Direct Torque Control of Induction Machine Dr. Nik Rumzi Nik Idris Department of Energy Conversion, Faculty of Electrical Engineering, Universiti Teknologi Malaysia

Basic Principles of DTC High performance induction motor drives Field Oriented Control - FOC Direct Torque Control - DTC

Basic Principles of DTC Tref Voltage vector selector Voltage Source Inverter Stator flux and torque estimator ref + _ IM   T + Vdc - Sa Sb Sc Stator flux and torque control within hysteresis bands Voltage vector selected based on stator flux and torque demands

Basic Principles of DTC How the voltage vectors control the flux? How the voltage vectors control the torque? These questions will be answered in the following slides

Basic Principles of DTC Space vector equations of IM :

Basic Principles of DTC Space vector equations of IM :

Basic Principles of DTC Direct Flux Control From stator voltage equation : Neglecting drop across Rs :

Basic Principles of DTC Direct Flux Control Voltage vectors for 3-phase VSI

Basic Principles of DTC Dy Hysteresis Flux band 100 110 010 011 101 001 y s Direct Flux Control

Basic Principles of DTC Direct Flux Control q Sector IV Sector III 60o I d Sector II Sector V Sector VI Sector I

Basic Principles of DTC Direct Flux Control vs,3 vs,2 vs,4 Sector I Sector II Hysteresis band vs,6 vs,5 vs,16 If the flux in kth sector k +1 vector increases  k + 2 vector reduces  Voltage vector vs,2 and vs,3 in sector I Voltage vector vs,3 and vs,4 in sector II

Basic Principles of DTC Direct Flux Control ref + /2 ref - /2 /2 - /2 Flux error Flux Error Status 1 t ref  ref + _ 1  Flux error status Flux error

Basic Principles of DTC Direct Torque Control IM torque equation

Basic Principles of DTC Direct Torque Control It can be shown that Rotor flux follows the stator flux with a time constant r

Basic Principles of DTC Direct Torque Control t = t1 q q t = t1 + t Applying voltage vectors rotating in the same direction s s Rotate continuously Rotate continuously sr sr r d r d q t = t1 + t Applying voltage vectors in opposite direction or zero voltage vectors s Rotate continuously sr r d

Basic Principles of DTC Direct Torque Control Three cases are considered : Case 1 Forward active voltage vectors stator flux increases or decreases Increases sr Increases Torque

Basic Principles of DTC Direct Torque Control Case 2 Zero voltage vectors stator flux stops Decreases sr Decreases Torque

Basic Principles of DTC Direct Torque Control Case 3 Reverse active voltage vectors stator flux increases or decreases Decreases sr rapidly Decreases Torque rapidly

Basic Principles of DTC Direct Torque Control Torque reference Torque T/2 T/2 -1 T Tref + _ 1 T Torque error T Speed 1 Torque error status -1

Basic Principles of DTC By limiting the torque and flux within their hysteresis bands, de-coupling of torque and flux can be achieved

Basic Principles of DTC Tref Voltage vector selector Voltage Source Inverter Stator flux and torque estimator ref + _ IM   T + Vdc - Sa Sb Sc Stator flux and torque control within hysteresis bands Voltage vector selected based on stator flux and torque demands

Basic Principles of DTC Selection table for optimum switching pattern

Stator Flux and Torque Estimation Accurate estimation to ensure proper operation and stability Various methods proposed voltage model current model closed-loop observer

Stator Flux and Torque Estimation Stator flux- voltage model Problems: dc drift stator resistance variation

Stator Flux and Torque Estimation In d-q form

Implementation of DTC Basic I/O requirements: Phase Current measurement DC Link Voltage measurement Speed measurement from Incremental Encoder for closed-loop speed control (optional) Fast processor to reduce torque ripple

Implementation of DTC

Experimental Results From oscilloscope 55s sampling, 240V, ¼ HP IM Step speed reference Speed Current Torque d-flux

Experimental Results From oscilloscope 55s sampling, 240V, ¼ HP IM Square wave speed reference Speed Current Torque