Depending on the circuit elements employed in the power factor correction technique, power factor correction can be subdivided into passive PFC, active PFC, and harmonic injection. Some time-special topologies are to be employed to reduce the component count. All of these techniques are explained here with simulation examples.
Passive PFC This correction method is the simplest and is employed by most power processor circuits, such as switching power supplies and advanced motor drives. It is very effective in lowering and limiting
FIGURE 10.8 Input voltage and current of the SRM driver with DBR.
current harmonic distortions. By proper design, it is also effective in achieving good power factors on the line side. These passive PFC circuits, as passive filters, can be added to either the AC or DC side. Generally, they are placed between the diode bridge rectifier mentioned before and DC link capacitor, i.e., on the DC side. Figures 10.9 and 10.10 show the passive PFC circuit and its effect on the supply line and voltage waveforms in simulation results.
As mentioned in the previous section, the nonlinear load power factor depends on displacement power factor too. Generally, a capacitor is added on the AC side to improve DPF. It can be seen from Fig. 10.10 that both phase current and voltage are not changed, but the supply current is very much in phase with the supply voltage. In other words, the power factor is improved considerably. In addition, peak of the input side current is reduced a lot. Thus, THD is also reduced to a great extent.
FIGURE 10.9 SRM drive with passive PFC along with DBR at its front end.
It is observed that the power factor with this method is not improved very close to unity. The size of the passive elements, i.e., inductor and capacitor, is also an issue. When more than one conversion stage is involved in the application, relative stability and EMI are the factors to be kept in mind while designing and inserting passive PFC circuits, specifically magnetic components, in the system. However, low cost and simple implementation make this technique attractive to many manufacturers.
Active PFC As this technique involves active switching elements, it is known as active PFC. There are many ways of achieving active power factor correction. A boost converter is the most popular and simplest one. Figures 10.11 and 10.12 show the power stage diagram and simulation results for an SRM drive with boost converter.
From the waveforms shown in Fig. 10.12, it is clear that by using a DC/DC boost converter, a power factor of almost unity can be achieved, and the load to the converter appears almost resistive. No current peak is visible in the supply current waveform. Here, a boost converter switch is controlled, keeping output voltage of the converter
FIGURE 10.10 Phase voltage and current waveforms and supply voltage and current waveforms with passive PFC.
FIGURE 10.11 SRM drive with boost converter along with DBR at its front end.
regulated, but this has nothing to do with controlling the switches of the main SRM drive circuit. During dynamic conditions, it should be observed that the overall system is not becoming unstable. Sometimes all the switches in a system are synchronized to avoid this problem. By modulating the duty cycle of the boost converter switch, the input current can be controlled to track the input voltage. With low distortion and accurate tracking between current and voltage, the power factor obtained from adding a front-end boost converter is typically higher than 0.99, and the input current THD is normally less than 5%. Limitation of the boost converter is that the output voltage should be always greater than the maximum peak supply voltage. To alleviate this problem, another PFC circuit has been developed called the buck-boost converter (Figs. 10.13 and 10.14). This active PFC topology can deliver output voltage both less and greater than the supply voltage depending on the line and load situation. Input voltage can also be tracked by controlling the current in a specific manner. Figures 10.13 and 10.14 show a buck-boost converter at the front end of the SRM drive and its simulation results.
FIGURE 10.12 Phase voltage and current waveforms and supply voltage and current waveforms.
FIGURE 10.13 Buck-boost converter at the front end of the SRM drive.
The boost and buck-boost converters seem to be overcompensating, if satisfying the standards is the only concern. The issue of choosing a passive or active PFC seems to be the tradeoff between cost and effectiveness. The effectiveness means the extent that harmonics are eliminated or reduced, not how well the method complies with the standard. However, with the trend of continuous reduction in semiconductor cost, this barrier will also soon be removed.
Specifically for the SRM, to achieve a high power factor, changes are proposed in the C-dump converter (Fig. 10.15). It has been proved that a very high power factor can be achieved using this SRM drive. This is almost the same C-dump converter discussed earlier with some changes. This also falls in the category of active PFC, or one might say it is a hybrid PFC technique. This is because the switches in the drive circuits are controlled to achieve not only the required phase current and voltage, but also a very high power factor. It can also be seen from the power stage diagram shown in Fig. 10.15 that the mutual inductor just before the DC link capacitor appears almost identical to the passive PFC circuit shown previously in this section.
FIGURE 10.14 Phase voltage and current waveforms and supply voltage and current waveforms.
FIGURE 10.15 SRM driver with modified C-dump converter.
Figures 10.16 and 10.17 show the operating modes and simulation results, respectively, of the same converter. A very high power factor without noticeable current distortion can be observed from the simulation results.
Harmonic Injection/External Compensation This method is generally used to filter the harmonics from the line but not to improve the power factor. However, it can be employed for power factor correction. The sizes of passive components increase when they have to remove low harmonic components (source current and voltage are low-frequency signals). This filter is usually configured to plug into an outlet and serve as a plug-in point for 2-to-4 electronic devices. There are three main types of this filter: parallel resonant, series resonant, and series-parallel resonant.
POWER FACTOR CORRECTION METHODS (Electric Motor)