Efficiency Improvement of Large Direct-Online Started Induction Motors : Industrial Design and Material Combinations for Caged Rotors

Sammanfattning

This project’s research area is electrical machines and drives, and more specifically medium voltage induction machine design, for large load starting- torque applications. When squirrel-cage induction machines are required to start large load torques, at a relatively low starting current, their rotor bars are designed with high resistivity materials to produce an effective electro- mechanical torque, since low-resistivity bars will not manage to accelerate the load to its full speed. However, a compromise must be struck when using high- resistivity materials. Higher resistances lead to higher power consumption and greater ohmic losses at nominal operation/load. Thus, changing to an intermediate resistivity solution (which is still able to start the load) should directly produce lower power consumption at any operating condition. The purpose of this project is to perform a thermal analysis on a given IM designed with multimaterial rotor bars. Since the current density in the slot will vary along the bar height (due to the conductivity difference between materials), the loss density will also vary and this will affect the bar temperature distribution. To fulfill the purpose of the project and its different goals, a research process, separated into five consecutive steps, was implemented. First, a literature review was conducted, where relevant previous work and theory was reviewed. Then, the three main phases of the project were conducted, namely developing the multimaterial analytical model, the electromagnetic, and the thermal finite element method models. Finally, the results of each simulation were analyzed and compared. Simulation results show that multimaterial rotor bars improve the starting characteristics for the reference induction machine, when compared with a design with rotor bars made with pure low-resistivity materials. Additional results show that the machine efficiency increases compared to a design with rotor bars made with pure high-resistivity materials. Steady-state Thermal simulations show that temperature distribution, in the rotor and bars are within normal operation limits for electrical machines, increasing confidence in the potential success of a prototype. Therefore, an induction machine with multimaterial rotor bars may be able to start and drive greater loads and produce fewer losses than their counterparts designed with rotor bars made of a single material, respectively. With this additional feature, induction machine designers may gain a greater flexibility for meeting restrictive torque customer requirements and efficiency legislation.