ASD EXAMPLE (Electric Motor)

10.1
As an example, a C-dump converter for an advanced switched reluctance motor (SRM) drive is investigated. The SRM cannot be operated by directly connecting to the mains. A power electronic interface is absolutely necessary for running it optimally. Various power electronic topologies have been proposed. C-dump converter topology is one of the popular topologies to drive SRMs. As this topology contains maximum number of passive elements and complexity, it is chosen to verify the PFC function for advanced drives
as a worst-case scenario. A power stage diagram and operating modes circuit reductions are shown in Figs. 10.4 and 10.5.
Three inductors grouped together represent three phases of the machine. The C-dump converter is operated in such a way that the required phase is energized and de-energized at required instances, i.e., aligned and unaligned positions of the rotor. Both hysteresis current control and PWM control are possible. Performance of the PFC circuit remains the same in all operating conditions. Even with different drives of different machines, this general PFC approach is equally effective.
The switches are turned on when the rotor is at the unaligned position, and they should not be conducting after the rotor phase passes the aligned position with the stator. There are various modes possible depending on particular applications. Here, when switches are turned on, the phase inductor and dump inductor store energy, and when the switches are turned off, both of them release the energy.
SRM drive with C-dump converter.
FIGURE 10.4 SRM drive with C-dump converter.
tmp7E8_thumb1Single-phase power stage and operating state diagrams of C-dump SRM drive.
FIGURE 10.5 Single-phase power stage and operating state diagrams of C-dump SRM drive.
The dump capacitor charges in a half cycle and discharges in another half cycle. The voltage level of this capacitor should be higher than the DC link voltage. Figure 10.6 shows the phase voltage and current waveforms when the drive is operated with the DC supply.
However, this is not the case with the home or commercial applications of many drive systems. Those drive systems pass through
Phase current and voltage of SRM with DC supply at the front end.
FIGURE 10.6 Phase current and voltage of SRM with DC supply at the front end.
one or more power conversion stages. In case of advanced DC drive systems, the AC supply of the utility is usually converted using a simple diode bridge rectifier (DBR). As mentioned before, when only a DBR is connected between the drive and utility, the smoothing capacitor gets charged and discharged during the high line periods (short time intervals), and high current spikes occur. This deteriorates both power factor and overall system performance. Figure 10.7 explains the power stage of such uncontrolled DC link. Waveforms are depicted in Fig. 10.8. From the supply voltage and current waveforms shown in Fig. 10.8, it is clear that high distortion occurs on the supply side.
The effects of poor power factor are illustrated in this section. This has encouraged various organization to introduce standards for allowable harmonics and minimum power factors of power electronic systems. By doing so, power quality of the overall power system can be improved by a considerable amount. In addition, material cost
SRM drive with DBR at its front end as uncontrolled DC link.
FIGURE 10.7 SRM drive with DBR at its front end as uncontrolled DC link.
saving can be realized in home appliance systems. The next section discusses different basic methods of power factor correction.


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