Direct torque controlFrom Wikipedia, the free encyclopedia

Direct torque control (DTC) is one method used in variable frequency drives to control the torque (and thusfinally the speed) of three-phase AC electric motors. This involves calculating an estimate of the motor's magneticflux and torque based on the measured voltage and current of the motor.

Contents

1 DTC control platform

2 History

3 References

4 See also

DTC control platform

Stator flux linkage is estimated by integrating the stator voltages. Torque is estimated as a cross product ofestimated stator flux linkage vector and measured motor current vector. The estimated flux magnitude and torqueare then compared with their reference values. If either the estimated flux or torque deviates from the referencemore than allowed tolerance, the transistors of the variable frequency drive are turned off and on in such a waythat the flux and torque errors will return in their tolerant bands as fast as possible. Thus direct torque control isone form of the hysteresis or bang-bang control.

Overview of key competing VFD control platforms:

VFD

Scalar controlV/f (Volts per frequency)

Vector control

FOC (Field-oriented control)

DTC (Direct torque control)DSC (Direct self control)

SVM (Space vector modulation)

The properties of DTC can be characterized as follows:

Torque and flux can be changed very fast by changing the references

High efficiency & low losses - switching losses are minimized because the transistors are switched only

when it is needed to keep torque and flux within their hysteresis bands

The step response has no overshoot

No coordinate transforms are needed, all calculations are done in stationary coordinate system

No separate modulator is needed, the hysteresis control defines the switch control signals directly

There are no PI current controllers. Thus no tuning of the control is required

The switching frequency of the transistors is not constant. However, by controlling the width of the tolerance

bands the average switching frequency can be kept roughly at its reference value. This also keeps the

current and torque ripple small. Thus the torque and current ripple are of the same magnitude than with

vector controlled drives with the same switching frequency.

Due to the hysteresis control the switching process is random by nature. Thus there are no peaks in the

current spectrum. This further means that the audible noise of the machine is low

The intermediate DC circuit's voltage variation is automatically taken into account in the algorithm (in voltage

integration). Thus no problems exist due to dc voltage ripple (aliasing) or dc voltage transients

Synchronization to rotating machine is straightforward due to the fast control; Just make the torque

reference zero and start the inverter. The flux will be identified by the first current pulse

Digital control equipment has to be very fast in order to be able to prevent the flux and torque from deviating

far from the tolerance bands. Typically the control algorithm has to be performed with 10 - 30 microseconds

or shorter intervals. However, the amount of calculations required is small due to the simplicity of the

algorithm

The current measuring devices have to be high quality ones without noise because spikes in the measured

signals easily cause erroneous control actions. Further complication is that no low-pass filtering can be used

to remove noise because filtering causes delays in the resulting actual values that ruins the hysteresis

control

The stator voltage measurements should have as low offset error as possible in order to keep the flux

estimation error down. For this reason the stator voltages are usually estimated from the measured DC

intermediate circuit voltage and the transistor control signals

In higher speeds the method is not sensitive to any motor parameters. However, at low speeds the error in

stator resistance used in stator flux estimation becomes critical

Summarizing properties of DTC in comparison to field-oriented control, we have:[1][2][3]

Comparison property DTC FOC

Dynamic response to torque Very fast Fast

Coordinates reference frame alpha, beta (stator) d, q (rotor)

Low speed (< 5% of nominal)behavior

Requires speed sensor forcontinuous braking

Good with position or speed sensor

Controlled variables torque & stator fluxrotor flux, torque current iq & rotor fluxcurrent id vector components

Steady-state torque/current/fluxripple & distortion

Low (requires high qualitycurrent sensors)

Low

Parameter sensitivity, sensorless Stator resistance d, q inductances, rotor resistance

Parameter sensitivity, closed-loopd, q inductances, flux (nearzero speed only)

d, q inductances, rotor resistance

Rotor position measurement Not required Required (either sensor or estimation)

Current control Not required Required

PWM modulator Not required Required

Coordinate transformations Not required Required

Switching frequencyVaries widely around averagefrequency

Constant

Switching lossesLower (requires high qualitycurrent sensors)

Low

Audible noisespread spectrum sizzlingnoise

constant frequency whistling noise

Control tuning loops speed (PID control)speed (PID control), rotor flux control (PI),id and iq current controls (PI)

Complexity/processing requirements Lower Higher

Typical control cycle time 10-30 microseconds 100-500 microseconds

The direct torque method performs very well even without speed sensors. However, the flux estimation is usuallybased on the integration of the motor phase voltages. Due to the inevitable errors in the voltage measurement andstator resistance estimate the integrals tend to become erroneous at low speed. Thus it is not possible to controlthe motor if the output frequency of the variable frequency drive is zero. However, by careful design of the controlsystem it is possible to have the minimum frequency in the range 0.5 Hz to 1 Hz that is enough to make possibleto start an induction motor with full torque from a standstill situation. A reversal of the rotation direction is possibletoo if the speed is passing through the zero range rapidly enough to prevent excessive flux estimate deviation.

If continuous operation at low speeds including zero frequency operation is required, a speed or position sensorcan be added to the DTC system. With the sensor, high accuracy of the torque and speed control can bemaintained in the whole speed range.

History

DTC was patented by Manfred Depenbrock in the US[4] and in Germany,[5] the latter patent having been filed onOctober 20, 1984, both patents having been termed direct self-control (DSC). However, Isao Takahashi andToshihiko Noguchi described a similar control technique termed DTC in an IEEJ paper presented in September

1984[6] and in an IEEE paper published in late 1986.[7] The DTC innovation is thus usually credited to all threeindividuals.

The only difference between DTC and DSC is the shape of the path along which the flux vector is controlled, the

former path being quasi-circular whereas the latter is hexagonal such that the switching frequency of DTC ishigher than DSC. DTC is accordingly aimed at low-to-mid power drives whereas DSC is usually used for higher

power drives.[8] (For simplicity, the rest of the article only uses the term DTC.)

Since its mid-1980s introduction applications, DTC have been used to advantage because of its simplicity and veryfast torque and flux control response for high performance induction motor (IM) drive applications.

DTC was also studied in Baader's 1989 thesis, which provides a very good treatment of the subject.[9]

The first major successful commercial DTC products, developed by ABB, involved traction applications late in the1980s for German DE502 [1] (http://commons.wikimedia.org/wiki/Category:MaK_DE_502)[2] (http://www.loks-aus-kiel.de/index.php?nav=1400726&lang=1) and DE10023 [3] (http://www.loks-aus-kiel.de

/index.php?nav=1400728) diesel-electric locomotives[10] and the 1995 launch of the ACS600 drives family.

ACS600 drives has since been replaced by ACS800[11] and ACS880 drives.[12] Vas,[13] Tiitinen et al.[14] and

Nash[15] provide a good treatment of ACS600 and DTC.

DTC has also been applied to three-phase grid side converter control.[16][17] Grid side converter is identical instructure to the transistor inverter controlling the machine. Thus it can in addition to rectifying AC to DC also feedback energy from the DC to the AC grid. Further, the waveform of the phase currents is very sinusoidal and powerfactor can be adjusted as desired. In the grid side converter DTC version the grid is considered to be a big electricmachine.

DTC techniques for the interior permanent magnet synchronous machine (IPMSM) were introduced in the late

1990s[18] and synchronous reluctance motors (SynRM) in the 2010s.[19]

DTC was applied to doubly fed machine control in the early 2000s.[20] Doubly-fed generators are commonly usedin 1-3 MW wind turbine applications.

Given DTC's outstanding torque control performance, it was surprising that ABB's first servo drive family, the

ACSM1, was only introduced in 2007.[21]

From the end of 90's several papers have been published about DTC and its modifications such as space vector

modulation,[22] which offers constant switching frequency.

In light of the mid-2000s expiration of Depenbrock's key DTC patents, it may be that other companies than ABBhave included features similar to DTC in their drives.

References

^ Garcia, X.T.; Zigmund, B.; Terlizzi, A.; Pavlanin, R.; Salvatore, L. (Mar 2006). "Comparison Between FOC and DTC

strategies for Permanent Magnet" (http://advances.uniza.sk/index.php/AEEE/article/view/179). Advances in Electrical and

Electronic Engineering 5 (1 -2): Vol 5, No 12 (2006): March June.

1.

^ Merzoug, M. S.; Naceri, F. (Sep 2008). "Comparison of Field Oriented Control and Direct Torque Control for PMSM"

(https://www.waset.org/journals/waset/v21/v21-54.pdf). World Academy of Science, Engineering and Technology 21 2008.

Merzoug (21): 209304.

2.

^ Kazmierkowski, M. P.; Franquelo, L.; Rodriguetz, J.; Perez, M.; Leon, J. (Sep 2011). "High Performance Motor Drives"

(http://ieeexplore.ieee.org/xpl/articleDetails.jsp?tp=&arnumber=6042559&contentType=Journals+%26+Magazines&

sortType%3Dasc_p_Sequence%26filter%3DAND%28p_IS_Number%3A6042545%29). IEEE Industrial Electronics

Magazine Sept 2011 5 (3): 626. doi:10.1109/mie.2011.942173 (https://dx.doi.org/10.1109%2Fmie.2011.942173).

3.

^ Depenbrock, Manfred. "US4678248 Direct Self-Control of the Flux and Rotary Moment of a Rotary-Field Machine"

(http://www.google.com/patents/US4678248).

4.

^ Depenbrock, Manfred. "DE3438504 (A1) - Method and Device for Controlling of a Rotating Field Machine"

(http://worldwide.espacenet.com/publicationDetails

/biblio;jsessionid=93A242BB138BECE753C72D97DEBCEEFF.espacenet_levelx_prod_3?FT=D&date=19860424&

DB=EPODOC&locale=en_EP&CC=DE&NR=3438504A1&KC=A1&ND=4). Retrieved 13 November 2012.

5.

^ Noguchi, Toshihiko; Takahashi, Isao (Sep 1984). "Quick Torque Response Control of an Induction Motor Based on a

New Concept". IEEJ: pp. 6170.

6.

^ Takahashi, Isao; Noguchi, Toshihiko (Sep 1986) (SepOct 1986). "A New Quick-Response and High-Efficiency Control

Strategy of an Induction Motor" (http://ieeexplore.ieee.org/xpl/articleDetails.jsp?tp=&arnumber=4504799&

contentType=Journals+%26+Magazines&refinements%3D4293480702%26queryText%3DIsao+Takahashi). IA-22 (5):

820-827. IEEE Trans. on Industry Applications. Retrieved 13 November 2012.

7.

^ Foo, Gilbert (2010). Sensorless Direct Torque and Flux Control of Interior Permanent Magnet Synchronous Motors at

Very Low Speeds Including Standstill. Sydney, Australia: The University of New South Wales.

8.

^ Baader, Uwe (1988). Die Direkte-Selbstregelung (DSR) : e. Verfahren zur hochdynam. Regelung von

Drehfeldmaschinen (in German). (https://suchen.ub.rub.de/Record/1146510_bo) (Als Ms. gedr. ed.). Dsseldorf: VDI-Verl.

ISBN 3-18-143521-X.

9.

^ Jnecke, M.; Kremer, R.; Steuerwald, G. (912 Oct 1989). "Direct Self-Control (DSC), A Novel Method Of Controlling

Asynchronous Machines In Traction Applications". Proceedings of the EPE'89 1: 7581.

10.

^ "ACS800 - The New All-compatible Drives Portfolio" (http://www.abb.com/product/us/9AAC133421.aspx). Retrieved

14 November 2012.

11.

^ Lnnberg, M.; Lindgren, P. (2011). "Harmonizing drives - The driving force behind ABB's all-compatible drives

architecture" (http://abblibrary.abb.com/global/scot/scot271.nsf/0/d0e4255c4bf47b14c1257958005844d1/$file

/62-65%202m152_eng_72dpi.pdf). ABB Review (2): 6365.

12.

^ Vas, Peter (1998). Sensorless Vector and Direct Torque Control (Repr. ed.). Oxford [u.a.]: Oxford Univ. Press.

ISBN 0198564651.

13.

^ Tiitinen, P.; Pohjalainen, P.; Lalu, J. (May 1995). "The Next Generation Motor Control Method: Direct Torque Control

(DTC)" (http://www.epe-association.org/epe/documents.detail.php?documents_id=3237). EPE Journal 5 (1): 1418.

doi:10.1109/pedes.1996.537279 (https://dx.doi.org/10.1109%2Fpedes.1996.537279). Retrieved 14 November 2012.

14.

^ Nash, J.N. (Mar 1997). "Direct Torque Control, Induction Motor Vector Control Without an Encoder"

(http://ieeexplore.ieee.org/xpl/articleDetails.jsp?tp=&arnumber=567792&contentType=Journals+%26+Magazines&

queryText%3DDirect+Torque+Control%2C+Induction+Motor+Vector+Control+Without+an+Encoder). IEEE Trans. on

Industry Applications 33 (2): 333341. doi:10.1109/28.567792 (https://dx.doi.org/10.1109%2F28.567792).

15.

^ Harmoinen, Martti; Manninen, Vesa; Pohjalainen, Pasi; Tiitinen, Pekka (17 Aug 1999). "US5940286 Method for

Controlling the Power To Be Transferred Via a Mains Inverter" (http://www.google.com/patents/US5940286). Retrieved

13 November 2012.

16.

^ Manninen, V. (1921 Sep 1995). "Application of Direct Torque Control Modulation to a Line Converter.". Proceedings of

EPE 95, Sevilla, Spain: 1,2921,296.

17.

^ French, C.; Acarnley, P. (1996). "Direct torque control of permanent magnet drives" (http://ieeexplore.ieee.org

/xpl/articleDetails.jsp?tp=&arnumber=536869&contentType=Journals+%26+Magazines&

queryText%3DDirect+torque+control+of+permanent+magnet+drives). IEEE Transactions on Industry Applications 32 (5):

10801088. doi:10.1109/28.536869 (https://dx.doi.org/10.1109%2F28.536869). Retrieved 15 November 2012.

18.

^ Lendenmann, Heinz; Moghaddam, Reza R.; Tammi, Ari (2011). "Motoring Ahead" (https://web.archive.org

/web/20140107064419/http://search.abb.com/library/Download.aspx?DocumentID=9AKK105408A0223&

LanguageCode=en&DocumentPartId=&Action=Launch). ABB Review. Retrieved 7 January 2014.

19.

^ Gokhale, Kalyan P.; Karraker, Douglas W.; Heikkil, Samuli J. (10 Sep 2002). "US6448735 Controller for a Wound Rotor

Slip Ring Induction Machine" (http://www.google.com/patents/US6448735). Retrieved 14 November 2012.

20.

^ "DSCM1 - High Performance Machinery Drives" (https://web.archive.org/web/20111018033817/http://www05.abb.com

/global/scot/scot201.nsf/veritydisplay/6bb6fb35783c76e3c125765f0065cb59/$file

/ACSM1technicalcatalogueREVE_EN.pdf). Retrieved 18 October 2011.

21.

^ Lascu, C.; Boldea, I.; Blaabjerg, F. (1215 Oct 1998). "A modified direct torque control (DTC) for induction motor

sensorless drive.". Proceedings of IEEE IAS 98, St. Louis, MO, USA 1: 415422. doi:10.1109/ias.1998.732336

(https://dx.doi.org/10.1109%2Fias.1998.732336).

22.

See also

Vector control (motor)

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Categories: Electric motors

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