This paper presents a method for reducing the simulation execution time associated with modelling a Permanent Magnet Synchronous Motor and Voltage Source Inverter (PMSM-VSI) based drive system. The proposed method is based on an analytical estimation of the 'average' ac voltage across each phase of the PMSM during each PWM switching period and then using these averages as piecewise-linear voltage sources for each motor phase. The model reproduces the functionality of the VSI and outputs 3 phase ac phase currents from a DC voltage source based on the existing PMSM motor and digital controller models such that machine and controller performance/limitations can still be evaluated and optimised. The VSI based model and average voltage model have been implemented in the PORTUNUS simulation package. Simulation results for a small <1kW PMSM shows excellent agreement of the average voltage model with the switching model over the complete operating range, including the non-sinusoidal phase current field-weakening region, but with the execution time of the average voltage model being typically five times faster.