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This paper presents the analysis and validation of a real-time ac drive simulator. Real-time simulation has been used for over a decade in power systems engineering to test controllers thoroughly, efficiently, and safely. The development and testing of large ac drives could benefit from this type of simulation, but real-time power system simulators have modeling restrictions preventing stable and accurate simulation of isolated drives switching at high frequencies. These restrictions can be overcome by combining in a single model both the power converter and motor models. After discussing the salient issues related to the real-time simulation of ac drives, a prototype real-time simulator for drives is described. Its ability to accurately emulate the behavior of large drives is demonstrated through two case studies. A first case study demonstrates the feasibility of simulating a drive of typical complexity in real-time. The second case study demonstrates how the controller for a PWM VSI fed induction motor, switching at 4 kHz, is designed, implemented in a microcontroner and tested in real-time using a simulated inverter, motor and load. Comparisons against results obtained with another simulation tool, which uses extremely accurate variable-step integration algorithms, demonstrates the validity of our approach. Index Terms--AC motor drives, hardware-in-the-loop testing, real time simulation, state space methods.