THE BASIC IDEA OF VECTOR CONTROL

In the so called “scalar control methods” for induction machines, the motor model is considered just for steady state. Therefore, it is expected that a controller based on these methods can not achieve the best performance during transients. This is the basic drawback of scalar control methods for induction machines

In “vector control methods,” the motor model considered is valid for transient .The reference frame used in the FOC is one whose real axis coincides with the rotor flux vector. This frame is not static and does not have a constant speed during transients. Actually, it was not a commonly used reference frame for the analysis of electric machines. The great advantage of this “noninertial” frame is that for impressed stator currents, this method allows independent flux and torque controls as in a separately excited dc machine. Impressed stator currents, that is currents controlled by a fast current loop that can be implemented using cheap hall effect current sensors and power electronics, are usual in the industry practice. Furthermore, the control proposed by Blaschke is the well-known control used for separately excited dc machines, and his theory could be named “field oriented modeling.”

Vector control, also called field-oriented control (FOC), is a variable-frequency drive (VFD) control method in which the stator currents of a three-phase AC electric motor are identified as two orthogonal components that can be visualized with a vector. One component defines the magnetic flux of the motor, the other the torque. The control system of the drive calculates the corresponding current component references from the flux and torque references given by the drive's speed control. Typically proportional-integral (PI) controllers are used to keep the measured current components at their reference values. The pulse-width modulation of the variable-frequency drive defines the transistor switching according to the stator voltage references that are the output of the PI current controllers

With FOC, direct axis component of the stator current is analogous to

field current and quadrature axis component of stator current is analogous

to armature current of a DC machine, therefore torque can be expressed as: Td = k Isq I sd

Comparison between Scalar & Vector Control
S. No.	Scalar Control	Vector Control
1	Simple	Complex
2	Slow response to transient	Operate with fast response
3	Low price	High Price Control
4	Low Performance	Speed regulation is very good
5	poor transient response	excellent transient response.
6	unsatisfactory speed accuracy at low speed regions	speed accuracy at low speed regions is good
7	low performance but stable	high performance
8	speed regulation is not good	speed regulation is very good
9	v/f control is scalar control	Direct and Indirect vector control are the type of vector control
10	Only magnitude can be controlled.	Magnitude and phase angle both controlled.
11	No any hall effect sensor required.	Costly hall effect sensor required.