Inductive load of single-phase half-wave controllable rectifier circuit

Inductive load of single-phase half-wave controllable rectifier circuit

In practical applications, in addition to the resistive loads mentioned in the previous article, inductive loads are often encountered, such as excitation windings of various motors, various inductive coils, etc. An inductive load contains both an inductance and a resistance, so it can be represented by an inductance L and a resistance R in series. Since the inductance hinders the change of the current, the current in the inductance cannot change abruptly. When the current flowing through the inductance changes, an induced electromotive force is generated at both ends of the inductance to prevent the current from changing. When the current increases, the polarity of the induced EMF prevents the current from increasing; when the current decreases, the polarity of the induced EMF prevents the current from decreasing. Therefore, the working conditions of the controllable rectifier circuit with an inductive load and a resistive load are quite different.

Working principle and waveform

The circuit diagram and waveform diagram of the single-phase half-wave controllable rectifier circuit with inductive load are shown in Figure 1.

Figure 1 - Circuit diagram and waveform diagram of single-phase half-wave controlled rectifier circuit with inductive load
Figure 1 – Circuit diagram and waveform diagram of single-phase half-wave controlled rectifier circuit with inductive load

When ωt1=α, the thyristor VT is triggered and turned on, the AC voltage u2 is immediately added to the load (Ld and Rd), and the rectified output voltage ud appears on the load immediately. However, due to the action of the inductance Ld, an induced electromotive force (the polarity of which is up positive and down negative in Figure 1a) is generated, which hinders the current change. The current in the inductance (that is, the load current) cannot be abruptly changed, but can only gradually increase from zero. When the current rises to a maximum value, the induced EMF is zero, and then when the current decreases, the induced EMF also changes polarity (up negative and lower positive in Figure 1a). When the AC voltage u2 drops to zero, due to the induced electromotive force of the inductor, the thyristor VT is still turned on by the positive voltage, even if the AC voltage u2 changes from zero to negative, as long as |eL|>|u2|, the thyristor VT is still subjected to the forward voltage, the thyristor will continue to conduct, and the rectified output voltage ud on the load will have a negative value. Until the anode current of the thyristor is less than the holding current, the thyristor VT is turned off and bears the reverse voltage immediately.

From the above analysis, a basic analysis method of power electronic circuits can be summarized. In fact, there are nonlinear power electronic devices in power electronic circuits. If the turn-on and turn-off processes are ignored, the device can be idealized, and the circuit can be simplified as a piecewise linear circuit, and each state of the device corresponds to a linear circuit topology. The above method is used for the analysis of single-phase half-wave circuit, that is, when VT is in an off state, it is equivalent to the circuit being disconnected at VT, and id=0; when VT is in an on state, it is equivalent to a short circuit at VT.

As can be seen from the waveform diagram in Figure 1, with an inductive load, the waveforms of the rectified output voltage ud and current id are very different from those of a resistive load. Due to the action of the inductance Ld, the rectified output voltage ud will have a negative voltage for a period of time, which reduces the average value of the rectified output voltage Ud. The larger the inductance Ld is, the larger the negative voltage part is, and the more the average value Ud of the rectified output voltage decreases. When the inductance Ld is large and meets the condition of ωLd>Rd (usually ωLd>10Rd), the positive and negative areas of the rectified output voltage ud waveform on the load are nearly equal, and the average value of the rectified output voltage Ud≈0. It can be seen that when a single-phase half-wave controllable rectifier circuit is used for a large inductive load, no matter how α is adjusted, the average value of the rectified output voltage Ud is always small, so this circuit is not actually used. When the actual single-phase half-wave controllable rectifier circuit has an inductive load, a freewheeling diode is connected in parallel at both ends of the load.