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This paper investigates field weakening in speed sensorless vector controlled induction motor drives. The sensorless drive uses direct field oriented control with a MRAS based speed and flux observer. The conventional case of field weakening for above rated speed operation is illustrated where the rotor time constant parameter is a function of flux and is derived from self-commissioning. A novel use of field weakening at zero or very low speed is introduced in order to maintain sensorless field orientation at zero/low speed under a wide range of loads.