Electric vehicle(EV) is a propitious engineering revolution to replace conventional motor vehicles to dismantle the current level of environmental pollution. Engineers are already focusing on developing its current state of efficiency and new ideas are emerging every now and then. One of the bigger challenges in such vehicles revolves around the control method adjustable speed drives used in conjunction.
The popular approach was to use a time-averaged torque control of the motor. The control of torque in dc commutator motors is based on the classical theory in which torque is proportional to the vector current of armature current and flux. Under sufficiently light load the commutator maintains the spatial orthogonality of these quantities so that the vector product becomes a scalar product. Since both the flux and the current are normally constant, the instantaneous torque is also constant. There is ideally no torque ripple and the torque is varied by changing the current or the flux or both. In ac sinusoidal machines the situation is similar. The actual phase currents and flux linkages are not constant, but can be made to appear constant by means of a reference-frame transformation that has appropriately been called the commutator transformation (e.g: Park's transformation). The instantaneous torque of an ideal ac sinusoidal machine is constant and is proportional to the vector product of current and flux linkages in the transformed reference frame. The transformation unifies the theory of the dc and ac systems for dc and ac drives that have closely similar structures when viewed from the appropriate reference frame. These principles are put into practice in the so-called "vector control" of ac motor drives.
As the dc commutator motors and ac sinusoidal motors have an instantaneous torque equal to their time-averaged torque in the steady state, the time-averaged control scheme is suitable for such machines. But the use of motors with varying instantaneous torque is increasing rapidly. As a consequence, instantaneous torque control is becoming a viable adaption for the upcoming future of this technology. The speed and functionality of the digital integrated circuits will have a further leap in adjustable control of speed drives through the prescribed instantaneous analysis. On the other hand it has the potential to provide a faster feedback response to the control unit. Also, it can remove the ripple torques over a wide spectrum of instabilities including shaft resonances with software controlled accuracy in principle. Hence engineers are shifting towards this innovative controlling method and research is progressing swiftly.
Time-averaged and instantaneous torque control both methods have their suitability to adjustable speed drives of modern electric vehicles. But which will dictate the next step of advancement in control mechanism remains a debate to be resolved for the future.
