The problem of estimating the currents in the windings of a three phase brushless DC machine from measurements of a single drive input current is discussed in this paper. The proposed solution involves the application of a state observer, which uses errors between the actual and estimated drive input currents to update its estimates of winding currents. Such estimation is possible only if the winding currents are observable (i.e. knowledge of the drive input currents is sufficient to derive the winding currents), and it is shown that observability depends on drive operating condition, PWM switching strategy and machine winding connection. All of these factors are discussed for the case of current measurement in the DC link and a novel measurement strategy involving the measurement of total switching device current. An important advantage of the single current measurement method is that measurements are made equally on all three phases of the machine, so that the effects of sensor imbalance are eliminated.