As a core component of power electronic equipment, the AC voltage regulator's core function is to dynamically adjust the amplitude and waveform of the output voltage to meet the stringent power quality requirements of different loads. The silicon controlled rectifier (SCR), a key power device in the regulator, directly determines the distortion of the output voltage waveform and energy transfer efficiency through its conduction angle control, making it one of the core parameters affecting regulator performance.
In the topology of an AC voltage regulator, the SCR is typically connected to the main circuit in anti-parallel configuration. Voltage chopping regulation is achieved by controlling its conduction timing. When an AC power supply input is a sine wave, the SCR triggers conduction with a certain delay after the voltage crosses zero. At this point, the load-side voltage waveform is truncated, with only the latter half retained. The magnitude of the conduction angle directly determines the proportion of the truncated waveform: the smaller the conduction angle, the more truncated portion, and the lower the effective value of the output voltage; conversely, as the conduction angle increases, the output voltage approaches a complete sine wave. This discontinuous conduction mechanism results in an output waveform exhibiting obvious pulse characteristics, rather than an ideal sine wave, thus introducing harmonic components.
The impact of the conduction angle on the output waveform is primarily reflected in the harmonic distribution. Due to the chopping effect of the thyristor, the output voltage contains odd harmonics (such as the 3rd, 5th, and 7th harmonics) in addition to the fundamental frequency. The amplitude of these harmonics has a non-linear relationship with the conduction angle: when the conduction angle approaches 180°, the harmonic content decreases significantly, and the waveform approaches a sine wave; however, when the conduction angle decreases, the amplitude of lower harmonics (especially the 3rd harmonic) increases sharply, leading to an increase in waveform distortion (THD). This harmonic pollution not only reduces the operating efficiency of the load equipment but may also interfere with other electronic devices through conduction or radiation, causing electromagnetic compatibility problems.
Furthermore, changes in the conduction angle also affect the phase characteristics of the output voltage. Ideally, the output voltage should be in phase with the input voltage, but the delayed triggering of the thyristor causes phase lag. When the conduction angle is small, the phase lag is more pronounced, which significantly reduces the power factor (PF) of the regulator in inductive or capacitive load scenarios. A deteriorating power factor not only increases line losses but can also trigger malfunctions in the power grid's reactive power compensation devices, affecting the stability of the entire distribution system. Therefore, modern AC voltage regulators often optimize phase characteristics through phase compensation technology or the use of bidirectional thyristors (TRIACs), but conduction angle control remains fundamental.
The dynamic adjustment capability of the conduction angle is also an important indicator of regulator performance. During sudden load changes or input voltage fluctuations, the regulator needs to quickly adjust the conduction angle to maintain stable output voltage. If the conduction angle adjustment lags, the output voltage will overshoot or drop, causing abnormal operation of sensitive loads (such as precision instruments and motor drives). For example, during motor startup, if the conduction angle fails to increase in time to provide sufficient torque, the motor may stall due to undervoltage; conversely, when the load decreases, if the conduction angle fails to decrease in time, the output voltage will rise, potentially damaging equipment. Therefore, the regulator's control algorithm needs high response speed and accuracy to achieve dynamic optimization of the conduction angle.
From an energy efficiency perspective, the choice of conduction angle directly affects the regulator's loss distribution. A thyristor exhibits a conduction voltage drop (typically 1-2V) in its on-state, and its power consumption is proportional to the on-current. When the conduction angle is small, although the conduction time per cycle is shortened, the current amplitude needs to increase to maintain output power, potentially leading to increased conduction losses. Simultaneously, harmonics generated by chopping can induce additional losses in the load (such as motor iron losses and capacitor heating). Therefore, optimizing the conduction angle requires comprehensive consideration of output waveform quality and energy efficiency, typically achieved through multi-level technology or soft-switching techniques to reduce losses.
In practical applications, the conduction angle control of an AC voltage regulator also needs to consider electromagnetic interference (EMI) suppression. The rapid switching action of the thyristor generates high-frequency noise; if the conduction angle adjustment is too frequent or drastic, EMI levels may exceed limits. To address this, regulators often employ filtering circuits (such as LC filters) or frequency modulation techniques to disperse harmonic energy, but these measures increase system complexity and cost. Therefore, a reasonable design of the conduction angle requires a balance between performance, cost, and reliability to meet the needs of different application scenarios.
