Abstract
Field oriented control is an established technique for rapid control of torque in induction motors. The controller tracks the orientation of the rotor flux, which rotates at synchronous frequency. The component of stator current in phase with this flux (known as the "flux current"), can be used to maintain a constant flux. Under these conditions, torque is directly proportional to the quadrature component of stator current, or "torque current". It has not proved cost-effective to measure either the rotor flux orientation or the motor torque directly. However both can be estimated from a combination of voltages and/or currents and position (or speed). The standard mathematical model uses the resistances and inductances of the motor equivalent circuit. These parameters may vary with temperature, motor operating speed and load. The underlying cause, range and timescale of these variations is examined, along with techniques for tracking the changes on-line. Detailed off-line characterisation results are presented for the test motor, in order to determine how accurately the parameters can be identified in practice. A number of standard torque and flux estimators have been analysed and implemented. Experimental results are presented for a 5.5kW motor drive system. Parameter errors and delays within the controller, which cause an error in the orientation of the stator currents, are shown to affect the motor performance. The motor is incorrectly fluxed, which may reduce its efficiency and peak torque capability in the steady state. In addition, any change in demanded torque is coupled into the flux current, exciting the natural response of the motor. This is characterised by damped oscillations at slip frequency, decaying at a rate determined by the rotor time constant. The implications for closed loop torque and speed control are discussed.