Abstract: This article introduces the working principle of a general-purpose thyristor soft starter controller, namely the phase-shifting principle. The phase-shifting principle is the control method commonly used by all thyristor soft starters. Under this control method, the starting current of the motor is small, the starting is smooth, and it can meet the needs of various loads.
Keywords: thyristor trigger current, power factor, angle
1 Introduction
Three-phase AC asynchronous motors are widely used in various industries due to their simple structure, low cost, and reliable operation. However, direct starting of these motors generates excessive starting current, especially for high-power motors. This large starting current severely impacts the power grid, degrading power quality and affecting the normal operation of other equipment. Furthermore, the mechanical shock caused by the starting torque reduces the motor's lifespan. Therefore, a soft starter needs to be connected in series between the motor and the power supply to solve this problem during the starting process.
With the rapid development of power electronics technology, thyristor soft starters have emerged. Due to their small size, compact structure, maintenance-free operation, safety, and reliability, they offer fully intelligent control, comprehensive functions, and rich menus. They also boast good starting repeatability and comprehensive protection. Therefore, they are gradually replacing traditional soft starter methods and becoming a new leader in the field of soft starting.
This article first describes the phase-shifting start method, which is currently the most commonly used soft start method for thyristors.
2 System Overview
By utilizing the switching characteristics of thyristors, the on-time of the thyristors is changed by adjusting their firing angle, thereby controlling the output voltage at the motor terminals and thus controlling the motor's starting characteristics. When the voltage at the motor terminals of the thyristors is the same as the input voltage, i.e., after the motor starting process is complete, the AC contactor (or circuit breaker) is energized (as shown in Figure 1, i.e., QF2 is energized), short-circuiting all the thyristors. At this point, the motor is directly connected to the power grid.
Figure 1. Schematic diagram of the main circuit of the thyristor soft starter
In Figure 1, QS is a high-voltage disconnector, QF1 and QF2 are vacuum circuit breakers (QF2 is sometimes also a contactor when the current is small), SCR is a (common) thyristor, and M is a medium-voltage motor. QF1 is responsible for switching the main circuit on and off, and QF2 is responsible for bypassing the power devices. In the SCR soft starter, there are 6 groups of SCRs, each group containing 10 SCRs in series (the value of m is determined by the voltage level and the withstand voltage of the thyristor).
3. Working Principle
3.1 Power Factor Angle
Because the motor is an inductive load, the current lags behind the voltage. When the voltage crosses zero, the current does not yet cross zero; it crosses zero only after a certain delay. The thyristor turns off only when the current crosses zero. We call this angle between the voltage zero-crossing point and the current zero-crossing point the power factor angle φ (as shown in Figure 2).
Figure 2 Relationship between firing angle and conduction angle
3.2 Conduction Angle and Firing Angle
(1) Conduction angle and firing angle
When a thyristor is working, the magnitude of its output voltage is determined by the thyristor's conduction angle, which in turn is determined by the firing angle and the power factor angle.
As shown in Figure 2, α is the trigger angle and q is the conduction angle.
The relationship between the angles is: q = -α + φ
(2) Initial trigger angle
The initial firing angle is a necessary condition for establishing current in the motor. The initial firing angle is generally between 40° and 60°, and this value varies depending on the motor and the load.
3.3 Phase sequence and its detection
(1) Phase sequence
Phase sequence refers to the order in which the instantaneous value of alternating current changes from negative to positive and passes through zero. Phase sequence detection is extremely important in thyristor soft starting; only by determining the phase sequence can the correct trigger pulse be issued to control the thyristor's conduction sequence.
(2) Phase sequence detection
Phase sequence detection is performed before the thyristors are turned on and before the trigger pulse is emitted. This system achieves phase sequence detection by judging the voltage drop signals of three anti-parallel thyristors. Let the voltage drop signal of phase A thyristor be Va, the voltage drop signal of phase B thyristor be Vb, and the voltage drop signal of phase C thyristor be Vc. When the voltage crosses zero, the voltage drop is 0. The voltage drop signals Va, Vb, and Vc strictly follow the phase sequence law of three-phase AC, with a period of 180° and a phase difference of 120° between each phase.
Using Va as a reference, timing begins when Va starts to drop. If the signal arriving after 60° is Vc and the signal arriving after 120° is Vb, then the sequence is considered positive. Conversely, if the signal arriving after 60° is Vb and the signal arriving after 120° is Vc, then the sequence is considered negative.
3.4 Pulse Trigger
(1) Trigger synchronization
To accurately control the output voltage Ud of the main circuit, the SCR must receive a trigger signal with the same frequency as the SCR main circuit. In the three-phase circuit (A, B, C), the trigger signals of the positive-phase thyristors are 120° out of phase, and the trigger signals of the negative-phase thyristors are also 120° out of phase. The trigger pulses of two anti-parallel thyristors in the same phase are 180° out of phase. From a macroscopic perspective, in a three-phase AC voltage regulator circuit, the controller issues a trigger pulse every 60°.
(2) Trigger pulse width
Thyristor triggering involves a process; the thyristor requires a certain amount of time to turn on, it's not instantaneous. The thyristor can only turn on when the anode current (main circuit current) rises above the thyristor's latching current IL . Therefore, the trigger pulse signal must have a certain width to ensure reliable thyristor conduction. For example, the typical thyristor turn-on time is around 6μs, so the trigger pulse width should be at least 6μs, usually 20-50μs. For large inductive loads, since the current rises more slowly, the trigger pulse width should be even greater; otherwise, the main circuit current may not have risen above the thyristor's latching current before the pulse ends, causing the thyristor to turn off again. Therefore, the pulse width should not be less than 300μs, usually 1ms, equivalent to 18° electrical angle of a 50Hz sine wave.
3.5 Phase-shifting voltage regulation process
Since asynchronous motors are inductive loads, according to power electronics, when an AC voltage regulator circuit drives an inductive load, it can only regulate voltage when the firing angle α is greater than the power factor angle φ of the inductive load. This is because when α < φ, the current conduction time will always remain at 180°, the same as when α = φ. In this case, phase control has no voltage regulation effect, and even if the thyristor trigger pulse is not wide enough, only one direction of the thyristor may be working, potentially introducing a DC component into the load and endangering the thyristor's safety. Therefore, when using phase-controlled thyristor circuits, wide-pulse triggering or dual-narrow-pulse triggering must be used, and the phase shift range must be limited to φ ≤ α < 180°.
The output voltage of the thyristor is between the conduction angle q By observing the waveform between the two points and changing the q- angle, the input voltage of the motor can be adjusted. The thyristor's output voltage is related to both angle α and angle φ. For a constant load impedance, angle q is constant, and adjusting angle α can change the thyristor's output voltage. However, the motor's power factor angle is a function of motor speed. During motor startup, the power factor angle changes continuously as the speed increases. Therefore, adjusting the thyristor's firing angle α must take into account the changes in angle φ. Only in this way can the motor input voltage be made to change according to a predetermined pattern.
4 Implementation Scheme
During soft starting, the DSP uses the current zero-crossing point as the trigger condition, sending a trigger pulse each time it detects a current zero-crossing point. First, the initial value of the trigger angle α is set according to the motor's parameter characteristics. During starting, the trigger angle α continuously moves forward until it reaches the power factor angle φ, at which point the trigger angle equals the power factor angle. The trigger pulse issued at this point coincides with the current zero-crossing point, which is the moment the thyristor can be turned off. Because the trigger pulse has a certain width, it can continue until after the current zero-crossing point, thus causing the thyristor to conduct at full voltage. The moment the thyristor conducts at full voltage marks the end of the soft starting process, at which point the bypass vacuum contactor can be switched to short-circuit the thyristor. To avoid secondary current surges, the trigger pulse is stopped after a delay, at which point the entire soft starting process is completely finished.
5. Actual control effect
This paper uses the power factor angle as the control basis for soft starting, which can track changes in the motor's power factor in real time and adjust the trigger angle accordingly. Therefore, most thyristor soft starter control methods adopt this phase-shift control method. The phase-shift starting method based on the power factor angle introduced in this paper has achieved smooth motor starting in practical applications, providing users with satisfactory soft starter products.
6. Conclusion
This article introduces a thyristor phase-shifting start method for controlling a motor using the power factor angle as a feedback signal. This control strategy can more accurately determine the turn-off time of the thyristor, thereby enabling more precise issuance of trigger pulses to control the soft-start process of the motor.
