Direct torque control

Last updated

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

Contents

DTC control platform

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

DTC block diagram.JPG

Overview of key competing VFD control platforms:

VFD
Scalar control

V/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:

These apparent advantages of the DTC are offset by the need for a higher sampling rate (up to 40 kHz as compared with 6–15 kHz for the FOC) leading to higher switching loss in the inverter; a more complex motor model; and inferior torque ripple. [1]

The direct torque method performs very well even without speed sensors. However, the flux estimation is usually based on the integration of the motor phase voltages. Due to the inevitable errors in the voltage measurement and stator resistance estimate the integrals tend to become erroneous at low speed. Thus it is not possible to control the motor if the output frequency of the variable frequency drive is zero. However, by careful design of the control system it is possible to have the minimum frequency in the range 0.5 Hz to 1 Hz that is enough to make possible to start an induction motor with full torque from a standstill situation. A reversal of the rotation direction is possible too 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 sensor can be added to the DTC system. With the sensor, high accuracy of the torque and speed control can be maintained in the whole speed range.

History

DTC was patented by Manfred Depenbrock in the US [2] and in Germany, [3] the latter patent having been filed on October 20, 1984, both patents having been termed direct self-control (DSC). However, Isao Takahashi and Toshihiko Noguchi described a similar control technique termed DTC in an IEEJ paper presented in September 1984 [4] and in an IEEE paper published in late 1986. [5] The DTC innovation is thus usually credited to all three individuals.

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 is higher than DSC. DTC is accordingly aimed at low-to-mid power drives whereas DSC is usually used for higher power drives. [6] (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 very fast 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. [7]

The first major successful commercial DTC products, developed by ABB, involved traction applications late in the 1980s for German DE502 and DE10023 diesel-electric locomotives [8] and the 1995 launch of the ACS600 drives family. ACS600 drives has since been replaced by ACS800 [9] and ACS880 drives. [10] Vas, [11] Tiitinen et al. [12] and Nash [13] provide a good treatment of ACS600 and DTC.

DTC has also been applied to three-phase grid side converter control. [14] [15] Grid side converter is identical in structure to the transistor inverter controlling the machine. Thus it can in addition to rectifying AC to DC also feed back energy from the DC to the AC grid. Further, the waveform of the phase currents is very sinusoidal and power factor can be adjusted as desired. In the grid side converter DTC version the grid is considered to be a big electric machine.

DTC techniques for the interior permanent magnet synchronous machine (IPMSM) were introduced in the late 1990s [16] and synchronous reluctance motors (SynRM) in the 2010s. [17]

DTC was applied to doubly fed machine control in the early 2000s. [18] Doubly fed generators are commonly used in 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. [19] In fact, since DTC's implementation requires more sophisticated hardware to provide comparable performances to the FOC, its first industrial application came much later.

From the end of 1990s several papers have been published about DTC and its modifications such as space vector modulation, [20] 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 ABB have included features similar to DTC in their drives.[ citation needed ]

See also

Related Research Articles

<span class="mw-page-title-main">Electric motor</span> Machine that converts electrical energy into mechanical energy

An electric motor is an electrical machine that converts electrical energy into mechanical energy. Most electric motors operate through the interaction between the motor's magnetic field and electric current in a wire winding to generate force in the form of torque applied on the motor's shaft. An electric generator is mechanically identical to an electric motor, but operates in reverse, converting mechanical energy into electrical energy.

<span class="mw-page-title-main">Pulse-width modulation</span> Electric signal modulation technique used to reduce power load

Pulse-width modulation (PWM), or pulse-duration modulation (PDM), is a method of controlling the average power delivered by an electrical signal. The average value of voltage fed to the load is controlled by switching the supply between 0 and 100% at a rate faster than it takes the load to change significantly. The longer the switch is on, the higher the total power supplied to the load. Along with maximum power point tracking (MPPT), it is one of the primary methods of controlling the output of solar panels to that which can be utilized by a battery. PWM is particularly suited for running inertial loads such as motors, which are not as easily affected by this discrete switching. The goal of PWM is to control a load; however, the PWM switching frequency must be selected carefully in order to smoothly do so.

<span class="mw-page-title-main">Induction motor</span> Type of AC electric motor

An induction motor or asynchronous motor is an AC electric motor in which the electric current in the rotor needed to produce torque is obtained by electromagnetic induction from the magnetic field of the stator winding. An induction motor can therefore be made without electrical connections to the rotor. An induction motor's rotor can be either wound type or squirrel-cage type.

<span class="mw-page-title-main">Synchronous motor</span> Type of AC motor

A synchronous electric motor is an AC electric motor in which, at steady state, the rotation of the shaft is synchronized with the frequency of the supply current; the rotation period is exactly equal to an integral number of AC cycles. Synchronous motors use electromagnets as the stator of the motor which create a magnetic field that rotates in time with the oscillations of the current. The rotor with permanent magnets or electromagnets turns in step with the stator field at the same rate and as a result, provides the second synchronized rotating magnet field. A synchronous motor is termed doubly fed if it is supplied with independently excited multiphase AC electromagnets on both the rotor and stator.

<span class="mw-page-title-main">Brushless DC electric motor</span> Synchronous electric motor powered by an inverter

A brushless DC electric motor (BLDC), also known as an electronically commutated motor, is a synchronous motor using a direct current (DC) electric power supply. It uses an electronic controller to switch DC currents to the motor windings producing magnetic fields that effectively rotate in space and which the permanent magnet rotor follows. The controller adjusts the phase and amplitude of the DC current pulses to control the speed and torque of the motor. This control system is an alternative to the mechanical commutator (brushes) used in many conventional electric motors.

<span class="mw-page-title-main">DC motor</span> Motor which works on direct current

A DC motor is an electrical motor that uses direct current (DC) to produce mechanical force. The most common types rely on magnetic forces produced by currents in the coils. Nearly all types of DC motors have some internal mechanism, either electromechanical or electronic, to periodically change the direction of current in part of the motor.

<span class="mw-page-title-main">Power electronics</span> Technology of power electronics

Power electronics is the application of electronics to the control and conversion of electric power.

<span class="mw-page-title-main">Variable-frequency drive</span> Type of adjustable-speed drive

A variable-frequency drive, variable-speed drives, AC drives, micro drives, inverter drives, or drives) is a type of AC motor drive that controls speed and torque by varying the frequency of the input electricity. Depending on its topology, it controls the associated voltage or current variation.

<span class="mw-page-title-main">Cycloconverter</span> Electrical circuit that changes AC frequency

A cycloconverter (CCV) or a cycloinverter converts a constant amplitude, constant frequency AC waveform to another AC waveform of a lower frequency by synthesizing the output waveform from segments of the AC supply without an intermediate DC link. There are two main types of CCVs, circulating current type or blocking mode type, most commercial high power products being of the blocking mode type.

<span class="mw-page-title-main">Motor drive</span>

Motor drive means a system that includes a motor. An adjustable speed motor drive means a system that includes a motor that has multiple operating speeds. A variable speed motor drive is a system that includes a motor and is continuously variable in speed. If the motor is generating electrical energy rather than using it – this could be called a generator drive but is often still referred to as a motor drive.

<span class="mw-page-title-main">Reluctance motor</span> Type of electric motor

A reluctance motor is a type of electric motor that induces non-permanent magnetic poles on the ferromagnetic rotor. The rotor does not have any windings. It generates torque through magnetic reluctance.

<span class="mw-page-title-main">AC motor</span> Electric motor driven by an AC electrical input

An AC motor is an electric motor driven by an alternating current (AC). The AC motor commonly consists of two basic parts, an outside stator having coils supplied with alternating current to produce a rotating magnetic field, and an inside rotor attached to the output shaft producing a second rotating magnetic field. The rotor magnetic field may be produced by permanent magnets, reluctance saliency, or DC or AC electrical windings.

Doubly fed electric machines, also slip-ring generators, are electric motors or electric generators, where both the field magnet windings and armature windings are separately connected to equipment outside the machine.

<span class="mw-page-title-main">Motor soft starter</span> Device used with AC electrical motors

A motor soft starter is a device used with AC electrical motors to temporarily reduce the load and torque in the powertrain and electric current surge of the motor during start-up. This reduces the mechanical stress on the motor and shaft, as well as the electrodynamic stresses on the attached power cables and electrical distribution network, extending the lifespan of the system.

An induction generator or asynchronous generator is a type of alternating current (AC) electrical generator that uses the principles of induction motors to produce electric power. Induction generators operate by mechanically turning their rotors faster than synchronous speed. A regular AC induction motor usually can be used as a generator, without any internal modifications. Because they can recover energy with relatively simple controls, induction generators are useful in applications such as mini hydro power plants, wind turbines, or in reducing high-pressure gas streams to lower pressure.

In electrical engineering, electric machine is a general term for machines using electromagnetic forces, such as electric motors, electric generators, and others. They are electromechanical energy converters: an electric motor converts electricity to mechanical power while an electric generator converts mechanical power to electricity. The moving parts in a machine can be rotating or linear. Besides motors and generators, a third category often included is transformers, which although they do not have any moving parts are also energy converters, changing the voltage level of an alternating current.

Vector control, also called field-oriented control (FOC), is a variable-frequency drive (VFD) control method in which the stator currents of a three-phase AC or brushless DC electric motor are identified as two orthogonal components that can be visualized with a vector. One component defines the magnetic flux of the motor, the other the torque. The control system of the drive calculates the corresponding current component references from the flux and torque references given by the drive's speed control. Typically proportional-integral (PI) controllers are used to keep the measured current components at their reference values. The pulse-width modulation of the variable-frequency drive defines the transistor switching according to the stator voltage references that are the output of the PI current controllers.

<span class="mw-page-title-main">Braking chopper</span>

Braking choppers, sometimes also referred to as braking units, are used in the DC voltage intermediate circuits of frequency converters to control voltage when the load feeds energy back to the intermediate circuit. This arises, for example, when a magnetized motor is being rotated by an overhauling load and so functions as a generator feeding power to the DC voltage intermediate circuit. They are an application of the chopper principle, using the on-off control of a switching device.

<span class="mw-page-title-main">Korndörfer autotransformer starter</span>

In electrical engineering, the Korndorfer starter is a technique used for reduced voltage soft starting of induction motors. The circuit uses a three-phase autotransformer and three three-phase switches. This motor starting method has been updated and improved by Hilton Raymond Bacon.

<span class="mw-page-title-main">Voltage controller</span>

A voltage controller, also called an AC voltage controller or AC regulator is an electronic module based on either thyristors, triodes for alternating current, silicon-controlled rectifiers or insulated-gate bipolar transistors, which converts a fixed voltage, fixed frequency alternating current (AC) electrical input supply to obtain variable voltage in output delivered to a resistive load. This varied voltage output is used for dimming street lights, varying heating temperatures in homes or industry, speed control of fans and winding machines and many other applications, in a similar fashion to an autotransformer. Voltage controller modules come under the purview of power electronics. Because they are low-maintenance and very efficient, voltage controllers have largely replaced such modules as magnetic amplifiers and saturable reactors in industrial use.

References

  1. Hughes, Austin; Drury, Bill (2013). "Variable Frequency Operation of Induction Motors". Electric Motors and Drives. pp. 205–253. doi:10.1016/B978-0-08-098332-5.00007-3. ISBN   978-0-08-098332-5. S2CID   107929117.
  2. Depenbrock, Manfred. "US4678248 Direct Self-Control of the Flux and Rotary Moment of a Rotary-Field Machine".
  3. Depenbrock, Manfred. "DE3438504 (A1) - Method and Device for Controlling of a Rotating Field Machine" . Retrieved 13 November 2012.
  4. Noguchi, Toshihiko; Takahashi, Isao (Sep 1984). "Quick Torque Response Control of an Induction Motor Based on a New Concept". IEEJ Technical Meetings on Rotating Machine RM84-76. pp. 61–70.
  5. Takahashi, Isao; Noguchi, Toshihiko (September 1986). "A New Quick-Response and High-Efficiency Control Strategy of an Induction Motor". IEEE Transactions on Industry Applications. IA-22 (5): 820–827. doi:10.1109/tia.1986.4504799. S2CID   9684520.
  6. Foo, Gilbert (2010). Sensorless Direct Torque and Flux Control of Interior Permanent Magnet Synchronous Motors at Very Low Speeds Including Standstill (Thesis). Sydney, Australia: The University of New South Wales.
  7. Baader, Uwe (1988). Die Direkte-Selbstregelung (DSR), ein Verfahren zur hochdynamischen Regelung von Drehfeldmaschinen[Direct self-regulation (DSR), a process for the highly dynamic regulation of induction machines] (in German). VDI-Verlag. ISBN   978-3-18-143521-2.[ page needed ]
  8. Jänecke, M.; Kremer, R.; Steuerwald, G. (9–12 Oct 1989). "Direct Self-Control (DSC), A Novel Method Of Controlling Asynchronous Machines In Traction Applications". EPE Proceedings. 1: 75–81.
  9. "ACS800 - The New All-compatible Drives Portfolio" . Retrieved 14 November 2012.
  10. Lönnberg, M.; Lindgren, P. (2011). "Harmonizing drives - The driving force behind ABB's all-compatible drives architecture" (PDF). ABB Review (2): 63–65.[ permanent dead link ]
  11. Vas, Peter (1998). Sensorless Vector and Direct Torque Control. Oxford University Press. ISBN   978-0-19-856465-2.[ page needed ]
  12. Tiitinen, P.; Surandra, M. (1995). "The next generation motor control method, DTC direct torque control". Proceedings of International Conference on Power Electronics, Drives and Energy Systems for Industrial Growth. Vol. 1. pp. 37–43. doi:10.1109/pedes.1996.537279. ISBN   978-0-7803-2795-5. S2CID   60918465.
  13. Nash, J.N. (1997). "Direct torque control, induction motor vector control without an encoder". IEEE Transactions on Industry Applications. 33 (2): 333–341. doi:10.1109/28.567792.
  14. Harmoinen, Martti; Manninen, Vesa; Pohjalainen, Pasi; Tiitinen, Pekka (17 Aug 1999). "US5940286 Method for Controlling the Power To Be Transferred Via a Mains Inverter" . Retrieved 13 November 2012.{{cite journal}}: Cite journal requires |journal= (help)
  15. Manninen, V. (19–21 Sep 1995). "Application of Direct Torque Control Modulation to a Line Converter". Proceedings of EPE 95, Sevilla, Spain: 1292–1296.
  16. French, C.; Acarnley, P. (1996). "Direct torque control of permanent magnet drives". IEEE Transactions on Industry Applications. 32 (5): 1080–1088. doi:10.1109/28.536869.
  17. Lendenmann, Heinz; Moghaddam, Reza R.; Tammi, Ari (2011). "Motoring Ahead". ABB Review. Archived from the original on January 7, 2014. Retrieved 7 January 2014.
  18. Gokhale, Kalyan P.; Karraker, Douglas W.; Heikkil, Samuli J. (10 Sep 2002). "US6448735 Controller for a Wound Rotor Slip Ring Induction Machine" . Retrieved 14 November 2012.{{cite journal}}: Cite journal requires |journal= (help)
  19. "DSCM1 - High Performance Machinery Drives" (PDF). Archived from the original (PDF) on October 18, 2011. Retrieved 18 October 2011.
  20. Lascu, C.; Boldea, I.; Blaabjerg, F. (1998). "A modified direct torque control (DTC) for induction motor sensorless drive". Conference Record of 1998 IEEE Industry Applications Conference. Thirty-Third IAS Annual Meeting (Cat. No.98CH36242). Vol. 1. pp. 415–422. doi:10.1109/ias.1998.732336. ISBN   0-7803-4943-1.