Synchronized Switching Modulation to Reduce the DC-Link Current in SRM Drives

Abstract

Switched Reluctance Machines (SRM) are emerging as a possible alternative in terms of cost and supply stability to rare earth based electric vehicle traction systems. However, because of the huge amounts of energy stored and transferred back and forth between the DC source and the SRM, large DC-link capacitors must be used as buffers, which increases overall costs and size. This paper proposes a novel modulation technique which forces the exchange of energy between phases while decreasing the energy transfer between the DC bus and the SRM. This means lower DC bus currents (capacitor size and cost reduction) and lower Joule-effect conduction losses (better efficiency). The proposed modulation has been validated experimentally in a test bench and compared with the conventional torque-sharing function. INDEX TERMS Switched reluctance motors, motor current control, inverter modulation, capacitor current reduction.

A Methodology to Determine the Effect of a Novel Modulation in the Reliability of an Automotive DC-Link Capacitor

“…Nonetheless, some factors are still conditioning a more widespread adoption of SRM-based traction systems [14], such as the complex control due to its no-lineal features and the pulsed nature of its excitation current (derived in torque ripple, vibrations and noise), affecting negatively in the performance of the electric machine. Above all these, the large amount of magnetic energy stored in the windings (which is transferred back and forth between the DC source and the SRM through the power converter, causing considerable current peaks [15]) is an important pitfall to overcome.…”

“…Likewise, [23] proposes a method to take advantage of a phenomenon which favors the magnetic energy exchange between phases, and [24] applies the method to develop a technique which aims to maintain a constant average DClink current over a switching cycle. An enhanced modulation scheme has been published in [14], named Synchronized Switching Modulation (SSM), which takes advantage of the phenomenon synchronizing the phases during the overlap which favors the magnetic energy exchange between phases, thus decreasing the DC bus -SRM energy transfer.…”

“…The approach of [14] has two main benefits. On the one hand, the current peaks that DC-link capacitors must sink/source are significantly mitigated, which makes it possible to use lower capacitance, i.e.…”

“…Nevertheless, there is still another aspect not taken into account in [14]. This current reduction, provided by the use of the SSM, decreases the thermal stress and affects directly to the life-cycle of the capacitor.…”

“…In addition, the scheme allows to easily apply the methodology to analyze any control which affects the DC-link current. Secondly, the enhanced methodology is employed at the same time to complete the SSM analysis from [14] in a lifetime point of view. The methodology has been developed together with TDK, who has also collaborated in this work providing the automotive DC-link model and its electrothermal simulations.…”