Comparative Study on Bar Materials in Cage-Start Synchronous Motors: Influence on Starting Torque, Current and Thermal Behavior

This paper investigates the influence of conductor materials - copper (Cu), aluminum (Al), brass (CuZn), bronze (CuSn), and copper-clad aluminum (CCA) - on starting torque, current, and thermal behavior of cage-start synchronous motors. A 6 MW, 6 kV, 50 Hz salient-pole motor was analyzed using coupled electromagnetic-thermal FEM with 0.5 ms time step and refined air-gap mesh to evaluate torque-slip, current-time characteristics, Joule losses, and rotor bar temperature rise.Based on the normalized per-unit results, the maximum starting torque was 2.87 p.u. for Cu, 2.71 p.u. for CCA, and 2.46 p.u. for Al. The acceleration time (t95) values were 13.2 p.u., 10.9 p.u., and 11.5 p.u., respectively, while normalized Joule losses (E_J) were 1.00, 0.83, and 0.93 normalized units. Cu achieves maximum starting torque but longest acceleration and highest losses. Al provides smoother acceleration with reduced heating. Brass and Bronze offer superior starting torque but extended acceleration and elevated losses. CCA material presents optimal compromise: achieves 99% of Cu pull-out torque (2.71 vs. 2.87 p.u.), 17% lower losses (0.83 norm.), and 21% faster acceleration than Al (10.9 p.u.). Material selection significantly impacts starting dynamics and thermal stress. The CCA hybrid conductor provides superior performance-efficiency balance for high-power synchronous motors in industrial and power generation applications.