, the main soft starter products for electric motors are solid-state soft starters—thyristor soft starters—and frequency converters that also function as soft starters. Frequency converters are used when speed regulation is required in the production process. Thyristor soft starters are generally used when the starting load is light and there is no speed regulation requirement. Frequency converters are used only under heavy loads or when the load power is particularly high. Thyristor soft starters are the mainstream soft starter products in developed countries, and various well-known electrical companies have their own thyristor soft starter brands, each with its own unique features. Examples include GE's ASTAT intelligent motor soft starter; ABB's PST and PSTB series motor soft starters; Schneider Electric's ATS46 soft starter; and Siemens' 3RW22SIKOSTART soft starter, among others. Currently, research on thyristor-based three-phase AC voltage regulating circuits abroad has progressed from open-loop and closed-loop methods for controlling voltage and motor current to establishing more accurate and practical mathematical models to find control methods suitable for motor loads in three-phase AC voltage regulating circuits, thereby improving the performance of motor loads in three-phase AC voltage regulating circuits. On the other hand, with the development of power electronics technology, asynchronous motors are developing towards greater reliability, convenience, and miniaturization.
A soft starter is essentially a DC voltage regulator used to achieve soft starting, soft stopping, real-time monitoring, and various protection functions. To ensure the safe and reliable operation of the system, the powerful control capabilities of a microcontroller can be fully utilized. The main control circuit monitors key components and parameters of the system in real time, such as overvoltage, undervoltage, overcurrent, and overload. With the application of digital DC PWM voltage regulation technology and the use of a high-performance microcontroller as the control core of the system, soft starters can achieve advantages such as fast and accurate control, rapid response, stable operation, and reliability.
1. Starting principle of a three-phase asynchronous motor
Traditional starting technologies for three-phase AC asynchronous motors, such as stator series resistor/reactor starting, autotransformer reduced-voltage starting, star-delta reduced-voltage starting, and rotor series resistor or frequency-sensitive rheostat starting, have all been important applications in the development of AC motor starting technology. However, with the development of thyristor technology, three-phase AC voltage-regulating soft starters have become the mainstream product in the soft starter market due to their advantages such as good performance, product variety, continuous voltage adjustment, and closed-loop torque or current control.
To study the relationships between voltage, current, and torque variables during the starting of a three-phase asynchronous motor, and further analyze the relationship between the starting current, starting torque, and applied voltage, a mathematical model of the motor needs to be studied. For soft starting of the motor, a mathematical model based on a lumped parameter equivalent circuit is often used. Without changing the physical quantities in the stator windings or the electromagnetic performance of the asynchronous motor, by calculating the frequency and windings, the frequency, number of phases, and effective series turns per phase of the rotor windings are reduced to be the same as those of the stator windings. The equivalent circuit of the asynchronous motor can then be derived using the reduced basic equations. The T-shaped steady-state equivalent circuit of a three-phase asynchronous motor is shown in Figure 1.
Figure 1. Equivalent circuit of an asynchronous motor
Where r1 is the stator winding resistance, x1 is the stator winding leakage reactance, r2 is the rotor winding resistance referred to the stator, and x2 is the rotor winding leakage reactance referred to the stator. rm represents the excitation resistance corresponding to the stator core loss, and xm represents the excitation reactance of the core magnetic circuit corresponding to the main magnetic flux. U1 is the stator voltage vector, E1 is the stator induced electromotive force vector, i1 is the stator current vector, and im is the magnetocurrent vector. The mathematical model based on the T-shaped equivalent circuit is as follows:
2. Principles and Analysis of Soft Starter
2.1 Thyristor Voltage Regulation Principle
There are two control methods for thyristors: phase control, which regulates voltage by controlling the conduction angle of the thyristor; and cycle control, which uses the thyristor as a stationary contactor to alternately switch the power supply voltage on and off for several cycles, controlling the effective value of the output voltage by changing the ratio of the on-time to the off-time, thereby achieving voltage regulation. However, when cycle control is used on the stator of an asynchronous motor, the frequency of switching on and off cannot be too low. On the one hand, it will cause fluctuations in the motor speed; on the other hand, each current switching is equivalent to a restart process of the asynchronous motor. When the power is cut off, the magnetic field in the air gap of the motor will be maintained by the transient current in the rotor and will rotate with the rotor. The frequency of the electromotive force induced in the stator winding by the air gap magnetic field will change. When the current interruption interval is long, the electromotive force induced in the stator by this rotating magnetic field may have a large phase difference with the power supply voltage when it is reconnected, resulting in a large current surge that may endanger the safety of the thyristor. Therefore, in the voltage regulation control of asynchronous motors, thyristor voltage regulation generally adopts phase control. When phase control is used, the output voltage waveform is no longer a sine wave. Analysis shows that the output voltage does not contain even-order harmonics, and the third harmonic is the main component of the odd-order harmonics. Harmonics in asynchronous motors can cause additional losses and adverse effects such as torque ripple. Furthermore, since asynchronous motors are inductive loads, as we know from power electronics, when a thyristor AC voltage regulation circuit has an inductive load, only when phase shifting occurs...
This system's soft starter employs the thyristor voltage regulation principle, achieving various soft-start functions by adjusting the magnitude and phase of the voltage at the motor stator input terminal. The soft starter uses the main circuit shown in Figure 2. Three sets of anti-parallel thyristors are connected in series with the three-phase stator coils of the star-connected motor. This connection method results in fewer harmonics, superior voltage regulation performance, and a simple and reliable control system.
Figure 2 Schematic diagram of soft starter main circuit
2.2 Soft start method
The main function of a soft starter is to achieve soft starting and soft stopping, with soft stopping being essentially the reverse process of soft starting. Three-phase asynchronous motor soft starters offer multiple starting modes to meet various starting requirements. Details are provided below:
Current-limiting starting is a soft-start method that limits the starting current of a motor to a set value Im during startup. The starting waveform is shown in Figure 3. It is mainly used for reduced-voltage starting under light loads. The output voltage increases rapidly from zero until the output current reaches the preset current limit Im. Then, while maintaining the output current below this value, the voltage is gradually increased until the rated voltage is reached. The advantages of this starting method are low starting current and the ability to adjust the starting current limit Im as needed. Its disadvantages are that the starting voltage drop is difficult to determine during startup, resulting in insufficient utilization of the voltage drop space, loss of starting torque, and a relatively long starting time. This method is widely used and suitable for loads such as fans and pumps.
Figure 3 Current limiting startup waveform
3. Hardware Design
3.1 Selection of Main Circuit
In thyristor AC voltage regulation systems, thyristors can automatically turn off when the load current waveform crosses zero, eliminating the need for additional commutation circuits. Therefore, their main advantages are simple circuitry, small size of the voltage regulator, low cost, and ease of use and maintenance. This system employs thyristor phase-controlled voltage regulation technology, using the main circuit shown in Figure 4, which connects six thyristors in series in opposite parallel pairs within the motor's main power supply circuit.
Figure 4 AC voltage regulating main circuit
3.2 Main Circuit
The main circuit design of the soft starter is shown in Figure 5.
Figure 5 Main circuit
A voltage regulating circuit is composed of three sets of anti-parallel thyristors. A contactor is connected between the three sets of thyristors and the three-phase power supply. During soft start, the contactor opens; after soft start, the contactor closes. At the start of soft stop, the contactor switches back to the bidirectional thyristor terminals, and the soft starter engages the stop operation. This process is repeated to complete soft start and soft stop. A synchronization signal for the soft starter is obtained from the three-phase power supply side through an isolation circuit. On the thyristor output side (R, S, T), a lower amplitude three-phase voltage is obtained through resistor division, and then rectified before being sent to the microcontroller for fault detection. TA1, TA2, and TA3 represent the Hall sensor current output; this current signal is rectified and converted into a voltage signal, which is then input to the control loop.
4. Overcurrent protection circuit design
An excellent overcurrent protection circuit should be able to react quickly to overcurrent and operate accurately. The overcurrent protection circuit in this design is similar to the overvoltage protection circuit. The overcurrent protection signal is taken from the current feedback loop, and the rectification and filtering circuits are the same as those in the current feedback circuit. It is compared with a set value; once the set value is exceeded, a low-level signal is output and sent to the external interrupt port P3.3 of the auxiliary microcontroller U2. Then, the software processes the signal to implement pulse blocking, fault alarm, and system reset for the overcurrent-affected thyristor. The overcurrent value is generally set to 5.5 times the rated current. This is because the maximum current limiting amplitude is typically 5 times during current-limited startup, so a certain margin is needed to ensure that the circuit is not cut off during normal startup. The specific overcurrent protection circuit is shown in Figure 6.
Figure 6 Overcurrent detection circuit
5. Software design for trigger pulse control
The generation of the required thyristor shift trigger pulse by the microcontroller must include a synchronous voltage detection stage, a phase shift delay angle timing stage, and a trigger pulse timing allocation stage, which is similar to the method implemented by analog circuits.
When a positive transition occurs in the synchronization detection signal, it is inverted and sent to the CPU's INT0 in the form of a termination instruction. The CPU's internal T0 timer detects the period of the synchronization signal, and the T1 timer implements the timing control of the phase shift angle. P1.2~P.7 of port P1 are used to output trigger signals for the three-phase bridge rectifier circuit, while P1.0~P1.1 of port P1 are used for sampling via division instructions. Since all outputs of the MCS51 microcontroller are high-level during CPU power-on reset, a low-level signal should be used as the effective trigger signal to avoid drive signals to all thyristors during reset. That is, when the port output is low-level, it is converted to high-level by an external inverter to trigger the thyristor to conduct. The width of the output trigger pulse is also controlled by timer T1.
6. Conclusion
The future development direction of motor soft starters is towards greater intelligence and multi-functionality. Currently, soft starting still primarily takes the forms of voltage ramp soft starting and current-limiting soft starting. In the future, torque control starting will become an important starting method for motors. Looking further ahead, variable frequency soft starting will become the mainstream. This is because variable frequency soft starters can achieve large starting torque while limiting current (starting current not exceeding the motor's rated current), enabling various starting functions, including soft stopping.
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