Waveform coefficient in silicon controlled rectifier circuit

The ratio of the effective value of a certain voltage (or current) to its average value is called the waveform coefficient. The form factor is a noteworthy issue in silicon controlled rectifier circuits. To illustrate this problem, we first conduct an experiment based on the silicon-controlled half-wave rectifier circuit shown in Figure 1. The models and parameters of each component are for reference only. First adjust the R value to the maximum and turn on the power. At this time, the DC voltmeter indicates zero and the bulb does not light up. Then slowly reduce the R value, the voltmeter reading gradually increases, and the bulb gradually becomes brighter. We will find that when the DC voltmeter indicates 10 volts, the bulb has reached normal brightness, which means that the power consumption of the bulb has reached the rated power. If the voltage continues to increase, the bulb may burn out. Why is it that the reading of the voltmeter is far from reaching the rated voltage of the bulb of 36 volts, but the power consumption of the bulb has reached the rated power? The current flowing in the light is a one-way pulsating current, and the voltage at both ends of the bulb is a one-way pulsating voltage. Its waveform is shown as the solid line in Figure 2. The reading of the DC voltmeter is the average value of this pulsating voltage, and D is its effective value. Its effective value is much larger than the average value. According to electrical engineering knowledge, the effective value U of this periodic unidirectional pulsating voltage. It is the arithmetic square root (root mean square value) of the average value of the square of the instantaneous value within a period. That is, by different Q values ​​into equation (3), the corresponding K value is obtained, as shown in Table 1. It can be seen from Table 1 that when the phase shift angle of the thyristor changes from zero to n, the waveform coefficient K value gradually increases, and the increase speed becomes faster and faster. When it is close to I, the K value will increase rapidly (while both U and Uo will decrease rapidly.) Now let's look at the experimental results. According to formula (2), it can be calculated that when the DC voltmeter indicates 10 volts, it is U. =10 volts, CO$n=-0.7979, waveform coefficient K"3.57, Uo"35.7 volts. Uo is quite close to the rated voltage of the bulb, so the bulb reaches normal brightness. According to the same principle, it can be calculated that when G is the same, in the full-wave controllable rectification circuit with resistive load, the output pulsating voltage (waveform is shown as the solid line in Figure 3) is 1/2 times the coefficient. In the above calculations, the forward voltage drop when the thyristor is turned on is ignored. This article will not go into details about other forms of rectifier circuits and the shape coefficient of the output voltage when the load is inductive. From the above analysis, it can be seen that when using a thyristor for rectification, the reading of L on the DC voltmeter (or ammeter) is the average 1K value of the output voltage (or current). The reading cannot be directly substituted into the formula BU U2 L to calculate The power consumption on the load, this is because U in the formula is the effective value of the voltage on the load R, that is, U=Uo. If you want to reduce the waveform coefficient and make the effective value of the output voltage close to the average value, there are three measures to take: (1) Minimize the phase shift angle of the thyristor. For example, when Q: o, then K = I. 57 (single-phase half-wave): (2) When the load rated voltage is much lower than the effective value of the input AC voltage, use a transformer to step down the voltage first and then rectify it; (3) Try to use one-way controllable rectification or three-phase controllable rectification Rectifier circuit. If the influence of the waveform coefficient is ignored, even though the voltmeter reading is far from the rated voltage of the load, it is still possible to burn out the electrical appliance and cause undue losses. This must be noted. In practical applications, for convenience, we can estimate the form factor of different output DC voltages according to Table 2, thereby estimating the effective value of the output voltage. n in Table 2 is the reading U of the DC voltmeter. The ratio to the effective value U of the input AC voltage. That is, 23 (3), the corresponding waveform coefficient K can be obtained. For example, in the circuit shown in Figure 1, when the DC voltmeter indicates 50 volts, n=50/220"0.23. According to Table 2, it can be estimated that the waveform coefficient K is between 2.32 and 1.98 at this time. time. For the full-wave controllable rectification circuit, based on the same principle, it can be concluded that in the full-wave controllable rectification circuit, corresponding to different n values ​​(n reaches the maximum value of 0.9 when the thyristor is fully conductive) The waveform coefficient K.