As the open phase fault occurs in a multiphase induction machine, it continues running even though its dynamic properties drastically changes. If neutral point of stator winding is not grounded, stator phase voltages become unknown and make the modeling so complicated. Using suitable basis vectors in six-dimensional space, phase voltages are calculated efficiently in this paper. Then, machine model in five-dimensional space is used to assess electromechanical torque and speed. So the presented model is a combination of machine models in both six and five-dimensional spaces. In order to make a comparison, previous method to estimate phase voltages is presented too. A simulation has developed to confirm the proposed model.