unit 4-field oriented control of induction motor

U.Shajith Ali / AP/ EEE / SSNCE

Field Oriented Control of

Induction Motor


Vector versus Scalar Control of Induction Motors

• Scalar control involves controlling only the magnitude of the control

variables with no concern for the coupling effects between these variables.

Conversely, vector or field orientated control involves adjusting the

magnitude and phase alignment of the vector quantities of the motor.

• Scalar control, such as the Constant Volts/Hertz method when applied to an

AC induction motor is relatively simple to implement but gives a sluggish

response because of the inherent coupling effect due to torque and flux

being functions of current and frequency. Vector control de-couples the

vectors of field current and armature flux so that they may be controlled

independently to provide fast transient response.


• Accurate position control is not possible with scalar control since this

requires instantaneous control of the torque. This requires either,

instantaneous change to the stator currents, which is not possible due to

energy storage effects, or instantaneous change to the rotor current which in

the case of scalar control is controlled indirectly via the stator currents.

Similarly, whilst scalar control may provide acceptable steady state speed

control, precise and responsive speed control due to load changes requires

accurate and responsive torque control.

• The vector approach overcomes the sluggish transient response when using

scalar control of AC motors.


Field Orientated Control (FOC)Field Orientated Control (FOC)Field Orientated Control (FOC)Field Orientated Control (FOC)

• The Field Orientated Control consists of controlling the stator currents represented by a vector.

• This control is based on projections which transform a threephase time and speed dependent

system into a two co-ordinate (d and q co-ordinates) time invariant system.

• These projections lead to a structure similar to that of a DC machine control.

• Field orientated controlled machines need two constants as input references: the torque

component (aligned with the q co-ordinate) and the flux component (aligned with d co-ordinate).

• As Field Orientated Control is simply based on projections the control structure handles

instantaneous electrical quantities. This makes the control accurate in every working operation.


Thus in Induction motor, FOC

• the ease of reaching constant reference

(torque component and flux component of

the stator current)

• the ease of applying direct torque control


In implementation

• (the Clarke transformation) which outputs

a two co-ordinate time variant system

• (the Park transformation) which outputs a

two co-ordinate time invariant system


Vector Control Implementation Principle


Implementation of Vector Control


Two methods of vector control

1. Direct or feedback method

2. Indirect or feedforward method


Direct or Feedback Vector Method

• Direct vector control block diagram with rotor flux orientation


Flux Vector Estimation

Two methods:

1.Voltage model

2.Current model


Voltage Model


The torque expression is


Voltage model estimation


Current ModelThe rotor circuit equations


Therefore

But


Indirect or Feedforward Control


Derivations of indirect control


Block Diagram of Indirect Vector Control


Direct Torque Control


What is Direct Torque Control?What is Direct Torque Control?What is Direct Torque Control?What is Direct Torque Control?

• Direct Torque Control describes the way in which the control of torque and speed are directly based on the electromagnetic state of the motor, similar to a DC motor, but contrary to the way in which traditional PWM drives use input frequency and voltage. DTC is the first technology to control the “real” motor control variables of torque and flux.

Direct Torque and Flux Control (DTFC)

or

Direct Torque Control (DTC)

or

Direct Self Control (DSC)


• Direct control of torque and stator flux of a

drive by inverter space vector selection

through a look up table.


Control Strategy of DTC


Advantages of DTC

• Fast torque response

• Torque control at low frequencies

• Torque linearity

• No feedback current control

• Do not need a tachometer or encoder

• No traditional PWM is applied

unit 4-field oriented control of induction motor

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