Analysis of LCI synchronous motor drives with finite DC link inductance

Steady-state operation of a load commutated inverter (LCI) fed synchronous motor drive is analysed by means of a digital computer based solution of the synchronous machine equations. The solution permits calculation of current, voltage, and torque waveforms and includes the effects of finite DC link inductance, saliency of the rotor and all machine resistances. The continuity constraints for steady-state operation are exploited to enable the proper initial conditions to be calculated directly, without the need for iteration. Operating points are described by the firing angle γ₀, and the overlap angle μ, and are computed for an assumed DC supply voltage of 1.0 per unit. The linearity of the state equations allows the normalised solutions to be scaled to any desired value of torque, DC link etc. Sample calculations are provided, showing the influence of the DC link inductance on the torque, DC link current and voltage waveforms. These are then compared with experimentally measured data taken from a 3 kVA LCI fed synchronous motor drive, and a good level of agreement is achieved