Scalar to vector: general principles of modern induction motor control

Vector control principles have almost invariably been described in the past using an extremely mathematical approach. Such an approach obscures the physical concepts underlying the control technique. The authors avoid, where possible, a heavily mathematical treatment and instead give an appreciation of the basic concepts. Some fundamental ideas are reviewed, such as DC drives and conventional scalar control, with a specific example of the induction motor dynamic performance being given, before vector control is introduced. The basic idea in vector control is to determine the instantaneous position of the rotor flux vector. The stator current vector is then oriented to have a specified spatial relationship with the rotor flux vector. This controlled spatial relationship between rotor flux and stator current is maintained at every instant in time and is the key to the superior transient performance. Both direct and indirect schemes of vector control are possible, with differing requirements for sensing flux, speed and rotor position