The Engineering Meetings Board has approved this paper for p ublication. It has successfully completed SAE’s peer review process under the supervision of the session organizer. This process requires a minimum of three (3) reviews by industry experts. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form o r by any means, electronic, mechanical, photocopying, recording, or otherwise, without the prior written permission of SAE. ISSN 0148-7191 Positions and opinions advanced in this paper are those of the author(s) and not necessarily those of SAE. The author is solely responsible for the content of the paper. SAE Customer Service: Tel: 877-606-7323 (inside USA and Canada) Tel: 724-776-4970 (outside USA) Fax: 724-776-0790 Email: [email protected] SAE Web Address: http://www.sae.org Printed in USA 2009-01-0956 The Case for Induction Motors with Die-cast Copper Rotors for High Efficiency Traction Motors James L. Kirtley Jr. Massachusetts Institute of Technology Richard F. Schiferl Baldor/Dodge/Reliance Advanced Technology Dale T. Peters and Edwin F. Brush, Jr. Copper Development Association Inc. Copyright © 2009 SAE International ABSTRACT This paper considers the application of die-cast copper rotor induction motors in the drive system of parallel gas/electric hybrid vehicles and compares performance in a realistic driving scenario to that of a permanent magnet motor where efficiency is substantially reduced by PM drag loss. It is concluded from this analysis that the induction machine has a substantial advantage because it can be de-excited when it is not producing torque, eliminating no-load rotational magnetic and electrical loss. Application of die-cast copper rotor traction motors in the hybrid drive system of the latest generation of large U.S. Army severe-duty trucks is then considered. Results of two different electric motor designs are presented, one with a cast aluminum rotor cage and one with a die-cast copper rotor cage. The copper die-cast rotor motor is shown to be 23% lighter and 30% smaller than the aluminum rotor machine. INTRODUCTION Current energy economics and the prospect that energy prices will be a larger item in budgets than has been the case in the past has driven governments, industry and consumers to place enhanced value on energy efficiency in general. With regard to electric motors that utilize nearly 50% of U.S. electrical energy, the United States has been a leader in passing a series of energy-saving laws over the past twenty years. The 1992 Energy Policy Act (EPAct 1992) initiated requirements for minimum motor efficiency ratings. EPACT 2005 established NEMA Premium® efficiency ratings as the basis for federal electric motor purchases and, more recently, The Energy Independence and Security Act of 2007 expands the types of motors to which efficiency standards apply and increases the efficiency minimum that a large group of commonly used motors must meet to the NEMA Premium® level. The steadily rising efficiency minimums have driven a considerable effort by both domestic and foreign motor manufacturers to redesign the induction motor to achieve these elevated efficiency minimums. The largest energy losses in an induction motor are the I 2R losses in the stator windings and in the rotor conductor. The fraction of total full-load stator I2R loss decreases with increasing motor size while that due to rotor conductor (about 25% of the total loss) increases slightly with increasing motor power rating. Generally the designer seeks to reduce the resistance in these conductive paths. This can mean more copper in the stator windings. In the rotor, an obvious approach was to replace the aluminum in the squirrel c