SPEED CONTROL OF AN INDUCTION MOTOR DRIVE USING INDIRECT VECTOR CONTROL METHOD Presented by: Milred Millan Oram Regd. No: Branch: EE-A Guided By: Prof.N.Guru

Contents 1.Introduction 2.Torque Control of DC Motor 3.Equivalent Circuit of Induction Motor 4.Phasor Diagrams for Induction Motor 5.Principles of Vector Control 6.Indirect Vector Control of Induction Motor 7.Advantages 8.Disadvantages 9.Reference

Introduction  Induction motors are used as a variable speed drive in many industrial applications.  In order to achieve high performance, field-oriented control of induction motor drive is employed.  Using the field-oriented control, a highly coupled, nonlinear, multivariable induction motor can be simply controlled through linear independent decoupled control of torque and flux, similar to separately excited dc motors.  Because of DC machine like performance, vector control is also known as decoupling, orthogonal, or trans-vector control.  Differs from conventional methods because it doesn’t require mechanical speed or position sensors.

Torque Control of DC Motors  The basic structure and field flux and armature Magneto Motive Force(MMF) are shown below:  Have decoupled or independent control over torque and flux.  DC motor-like performance can be achieved with an induction motor if the motor control is considered in the synchronously rotating reference frame (d e -q e ) where the sinusoidal variables appear as dc quantities in steady state. Where, Te=developed torque I a =armature current I f =field current Ψ f =field flux Ψ a =armature flux

With vector control: i ds (induction motor)  I f (dc motor) i qs (induction motor)  I a (dc motor) Thus torque is given by: Where is peak value of sinusoidal space vector.

 This dc motor-like performance is only possible if i qs * only controls i qs and does not affect the flux, i.e. i qs and i ds are orthogonal under all operating conditions of the vector-controlled drive.  Thus, vector control should ensure the correct orientation and equality of the command and actual currents.

Equivalent Circuit of Induction Motor  The complex d e -q e equivalent circuit of an induction motor is shown in the below figure (neglecting rotor leakage inductance). where, Vs= stator terminal voltage Rs= stator resistance L m = magnetizing Inductance L ls = stator leakage Inductance = stator current i ds = direct axis component of stator current i qs = quadrature axis component of stator current

Equivalent Circuit of Induction Motor (cont’d)  Since the rotor leakage inductance has been neglected, the rotor flux =, the air gap flux.  The stator current vector I s is the sum of the i ds and i qs vectors. Thus, the stator current magnitude, is related to i ds and i qs by:

Phasor Diagrams for Induction Motor  The terminal voltage V s slightly leads the air gap voltage because of the voltage drop across the stator impedance. i qs contributes real power across the air gap but i ds only contributes reactive power across the air gap.

Principles of Vector Control The basic conceptual implementation of vector control is illustrated in the below block diagram: Note: The inverter is omitted from this diagram.

Principles of Vector Control (cont’d) There are two approaches to vector control: 1) Direct field oriented current control - here the rotation angle of the i qs e vector with respect to the stator flux  qr ’ s is being directly determined (e.g. by measuring air gap flux) 2) Indirect field oriented current control - here the rotor angle is being measured indirectly, such as by measuring slip speed.  Indirect vector control is similar to direct vector control except the unit vector signals (cos e and sin e ) are generated in a feed forward manner.

Indirect Vector Control The ds – qs axes are fixed on the stator, but the dr – qr axes, which are fixed on the rotor, are moving at speed wr as shown in fig., and synchronously rotating axes de – qe are rotating ahead of the dr – qr axes by positive slip angle θsl corresponding to slip frequency wsl. θe= angle between ds- de axis

Indirect Vector Control (cont’d)  There is a slip difference between the rotor speed and the synchronous speed given by: Since,, we can write:  We can use the d e -axis and q e -axis equivalent circuits of the motor to derive Control expressions.

Indirect Vector Control (cont’d) Dynamic d e - q e equivalent circuit of machine (a)q e -axis circuit, (b) d e -axis circuit

Indirect Vector Control (cont’d) The rotor circuit equations may be written as: The rotor flux linkage equations may be written as:

Indirect Vector Control (cont’d) Combining these with the earlier equations allows us to eliminate the rotor currents which cannot be directly obtained. The resulting equations are: where.

Indirect Vector Control (cont’d) For decoupling control the total rotor flux needs to be aligned with the d e -axis and so we want:  qr =0 => d qr /dt =0 If we now substitute into the previous equations, we get: and where has been substituted for  dr.

Indirect Vector Control (cont’d) For implementing the indirect vector control strategy, we need to take these equations into consideration as well as the equation: Note: A constant rotor flux results in the equation: so that the rotor flux is directly proportional to i ds in steady state.

Indirect Vector Control (cont’d) Qn. Why does vector control provide superior dynamic performance of ac motors compared to scalar control ? Ans. In scalar control there is an inherent coupling effect because both torque and flux are functions of voltage or current and frequency. This results in sluggish response and is prone to instability because of 5 th order harmonics. Vector control decouples these effects.

Advantages of indirect vector control Offers more precise control Used in high performance drives where oscillations in air gap flux linkages are intolerable, e.g. robotic actuators, centrifuges, servos, etc. Provide superior dynamic performance

Disadvantage Slip gain detuning: This is due primarily to variation in rotor resistance. This effect is illustrated below where, R r =actual rotor resistance and = estimated rotor resistance.

References 1.B.K. Bose, Prentice Hall - Modern Power Electronics And Ac Drives, Website: (ISSN , ISO 9001:2008 Certified Journal, Volume 2, Issue 12, December 2012) 3.F.BLASCHKE, “The principle of field orientation as applied to the new transvector closed-loop control system for rotating-field Machine.