Vector Controlled Induction Motor Drives

The reason that DC motors have high performance in motion control is that a separately exited DC motor permits the separate control of torque and flux. The linearity of the system makes a DC motor easier to control than an AC motor. However, the mechanical commutation of the DC machine causes higher failure

rates, even with frequent maintenance. Furthermore, commutation sparks constrain a DC motor from being applied in hazardous environments or in high power applications (i.e. megawatt level).

In order to improve the performance of an induction motor in motion control, a principle similar to that of separate excitation, was applied to induction motor control and is referred to as field orientation control.

Field orientation control, sometimes called vector control, is based on a vector transformation from a stationary reference frame to a rotating reference frame and vice versa when necessary. As its name implies, the principle of field orientation control is to establish a synchronous reference frame such that the d-axis (i.e. direct axis) coincides with the orientation of the total rotor flux linkage of the machine. In this frame, stator currents are decoupled into two orthogonal components: the torque component along the q-axis (i.e. quadrature axis) and the flux component along the d-axis. This decoupling permits the direct control of shaft torque while keeping the flux magnitude constant. In other words, the induction motor can be controlled like a separately excited DC motor. The key to the implementation of field orientation control is how to obtain the information about the instantaneous direction of the rotor flux vector. In general, there are two generic approaches. The first one is to utilize direct sensing of the air gap flux by the use of Hall probes, search coils or other measurement devices.

This technique is accurate and insensitive to variation in motor parameters. However it is expensive, intrusive and introduces sensor reliability issues. The second method is an indirect approach where the rotor flux is estimated from stator currents, stator voltage and/or rotor velocity. This approach uses a parameter model of induction machine to predict the rotor flux with the available measurements and is therefore sensitive to variation in motor parameters such as the rotor resistance and magnetizing inductance. Unfortunately motor parameters vary greatly with temperature, frequency and current amplitude. Therefore in order to achieve the same performance as the direct method, motor parameter must be estimated accurately and instantaneously.