Interconnected rotor induction motors

The stators of two identical wound-rotor induction motors are connected to a common supply to produce fields revolving in the same direction. The shafts are coupled mechanically and the rotor circuits are electrically connected together with the voltages initially phased so that no rotor current flows. By rotating one stator relative to the other through an angle ß the rotor voltages are no longer in anti-phase, rotor currents flow and the unit produces torque. Boucherot adopted a similar scheme in the construction of a composite machine with two stators and a common squirrel-cage rotor. The mid-points of the rotor bars were connected together through a resistive element so that currents always flowed in the rotor circuits and zero torque could not be obtained. Expressions for torque are derived for the shunt-interconnected rotor developed by Boucherot and for the purely series-interconnected motor investigated by the author. It is shown that, for the former, torque cannot be controlled from zero without voltage variation, whereas in the latter arrangement zero torque is always obtained when the phases of the induced rotor voltages are in opposition. The properties and applications of the series interconnected motor circuit are discussed. By the insertion of impedance the torque/speed characteristics of the motors can be modified retaining the zero torque position. Possible applications include small hoists, cranes or lifts, and positioning, manipulating or tensioning drives for which precise control is required.