Abstract: This invention relates to an apparatus for determining the power angle in electrical AC machines. The invention includes means for determining the synchronous rotating magnetic field of the AC machine and means for determining the angular velocity of the machine's rotor. The value of the angular velocity of the machine's rotor is subtracted from the value of the synchronous rotating magnetic field. The difference between the synchronous rotating magnetic field of the machine and the value of the angular velocity of the machine's rotor is multipled by a predetermined constant to produce an accurate measure of the power angle of the machine being tested.