A Direct Drive Sensorless Starting Scheme Based on Triple Current Threshold Method in Switched Reluctance Machines for Low-Cost Current Sampling Applications

This article proposes a new direct drive sensorless starting scheme in switched reluctance machines (SRMs). First, the performances in terms of accuracy, injected current pulse value, negative torque, total torque, and torque ripple of different current threshold methods are comprehensively compared, which is rarely reported in the literature. On this basis, with the advantages of high accuracy, low injected current, largest starting torque ability, and lowest torque ripple, the outgoing phase injection double current threshold method is selected as the best candidate. To solve the problem that the phase current cannot be detected under the demagnetization state in typical low-cost resistor-based current sampling applications, a triple current threshold method is proposed, whose first two thresholds represent the turn- ON and turn- OFF instants and the third threshold indicates that the demagnetization current has returned to zero. To extend the speed range of the proposed method, an analytical method combined with a look-up table searching method is proposed to accurately predict the current threshold value, which can realize injection as soon as the demagnetization current returns to zero. Finally, both steady state and starting experiments under different load conditions verify the effectiveness and accuracy of the proposed method.