This paper develops a version of the synthetic loading method, suitable for the testing of multiphase machines with multiple three-phase distributed windings. The method is at first discussed in general terms for a structure with k three-phase stator windings (i.e., total number of phases is n = 3k). Subsequent detailed development is described for a dual three-phase (six-phase) stator winding configuration. With a control architecture that allows the use of half of the three-phase windings as a motor and the other half as a generator, themachine (and/or the converter) can be tested under full rated power without the need for any mechanical load. Moreover, the power consumed from the grid is in essence equal only to the total losses of the system. Modeling, based on the double d-q approach, and the control layout that includes full cross-coupling decoupling are described for a permanent magnet (PM) synchronous machine. An experimental test rig with a double three-phase PM machine of 150 kW rating is detailed and the samples of experimental results are provided to verity the theoretical considerations.