Field-oriented control (FOC) or vector control methods for induction motor drives can be broadly classified as direct FOC or indirect FOC. Direct FOC measures the rotor flux directly using a sensor, while indirect FOC estimates the rotor flux using control equations and requires rotor speed measurement. Indirect FOC is more commonly used as it can operate from zero speed to high speeds. Both methods transform stator currents to a rotating reference frame to separately control flux using the direct axis current and torque using the quadrature axis current. One advantage of FOC is improved torque response and control at low frequencies and speeds.

1. decoupling between the motor flux and torque. Consequently, they can be separately controlled by the stator direct-axis and quadrature-axis
currents, respectively.
7. Classify field-oriented control (vector control) methods.
Field-oriented control (FOC) for the induction motor drive can be broadly classified into two types that are direct FOC and indirect FOC schemes.
Field orientation has emerged as a powerful tool for controlling ac machines such as inverter-supplied induction motors/synchronous motors. There
are essentially two general methods of vector control. One, called the direct or feed- back method, was invented by Blaschke and the other, known
as the indirect or feed forward method was invented by Hasse. The two methods differ in the way the rotor angle is determined.
Indirect Field-Oriented Control (IFOC)
In case of IFOC rotor flux vector is estimated using the field oriented control equations (current model) requiring a rotor speed measurement?
Among both schemes, IFOC is more commonly used because in closed-loop mode it can easily operate throughout the speed range from zero speed
to high-speed field-weakening. In indirect FOC, the angle is obtained by using rotor position measurement and machine parameter’s estimation.
The IFOC is a high-performance method used to control the IMs by controlling the stator currents. It’s based on converting the three-phase currents
of the stator into two orthogonal components (i.e. d-q coordinates). The flux is controlled by the d component of the current isd, whereas the q
component isq controls the motor torque. In general, the two current components isd and isq are compared with their reference values, then two PI
controllers process the error signals to calculate the reference values of the voltage components Vsd and Vsq. Clark transformation is used to
transform the reference voltages into stationary reference frame.
Direct Field-Oriented Control (DFOC)
In DFOC strategy rotor flux vector is either measured by means of a flux sensor mounted in the air-gap or by using the voltage equations starting
from the electrical machine parameters. In direct FOC the angle is obtained by the terminal voltages and currents.
In direct FOC the rotor angle or control vector is obtained by the terminal voltages & currents directly by using flux estimators. The direct vector
control is also known as feedback vector control scheme. Similar to Indirect Vector Control, various controllers have been implemented on direct
vector controlled induction motor drives also to improve the performance of the drive. While the direct method is inherently the most desirable
control scheme, it suffers from high cost and the unreliability of the flux measurement. Although the indirect method can approach the performance
of the direct measurement scheme, the major weakness of this approach is centered upon the accuracy of the control gains which, in turn, depend
heavily on the motor parameters assumed in the feed forward control algorithm.
Generally FOC has the following advantages
Improved torque response.
Torque control at low frequencies and low speed.
Dynamic speed accuracy.
Four quadrant operation.
Short-term overload capability.
Reduction in size of motor and cost.
Reduction in power consumption.
8. State an advantage of using vector control.
One advantage of field-oriented control or vector control is;
It increases efficiency.
Letting smaller motors replace larger ones without sacrificing torque and speed.
It offers higher, more dynamic performance in the case of speed and torque-controlled ac drives.
Better speed holding, better response to sudden load changes as well as greatly improved torque at low speeds.
Motors often run more efficiently and therefore cooler. Inverter has always offered simple, effective vector control which is easy to set up.
Scalar control method relies on keeping V/f ratio constant so as to maintain air gap flux at constant value. However, the vector control analysis of an
IM allows decoupled analysis where the flux and torque component can be independently controlled, just like in dc motor.
High accuracy of speed control.
Soft start and smooth motor rotation in full speed range.
Fast response to a load change: when there is a load change, there is practically no change in the speed.