Abstract—This paper discusses design and control techniques for fault tolerant permanent-magnet (PM) machines that are suitable for applications where safety is of paramount importance. Back-EMF based design and analysis approach for a multiphase air-core linear motor has been presented in this paper. Apart from the design approach, fault tolerant techniques for current control in both time and frequency domains have also been discussed. In time domain, an iterative learning based control method is also presented and is found to be very robust. Both open-circuit and short- circuit fault tolerant operation have been discussed in this paper. Index Terms— AC machines, adaptive control, control design, current control, fault tolerance, motor drives, optimal control, permanent magnet machines, torque control. I. NOMENCLATURE v s Synchronous speed of motor ܧ ௡ Peak value of harmonic component of back-EMF ܤ ௬ ሺݕ ,ݔሻ Normal component of flux density in (x,y) plane N Number of turns in a single coil L Length of motor  Pole pitch n Harmonic number  ௪ Winding factor of the coil assembly  ௜ Instantaneous current of phase ‘i’  ௜ ,  ௜ Instantaneous back-EMF of phase ‘i’ ܨFault matrix  כ Reference torque  כ Optimal reference current  ଴ כ Initial reference current  ௠ Machine torque This work was supported by the Office of Naval Research under award no. N000140910886 A. Mohammadpour, A. Gandhi and L. Parsa are with the Department of Electrical, Computer and Systems Engineering, Rensselaer Polytechnic Institute, Troy, NY 12180 (e-mail: gandha2@rpi.edu).  ௗ Oscillating torque disturbance  ௦௖ Short-circuit current ݎ ௙ Fault resistance  Fundamental electrical frequency ܧ ଵ , ܧ ଷ Peak value of fundamental and third harmonic components of back-EMFs ܫ ଵ , ܫ ଷ , ܫ ଷଵ , ܫ ଷଶ Peak value of fundamental and third harmonic components of currents  Instantaneous value of input power  ௜ Peak value of ‘i’th harmonic component of input power II. INTRODUCTION lectrical drives play significant roles in several safety critical applications such as aerospace, naval, transportation and others. These applications demand certain special features from the drives with the most important of them being the fault tolerant capability - the drives are expected to perform with a reasonable performance even when a fault occurs. Several faults can occur in an electrical drive system and are primarily classified into either mechanical or electrical faults. Electrical faults are among the most commonly occurring faults. In this paper, design and control of drives with regards to their fault tolerant ability to electrical faults are discussed. This paper is organized as follows. Section II discusses the design approaches that are carried out to provide fault tolerance to electrical machines. A back-EMF based modeling, design and analysis method for a multiphase linear air-core permanent magnet linear synchronous motor (PMLSM) is described in section II. Section III presents the various current control methods that have been developed to achieve fault tolerant operation in case of short-circuit or open-circuit faults. Section IV concludes the paper. III. DESIGN APPROACHES TO FAULT TOLERANCE The most common design methodology to include fault tolerance to an electrical machine is to increase the number of phases of the machine. Multiphase machines are Design and Control of Fault-Tolerant Permanent Magnet Machines Ali Mohammadpour, Student Member, IEEE, Arun Gandhi, Student Member, IEEE and Leila Parsa, Senior Member, IEEE E 978-1-4673-5658-9/13/$31.00 ©2013 IEEE 108