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

T ref + T _



ref + _



Voltage vector selector S a S b S c Voltage Source Inverter IM

 

+ V dc Stator flux and torque estimator

•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 :

v s



R s i s



d



s dt

0 

R r i

'

r



d



dt

'

r



j



r

 '

r

Basic Principles of DTC • Space vector equations of IM : 

s



L s i s



L m i

'

r

 '

r



L r i

'

r



L m i s

Basic Principles of DTC Direct Flux Control • From stator voltage equation :

d



s



v s



R s i s dt

• Neglecting drop across

R s

:  

s



v s



t

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

v s

(

t

)  2 3

V



S a

(

t

) 

S b

(

t

)

a



S c

(

t

)

a

2  ,

where a



e j

2 3 

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

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

Basic Principles of DTC Direct Flux Control If the flux in k th sector k +1 vector increases k + 2 vector reduces  

Hysteresis band

v s,4 Sector I v s,3 v s,2 Sector II v s,4 v s,3 v s,3 • Voltage vector vs,2 and vs,3 in sector I • Voltage vector vs,3 and vs,4 in sector II v s,4 v s,5 v s,6 v s,3 v s,2 v s,16

Basic Principles of DTC Direct Flux Control  ref + _ Flux error 

1

 

ref +



/2 Flux



ref -

 

/2 ref

Flux error status 

/2 Flux error 0 -



/2 Flux Error Status 1 0

t t t

Basic Principles of DTC Direct Torque Control • IM torque equation t elec  3 2 L  L s m L r  s  r sin  sr

Basic Principles of DTC Direct Torque Control • It can be shown that 

r r

 1 

L m L s p



r



s r

• Rotor flux follows the stator flux with a time constant  r

q

Basic Principles of DTC

Direct Torque Control

t = t 1



s



sr



r Rotate continuously

d

Applying voltage vectors rotating in the same direction

q

Applying voltage vectors in opposite direction or zero voltage vectors

q

t = t 1 +



t



s



sr



r Rotate continuously

d

t = t 1 +



t



s



sr



r Rotate continuously

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 Torque error T ref + _ T

1 0 -1



T

 T Speed Torque error status 0 -1 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

T ref + T _



ref + _



Voltage vector selector S a S b S c Voltage Source Inverter IM

 

+ V dc Stator flux and torque estimator

•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 Counterclockwise

Inc Flux

(0)

Dec Flux Inc T Dec T Inc T

(01) (00) (01) (1)

Dec T

(00) Sec I 100 000 110 111 Sec II 110 111 010 000 Sec III 010 000 011 111 Sec IV 011 111 001 000 Sec V 001 000 101 111 Sec VI 101 111 100 000 Clockwise

Inc Flux

(0)

Inc T

(10)

Dec T

(00)

Dec Flux

(1)

Inc T

(10)

Dec T

(00) Sec I 001 000 011 111 Sec II 101 111 001 000 Sec III 100 000 101 111 Sec IV 110 111 100 000 Sec V 010 000 110 111 Sec VI 011 111 010 000

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 

s

  (

v s



i R s

)

dt

• Problems: – dc drift – stator resistance variation

Stator Flux and Torque Estimation • Torque estimation T  1 .

5 p 2   s    s  T  3 2 p 2 Im i   s s – In d-q form T  1 .

5 p 2   sd i sq   sq i sd 

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