Field-Oriented Induction Motor Drive with Efficiency Optimisation

Variable speed induction motor drives based on field oriented control are going to cover increasing application areas both in industrial and household appliances, thereafter the energy consumption and the drive efficiency become key points. It is well known that the induction motor is a high efficiency machine when working close to its rated operating point. Unfortunately variable speed/torque operations can lead to efficiency drop, which cannot be avoided by the improvement in the machine design. Nevertheless, energy efficiency improvements can be obtained by a proper control strategy of the power converter. The proposed approaches in this area rely on the consideration that efficiency improvements can be obtained by controlling the balance between the copper and iron losses, i.e. the electromagnetic losses. This balance can be controlled by selecting the flux level in relation to the torque and speed of the machine. Thus, for any specific torque and speed there is a specific flux level which minimises the total losses and maximises the efficiency. This paper presents a modified controller for induction motor drives, capable to assure both efficiency optimisation at steady-state and maximum torque capability during transient operation. The scheme is depicted in Fig. 1. It is based on a rotor-flux field-oriented controller, where the flux (id*) and torque (iq*) command currents are provided, respectively, by an efficiency optimiser and the speed regulator. The efficiency optimisation is based on a mixed “analytical/measurement” approach. The current and voltage signals are employed for on-line measurement of the motor input power. As a first attempt, the efficiency optimiser calculates id* as a function of iq* by means of the relation providing the slip speed at maximum efficiency (for the given rotor speed). Such a relation is obtained by the T-form steady-state equivalent circuit of the induction motor, whereas the solution is calculated using a fast self-converging real-time algorithm (details will be given in the full paper). Then, the controller starts to make small step changes in the flux command current (id*) by setting the value which results in minimum input power. Since the output power is held constant by the speed regulator (for a fixed load torque), the system will always seek the maximum efficiency. The changes of the flux current command are enabled at constant speed operation only by the enable signal (en). Changes of (id*) are made slowly with respect to the response time of the drive in order to avoid perturbation of the steady state operation. During transients, i.e. when a large speed error is detected, the enable signal is set to “false” and the efficiency optimisation is disabled. In this case, the flux command current is increased to a level that produces high torque per stator ampere to assist in responding to the speed error. In the full paper test results showing the system performance and the efficiency improvements achieved by the proposed controller will be presented and discussed.