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Dr.-Ing. Amr Saleh

"A Fault Tolerant Strategy for the Induction Motor Field Oriented Controlled Drives after the Loss of one Inverter Phase" (2007)

Induction motors have always been known for their simple, economic and rugged construction but before the early seventies of the last century were not suitable for the adjustable speed drives due to the inherent complexity of control. With the advent of field oriented control (FOC), however the induction motor has become an attractive option for these types of drives.

It is known that the AC drive systems are very sensitive to different mode of failure occurring at the front-end rectifier, at the power inverter or at a control subsystem. In general, when one of these faults does occur, the system operation has to be interrupted for non programmed maintenance procedures. Whenever possible, the isolation of the fault is preferable since it keeps the drive partially operative. This leads the researchers to focus on schemes of fault tolerant AC drive systems.

This work contributes to the area of fault tolerant AC induction motor drives with field oriented control. It investigates the utilization of the machine as a single phase induction motor after the loss of one inverter phase due to a disconnection internal or external to the inverter and/or intentionally disconnected. Failures in the wiring or selective over current protection is an example of external disconnection while a transistor failure (misfiring or short circuit) requires intentional isolation.

The strategy proposed in this work assumes that for all such cases complete isolation of one machine phase will be essential and the motor will complete its operation as single phase motor with field oriented control in the manner of separate channels of torque and flux control. Considerable reduction of the maximum available torque due to single phase operation takes place. Limitations are also discussed and experimental results support the theoretical work.

Due to the lack of the single phase motor to the starting torque, starting strategy of the three phase induction motor from a single phase supply previously published in literature and new proposed strategy in this work are presented in order to be able to start the motor from zero speed or reverse the direction of rotation. Simulation results are also presented.