An ac motor for high-torque drive has a transformer with a magnetic circuit with a central limb. Either the two ends or the two connected electrodes of the secondary winding of the transformer project at an angle to the longitudinal axis of the secondary winding; an armature rotor dielectrically bridges the two ends or the two connected electrodes, moving parallel to the longitudinal axis. The rotor forms two series-connected capacitors with the two ends or the two connected electrodes. An LC circuit is formed by the two capacitors in series to the total effective inductance of the power source, the electric transformer, the two ends or the connected electrodes and the armature. The impedance of the LC circuit is varied by the frequency of the power source supplying the primary winding of the transformer, in order to push required electric power from the power source to the series LC circuit.