Abstract: This paper introduces the detailed process of phase-fixing operation using an oscilloscope combined with the characteristics of thyristor circuits, and recommends a practical method for quickly identifying transformer connection groups and connecting transformers according to these groups. Through dynamic analysis and testing of PI regulator performance using waveform analysis, it explores a method for testing and debugging the system using a slow-scan, long-persistence oscilloscope. Keywords: Phase-fixing connection group, slow-scan, long-persistence oscilloscope, dynamic debugging. Introduction With the development of AC variable frequency speed control technology, although the dominance of DC speed control has been broken, it still holds a certain position in the speed control field due to its excellent starting, braking, forward and reverse rotation, and speed regulation performance, especially in applications requiring high precision and speed. DC speed control systems have various structural forms. This paper takes the widely used thyristor three-phase fully controlled bridge irreversible, double-closed-loop speed control system as an example to introduce some methods and techniques for system debugging. Since DC speed control systems involve many electrical fields such as power electronics, transformers, motors, and electrical engineering, mastering their working principles and testing and debugging methods is highly beneficial to improving overall electrical capabilities. Applying their related principles and methods to learn and solve other automatic control systems provides a sense of lateral understanding, leading to greater efficiency. Dynamic debugging of the system refers to adjusting system parameters according to the load characteristics to meet dynamic performance indicators, such as rise time, maximum overshoot, settling time, and dynamic speed drop. This aims to fully utilize the motor's potential and adapt to production process requirements. 1. The Concept of Synchronization From the DC motor speed formula, we know that there are three ways to change n: namely, changing U, Ra, and Φ. Because voltage regulation speed control is widely used due to its superior performance, this article only introduces thyristor DC voltage regulation speed control, taking a three-phase fully controlled bridge as an example. As shown in Figure 1, the occurrence time of the trigger pulse Ug of each thyristor VT should maintain a certain phase relationship with the main circuit voltage, which is called synchronization. Synchronization involves the phase sequence of the main circuit power supply, the connection group of the rectifier transformer TR and the synchronization transformer TS, and the specific requirements of various types of triggers. Some also involve phase lag issues caused by filtering. This requires correctly selecting the synchronization transformer and verifying the phase sequence of TR and TS. 2. Determining and Correctly Connecting the Synchronization Transformer 2.1 First, determine the phase difference between TR and TS, and determine the connection group of TS. Example: TR connection group is D/Y-5, TS lags TR by 1500, from the vector diagram (as shown in Figure 2), the TS connection group is Y/Y-10. 2.2 The next key step is to accurately identify, select, or connect the transformer to meet the phasing requirements. While vector diagram analysis should be mastered, it is cumbersome in practical applications, requiring manual drawing, and often leads to errors in noisy work environments. Based on my experience, I have developed a method for quickly identifying and connecting transformers, which has been repeatedly verified using numerous vector diagrams and is indeed feasible. The method is as follows: ⑴ Remember the key points: The vector origin must be located, Y/Y-12 is the reference point, The primary side moves forward with the angle, The secondary side moves backward with the angle, The opposite polarities move backward with the opposite polarity, The star rotates and the angle shifts four points to align. ⑵ Explanation 1: Y/Y-12 is used as the vector reference, i.e., the starting origin. All other connection groups are derived from Y/Y-12; as shown in Figure 3. (3) Explanation 2: If the first connection is changed to a delta connection and the second connection remains a star connection, this group will shift phase by 30° phase difference, leading Y/Y-12 by 30° (1 point) and becoming D/Y-11, as shown in Figure 4(a). If the first connection is changed to an inverse delta connection and the second connection remains a star connection, it will lag behind Y/Y-12 by 30° and become D/Y-1, as shown in Figure 4(b), which is the so-called "original side leading by the first angle". (4) Explanation 3: If the first connection remains a star connection and the second connection is changed to a delta connection, a phase difference of 30° will also be generated. The leading and lagging relationship between the forward and inverse delta connections is the opposite of the first connection change situation, that is, the forward delta connection lags by 30° and the inverse delta connection leads by 30°, as shown in Figure 5, which is the so-called "order angle retreating by the first angle". (5) Explanation 4: If both the first and second connections are changed to delta connections, the total phase difference can be calculated by algebraically summing the phase differences generated by the changes in the first and second sides. As shown in Figure 6(a), the first phase is (-300), the second phase is (-300), (-300) + (-300) = -600, lagging by 600, i.e., 2 o'clock, connection group D/D-2. As shown in Figure 6(b), the first phase is (-300), the second phase is (+300), (-300) + (+300) = 00, the phase difference is 00, which is connection group G/G-12. ⑹ If the first and second phase terminals are opposite, i.e., opposite polarity, the difference of 6 points based on the above judgment is called "polarity opposite, reverse and retreat", as shown in Figure 7a and b, 12 o'clock + 6 o'clock = 6 o'clock, the connection group changes from 12 o'clock to 6 o'clock. ⑺ If there is phase shift, after the above judgment, move one phase in the phase sequence direction, lagging by 1200, add 4 points, as shown in Figure 8(a), 11 o'clock + 4 o'clock = 3 o'clock, the group number changes from 11 o'clock to 3 o'clock. Moving one phase forward by 120° in the reverse phase sequence direction reduces the number by 4 points. In Figure 8(b), 12 o'clock - 4 o'clock = 8 o'clock, and the group number changes from 12 o'clock to 8 o'clock. (8) In summary, the group can be visually determined from the transformer connection diagram; the connection diagram can also be drawn directly from the group without the need for vector diagram identification and connection. This method is accurate, convenient, and easy to understand. This quick identification method for connection groups is also applicable to the identification and connection of power transformers in D/Y, Y/D, and Y/Y configurations, but not to the identification and connection of Z-type configurations. 3 Determining the Phase Sequence of the Main Power Supply The waveform diagram of the three-phase AC power supply is shown in Figure 9. The phase sequence should be U→V→W→U, that is, phase V lags phase U by 120°, and phase W lags phase V by 120°. When the equipment is first run or after maintenance, it is inevitable that the phase sequence will be incorrect. It is necessary to check and adjust it to meet the requirements. The steps are as follows: 3.1 Open the main circuit load and close the trigger circuit. 3.2 Connect the oscilloscope probe CH1 to the main power supply PE, and connect the CH1 probe to the main power supply U-phase terminal. Adjust the oscilloscope X-axis sensitivity so that the sine wave occupies 3 divisions for half a cycle, with each division being 600. 3.3 Connect the probe CH2 to the power supply V-phase terminal. If the oscilloscope displays the waveform in Figure 10 (a), where the waveform of CH2 lags behind the waveform of CH1 by 1200, it can be known that CH1 measures the U-phase waveform, and CH2 measures the V-phase waveform. Otherwise, the waveform is displayed as in Figure 10 (b), where CH2 measures the W-phase. In this case, simply swap the W and V phases. 4. Check the phase relationship between the synchronous transformer and the rectifier transformer 4.1 First, check the phase sequence of the secondary side of the synchronous transformer. The method is the same as that of the main circuit and will not be repeated. 4.2 Although the synchronous transformer has a phase sequence, it does not mean that the phase is coordinated with the main circuit. It is also necessary to check the phase difference between TS and TR to see if they correspond one-to-one. Steps: ⑴ Short-circuit the neutral lines of the secondary side of TR and TS, and clamp them with CH1. (2) Connect CH1 to the U phase of TR and CH2 to the U phase of TS. (3) Observe whether the waveform meets the phase difference mentioned in 2.1 above. If it does, it means that TR and TS have a one-to-one phase correspondence. If it does not, without changing the phase sequence of TS, move the entire circuit to the primary side of TS, i.e., U→VV→WW→U, until the phase difference requirement between TR and TS is met. 5. Check the sequence of the measured trigger pulses . The thyristor conduction sequence in the main circuit shown in Figure 1 is: VT1→VT2→VT3→VT4→VT5→VT6→VT1. The corresponding trigger pulses are Ug1→Ug2→Ug3→Ug4→Ug5→Ug6→Ug1. Each pulse lags the previous phase by 60°. Steps: 5.1 Turn off the main circuit. The trigger circuit works and there is a pulse output. 5.2 Clamp the CH1 probe to the star point on the secondary side of TS. Connect CH1 to Usa and adjust half a cycle to 3 divisions. 5.3 Connect CH2 probe to Ug1 (if using a pulse transformer, connect to the test point). 5.4 Sequentially connect Ug1, Ug2, Ug3, Ug4, Ug5, and Ug6 pulses and display them on the oscilloscope, as shown in Figure 11 (a) for Ug1, (b) for Ug2, and (c) for Ug6. This indicates the pulse sequence is correct; otherwise, check the previous steps until correct. 6. Setting the Initial Phase and Phase Shift Range When the given value is zero, the trigger pulse should cause Ud=0. Generally, it should be set slightly greater than 90°. Steps: 6.1 Based on the phase difference between TR and TS mentioned above, and considering the specific requirements, calculate the phase relationship between TS and the pulse when the requirements are met. 6.2 Disconnect the main circuit and activate the trigger circuit. 6.3 Connect CH1 ground to the TS star point, connect CH1 probe to TS Usu, and connect CH2 to the pulse or test point. 6.4 Adjust the negative bias voltage to make Usa and the pulse satisfy the phase relationship in 6.1. 6.5 Apply control voltage and adjust it; when the voltage is applied, apply a limit. 7. Dynamic Detection and Debugging of the Dual-Closed-Loop DC Speed ​​Control System 7.1 System Composition and Working Principle The dual-closed-loop DC speed control system is a complex control system combining thyristor converter technology and automatic control technology; dynamic debugging is crucial. A typical principle circuit is shown in Figure XII. To ensure good dynamic and static characteristics of the speed and current dual-closed-loop speed control system, both speed and current regulators use PI regulators, and both speed and current use negative feedback closed loops. This requires that the polarities of the speed setpoint voltage Ug and the speed feedback voltage Un are opposite, and the polarities of the current setpoint voltage and the current feedback voltage Ufa are opposite. The polarity of the current regulator ACR output voltage Uc is determined by the phase-shifting characteristics of the thyristor triggering device. The speed setpoint voltage Ug is compared with the speed feedback voltage Ufs and then applied to the input of the speed regulator ASR. The motor speed is determined by the setpoint voltage Ug. The output voltage Us of the ASR is used as the current setpoint voltage and applied to the input of the current regulator. The output voltage Uc of the current regulator ACR is used as the control voltage and applied to the trigger. The thyristor-controlled rectifier outputs a rectified voltage to ensure the motor operates at the set speed. The ASR output limit corresponds to the maximum current setpoint and depends on the motor's overload capacity and the system's maximum acceleration requirement. The ACR output limit restricts the current. The current regulator is designed to be unsaturated, while the speed regulator is designed to be both saturated and unsaturated. When the regulator is saturated, the output is constant, and changes in the input no longer affect the output. Only by applying a reverse input can the regulator exit saturation. When the regulator is unsaturated, the output is less than the limit value. The proportional-integral action ensures that the input deviation is always zero in steady state. However, due to the integral retention (memory) effect of the ASR and ACR, both the ASR and ACR have constant output voltages. This achieves zero steady-state error in both speed and current. 7.2 PI Regulator Performance Testing and Waveform Analysis The PI regulator is the core of the system, and its performance principle diagram is shown in Figure 13. The relationship between input and output is as follows: The working process of the PI regulator: When the input voltage is suddenly applied, capacitor C is equivalent to a short circuit, thus acting as a proportional regulator. Therefore, the output quantity produces an immediate response jump. As the capacitor charges, the output voltage gradually increases, which is equivalent to an integral element. As long as U0 continues to increase, it will reach a stable state. Test method: (1) Connect the wires as shown in Figure 13. Set the coupling mode of the slow-scan, long-persistence oscilloscope to GND, and the vertical mode to dual-trace CHOP (intermittent). Adjust the two scanning baselines to appropriate positions using vertical displacement. Set the coupling mode to DC, adjust the Y-axis sensitivity to 2V, and the X-axis sensitivity to 1S. The brightness should not be too bright, and the focus should not be too sharp to avoid damaging the oscilloscope with prolonged observation. Connect CH1 to Ui and CH2 to U0. (2) Short-circuit the integrating capacitor to discharge it, ground the input terminal, and adjust the zero-adjustment potentiometer to make the output zero, achieving zero input and zero output. (3) Loosen the capacitor shorting wire and slowly increase the input voltage in the positive direction. The output voltage will also slowly increase in the negative direction. Then slowly adjust the input voltage to zero. The output waveform will remain approximately horizontal (due to capacitor leakage, the waveform will slowly approach the baseline). On the oscilloscope, you can see a waveform with a slowly moving spot leaving an afterglow. (4) If the input voltage drops suddenly, the output waveform will jump (proportional adjustment takes effect), and then slowly increase in the negative direction (integral adjustment takes effect). (5) If the input voltage value is proportionally amplified and exceeds the output saturation value, the output will jump to the saturation value. After this, the integral will seem to have no effect, but no matter how abruptly the input voltage changes before the sign changes, the output will remain unchanged. This is actually due to the capacitor. The output will decrease (absolute value) when the input voltage changes in the opposite direction (sign change). (6) Increasing the proportional constant increases the amplification factor. When the input voltage changes abruptly, the output change increases, and vice versa. The above waveform is shown in Figure 14. (7) Increasing the integrating capacitor lengthens the integration time, making the output waveform slope, and vice versa. Having understood the working principle of the PI controller, its waveform changes, and the impact of various parameters on the waveform, we can analyze the system's dynamic performance based on the waveform during system debugging, and correctly connect and adjust the parameters. In actual debugging, connect the oscilloscope probe to the input and output points of the ASR or ACR, observe the waveform, and analyze various performance indicators in accordance with the principles of the PI controller described above. The current waveform should be as shown in Figure 15. Generally, oscillation is only allowed 1-2 times, and overshoot should be within 10%. Adjust the proportional constant appropriately, and dynamically monitor the system in conjunction with the requirements of the driven load. Adjust the overshoot, stability, and speed drop requirements according to the load, measuring and adjusting simultaneously. Simulate some special situations that may occur in production for predictive debugging, such as stalled rotor, sudden no-load, and grid voltage fluctuations. The goal is to balance speed, stability, zero steady-state error, and minimal oscillation to meet system requirements and fully utilize the equipment's potential. 8. Conclusion This article introduces a simple method for phase-determining operation using an oscilloscope, and discusses some operational techniques and skills for testing the dynamic performance of a PI controller. It aims to deepen understanding and application of converter technology knowledge during the debugging process, improve transformer knowledge, oscilloscope application skills, and electronic technology experimental skills, ultimately enhancing overall electrical maintenance capabilities. The method for quickly identifying transformer connection groups described in this article is based on my personal experience. The input and output waveforms of the PI controller (Figure XIV) were repeatedly simulated and measured in experiments, synthesized point by point based on records. Any errors are welcome to be pointed out.