The author presents a field analysis for slip-ring induction motors with general external impedance asymmetry. The method is based upon a rotating-field theory approach including spatial magneto motive force harmonics. It is shown that a series of non-mains frequency voltages are induced in the stator winding for both balanced and unbalanced rotors. The method is also used to derive the harmonic equivalent circuit for a balanced three-phase slip-ring motor with a skewed rotor. The analysis is verified by comparing predicted torque and current characteristics with measured values obtained from a machine with a balanced rotor and with a rotor connected to asymmetrical external resistances.