Differential Electrical Shaft Combined with a Double Kramer Cascade

A new electrical transmission drive that insures an adjustable constant speed difference between a pair of mechanical shafts independent of load conditions or speed level is described in the paper. This generalized electrical shaft comprises two rotor interconnected induction machines, fed from different frequencies and two dc machines forming a double Kramer cascade. The use of the Kramer cascades offers many important advantages over the alternative simpler scheme of´using resistors in the rotor circuit, namely, higher efflciency, much lower speed dependence on load, and speed level adjustability in the subsynchronous range. The drive performance is analyzed by means of an equivalent circuit from which the current and torque expressions are derived. Good agreement is obtained between theoretical and experimental results.