DEVELOPMENT OF THE COPPER MOTOR ROTOR-MANUFACTURING CONSIDERATIONS AND MOTOR TEST RESULTS

1. INTRODUCTION Because the electrical conductivity of copper is nearly 60% higher than that of aluminum, one would expect the I2R losses in the rotor to be substantially lower if copper were substituted for aluminum as the conductive material of the squirrel cage structure. Motor modeling by several manufacturers has shown that motors with copper-containing rotors would have overall loss reductions of 15 to 20%. Aluminum has been the material of choice for all but very large motors because the intricate squirrel cage is readily manufactured by pressure die casting through the rotor lamination stack. The large motors (>250 Hp, 200 kW) and a few smaller special purpose motors with copper in the rotors are assembled by a slow and costly fabrication technique that is not economical for production of the millions of integral and fractional horsepower motors sold annually. Die casting of the copper will be required for rapid and cost-effective manufacture, but the process has not been practical because of short die life resulting from the high melting temperature of copper. Several phases of this development are summarized in this paper. The first phase concerned the manufacturability of the copper rotor and addressed the problem of die life in pressure die casting copper by surveying a number of candidate high temperature die materials and the optimum conditions for their use to maximize die life. The significant results of the die material study are outlined here; more complete accounts have been published elsewhere [1], [2]. Although the motor test results indicated that the copper rotor die castings were of high quality, subsequent examination of sectioned end rings showed some large pores. Porosity is a common problem in high pressure die casting. This prompted a study to minimize the occurrence of large pores by modeling of shot profiles and experimental confirmation of the model predictions. In the second phase, copper rotors were die cast for several major motor manufacturers for evaluation by dynamometer testing in their own facilities. This paper presents these data on performance of motors incorporating the die-cast copper rotors and compares performance to that of the same motor with an aluminum rotor. Available data from the literature on motors built with copper rotors are also summarized. Finally, in a phase still ongoing at this writing, frequency response modeling of the rotor bars is being used to guide the design of the slot pattern in the rotor to …