Direct Torque Control
Encyclopedia
Direct torque control is one method used in variable frequency drive
s 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.
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 from the reference more than allowed tolerance, the transistor
s of the variable frequency drive
are turned off and on in such a way that the flux and torque will return in their tolerance bands as fast as possible. Thus direct torque control is one form of the hysteresis or bang-bang control
.
This control method implies the following properties of the control:
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 integral
s 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.
ed by Manfred Depenbrock in filed originally on October 20, 1984 in Germany. He called it "Direct Self-Control" (DSC). However, Isao Takahashi and Toshihiko Noguchi presented a similar idea only few months later in a Japanese journal
. Thus direct torque control is usually credited to all three gentlemen.
The only difference between DTC and DSC is the shape of the path along which the flux vector is controlled to follow. In DTC the path is a circle
and in DSC it was a hexagon. Today DTC uses hexagon flux path only when full voltage is required at high speeds.
Since Depenbrock, Takahashi and Noguchi had proposed direct torque control (DTC) for induction
machines in the mid 1980s, this new torque control scheme has gained much momentum. From its introduction, the Direct Torque control or Direct Self Control (DSC) principle has been used for Induction Motor (IM) drives with fast dynamics. Despite its simplicity, DTC is able to produce very fast torque and flux control, if the torque and flux are correctly estimated.
Among the others, DTC/DSC was further studied in Ruhr-University in Bochum, Germany at the end of 80's. A very good treatment of the subject can be found from the doctoral thesis:
The first commercial application was in traction. At the end of 80's DSC was tried in German diesel-electric locomotives
DE502 http://commons.wikimedia.org/wiki/Category:MaK_DE_502http://www.loks-aus-kiel.de/index.php?nav=1400726&lang=1 and DE1003 http://www.loks-aus-kiel.de/index.php?nav=1400728 by ABB, please see paper:
The first major commercial application was, however, the ACS600 variable speed drive
by ABB that saw the daylight in 1995. ACS600 has later been replaced with ACS800 http://www.abb.com/product/us/9AAC133421.aspx. A good presentation of ACS600 and a full theoretical treatment of direct torque control can be found in the book:
Also dealing with DTC and ACS600 are:
DTC has also been applied to three-phase
grid side converter
control . 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 (which, actually, there are many in the grid!). A paper dealing with grid side converter DTC was presented already in 1995:
In the late 1990s DTC techniques for the Interior Permanent Magnet
Synchronous
Machine (IPMSM) appeared.
Further, in the beginning of 2000's DTC was applied to doubly fed machine
control . Doubly fed generators are today commonly used in wind turbine
applications.
Thinking of the outstanding torque control dynamics of the DTC it was somewhat surprising that the first servo drive using DTC, ABB's ACSM1,http://www.abb.com/product/seitp322/1b619d05b5719186c125734b0023cc8c.aspx?productLanguage=us&country=00 was introduced quite late, in 2007.
During 2000's several papers have been published about DTC. Also several modifications such as space vector modulated DTC that has constant switching frequency, has been presented.
Due to the expiring of the DTC patent by Depenbrock in 2004 it is to be expected that other companies than ABB will also start to sell drives using DTC.
Variable frequency drive
A variable-frequency drive is a system for controlling the rotational speed of an alternating current electric motor by controlling the frequency of the electrical power supplied to the motor. A variable frequency drive is a specific type of adjustable-speed drive...
s to control the torque
Torque
Torque, moment or moment of force , is the tendency of a force to rotate an object about an axis, fulcrum, or pivot. Just as a force is a push or a pull, a torque can be thought of as a twist....
(and thus finally the speed
Speed
In kinematics, the speed of an object is the magnitude of its velocity ; it is thus a scalar quantity. The average speed of an object in an interval of time is the distance traveled by the object divided by the duration of the interval; the instantaneous speed is the limit of the average speed as...
) of three-phase
Three-phase
In electrical engineering, three-phase electric power systems have at least three conductors carrying voltage waveforms that are radians offset in time...
AC electric motors
Electric motor
An electric motor converts electrical energy into mechanical energy.Most electric motors operate through the interaction of magnetic fields and current-carrying conductors to generate force...
. This involves calculating
Calculation
A calculation is a deliberate process for transforming one or more inputs into one or more results, with variable change.The term is used in a variety of senses, from the very definite arithmetical calculation of using an algorithm to the vague heuristics of calculating a strategy in a competition...
an estimate of the motor's magnetic flux
Magnetic flux
Magnetic flux , is a measure of the amount of magnetic B field passing through a given surface . The SI unit of magnetic flux is the weber...
and torque
Torque
Torque, moment or moment of force , is the tendency of a force to rotate an object about an axis, fulcrum, or pivot. Just as a force is a push or a pull, a torque can be thought of as a twist....
based on the measured voltage
Voltage
Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...
and current
Electric current
Electric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...
of the motor.
Method
StatorStator
The stator is the stationary part of a rotor system, found in an electric generator, electric motor and biological rotors.Depending on the configuration of a spinning electromotive device the stator may act as the field magnet, interacting with the armature to create motion, or it may act as the...
flux linkage
Flux linkage
Flux linkage is a property of a coil of conducting wire and the magnetic field through which it passes. It is determined by the number of turns in the coil and the flux of the magnetic field....
is estimated by integrating
Integral
Integration is an important concept in mathematics and, together with its inverse, differentiation, is one of the two main operations in calculus...
the stator
Stator
The stator is the stationary part of a rotor system, found in an electric generator, electric motor and biological rotors.Depending on the configuration of a spinning electromotive device the stator may act as the field magnet, interacting with the armature to create motion, or it may act as the...
voltages. Torque
Torque
Torque, moment or moment of force , is the tendency of a force to rotate an object about an axis, fulcrum, or pivot. Just as a force is a push or a pull, a torque can be thought of as a twist....
is estimated as a cross product
Cross product
In mathematics, the cross product, vector product, or Gibbs vector product is a binary operation on two vectors in three-dimensional space. It results in a vector which is perpendicular to both of the vectors being multiplied and normal to the plane containing them...
of estimated stator flux linkage
Flux linkage
Flux linkage is a property of a coil of conducting wire and the magnetic field through which it passes. It is determined by the number of turns in the coil and the flux of the magnetic field....
vector and measured motor current
Electric current
Electric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...
vector. The estimated flux magnitude and torque are then compared with their reference values. If either the estimated flux or torque deviates from the reference more than allowed tolerance, the transistor
Transistor
A transistor is a semiconductor device used to amplify and switch electronic signals and power. It is composed of a semiconductor material with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor's terminals changes the current...
s of the variable frequency drive
Variable frequency drive
A variable-frequency drive is a system for controlling the rotational speed of an alternating current electric motor by controlling the frequency of the electrical power supplied to the motor. A variable frequency drive is a specific type of adjustable-speed drive...
are turned off and on in such a way that the flux and torque will return in their tolerance bands as fast as possible. Thus direct torque control is one form of the hysteresis or bang-bang control
Bang-bang control
In control theory, a bang–bang controller , also known as a hysteresis controller, is a feedback controller that switches abruptly between two states. These controllers may be realized in terms of any element that provides hysteresis...
.
This control method implies the following properties of the control:
- 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 responseStep responseThe step response of a system in a given initial state consists of the time evolution of its outputs when its control inputs are Heaviside step functions. In electronic engineering and control theory, step response is the time behaviour of the outputs of a general system when its inputs change from...
has no overshootOvershoot (signal)In signal processing, control theory, electronics, and mathematics, overshoot is when a signal or function exceeds its target. It arises especially in the step response of bandlimited systems such as low-pass filters... - No coordinate transformsRotation (mathematics)In geometry and linear algebra, a rotation is a transformation in a plane or in space that describes the motion of a rigid body around a fixed point. A rotation is different from a translation, which has no fixed points, and from a reflection, which "flips" the bodies it is transforming...
are needed, all calculations are done in stationary coordinate system - No separate modulatorPulse-width modulationPulse-width modulation , or pulse-duration modulation , is a commonly used technique for controlling power to inertial electrical devices, made practical by modern electronic power switches....
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 spectrumFrequency spectrumThe frequency spectrum of a time-domain signal is a representation of that signal in the frequency domain. The frequency spectrum can be generated via a Fourier transform of the signal, and the resulting values are usually presented as amplitude and phase, both plotted versus frequency.Any signal...
. 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 (aliasingAliasingIn signal processing and related disciplines, aliasing refers to an effect that causes different signals to become indistinguishable when sampled...
) or dc voltage transients - SynchronizationSynchronization (alternating current)-Electricity generation:Electricity generation requires the connection of large numbers of alternators in parallel and additional alternators must be switched in when demand rises....
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 controlDigital controlDigital control is a branch of control theory that uses digital computers to act as system controllers.Depending on the requirements, a digital control system can take the form of a microcontroller to an ASIC to a standard desktop computer....
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 algorithmAlgorithmIn mathematics and computer science, an algorithm is an effective method expressed as a finite list of well-defined instructions for calculating a function. Algorithms are used for calculation, data processing, and automated reasoning...
has to be performed with 10 - 30 microsecondsSecondThe second is a unit of measurement of time, and is the International System of Units base unit of time. It may be measured using a clock....
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 noiseNoiseIn common use, the word noise means any unwanted sound. In both analog and digital electronics, noise is random unwanted perturbation to a wanted signal; it is called noise as a generalisation of the acoustic noise heard when listening to a weak radio transmission with significant electrical noise...
because spikes in the measured signals easily cause erroneous control actions. Further complication is that no low-pass filterLow-pass filterA low-pass filter is an electronic filter that passes low-frequency signals but attenuates signals with frequencies higher than the cutoff frequency. The actual amount of attenuation for each frequency varies from filter to filter. It is sometimes called a high-cut filter, or treble cut filter...
ing 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
The direct torque method performs very well even without speed sensors
Wheel speed sensor
A wheel speed sensor or vehicle speed sensor is a type of tachometer. It is a sender device used for reading the speed of a vehicle's wheel rotation...
. 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 integral
Integral
Integration is an important concept in mathematics and, together with its inverse, differentiation, is one of the two main operations in calculus...
s tend to become erroneous at low speed. Thus it is not possible to control the motor if the output
Output
Output is the term denoting either an exit or changes which exit a system and which activate/modify a process. It is an abstract concept, used in the modeling, system design and system exploitation.-In control theory:...
frequency of the variable frequency drive is zero. However, by careful design of the control system
Control system
A control system is a device, or set of devices to manage, command, direct or regulate the behavior of other devices or system.There are two common classes of control systems, with many variations and combinations: logic or sequential controls, and feedback or linear controls...
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
Induction motor
An induction or asynchronous motor is a type of AC motor where power is supplied to the rotor by means of electromagnetic induction. These motors are widely used in industrial drives, particularly polyphase induction motors, because they are robust and have no brushes...
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
Wheel speed sensor
A wheel speed sensor or vehicle speed sensor is a type of tachometer. It is a sender device used for reading the speed of a vehicle's wheel rotation...
or position sensor
Rotary encoder
A rotary encoder, also called a shaft encoder, is an electro-mechanical device that converts the angular position or motion of a shaft or axle to an analog or digital code. The output of incremental encoders provides information about the motion of the shaft which is typically further processed...
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
Direct torque control was patentPatent
A patent is a form of intellectual property. It consists of a set of exclusive rights granted by a sovereign state to an inventor or their assignee for a limited period of time in exchange for the public disclosure of an invention....
ed by Manfred Depenbrock in filed originally on October 20, 1984 in Germany. He called it "Direct Self-Control" (DSC). However, Isao Takahashi and Toshihiko Noguchi presented a similar idea only few months later in a Japanese journal
Academic journal
An academic journal is a peer-reviewed periodical in which scholarship relating to a particular academic discipline is published. Academic journals serve as forums for the introduction and presentation for scrutiny of new research, and the critique of existing research...
. Thus direct torque control is usually credited to all three gentlemen.
The only difference between DTC and DSC is the shape of the path along which the flux vector is controlled to follow. In DTC the path is a circle
Circle
A circle is a simple shape of Euclidean geometry consisting of those points in a plane that are a given distance from a given point, the centre. The distance between any of the points and the centre is called the radius....
and in DSC it was a hexagon. Today DTC uses hexagon flux path only when full voltage is required at high speeds.
Since Depenbrock, Takahashi and Noguchi had proposed direct torque control (DTC) for induction
Electromagnetic induction
Electromagnetic induction is the production of an electric current across a conductor moving through a magnetic field. It underlies the operation of generators, transformers, induction motors, electric motors, synchronous motors, and solenoids....
machines in the mid 1980s, this new torque control scheme has gained much momentum. From its introduction, the Direct Torque control or Direct Self Control (DSC) principle has been used for Induction Motor (IM) drives with fast dynamics. Despite its simplicity, DTC is able to produce very fast torque and flux control, if the torque and flux are correctly estimated.
Among the others, DTC/DSC was further studied in Ruhr-University in Bochum, Germany at the end of 80's. A very good treatment of the subject can be found from the doctoral thesis:
- Uve Baader: Die Direkte-Selbstregelung (DSR), Ein Verfahren zur hochdynamischen Regelung von Drehfeldmaschinen. Fortschr.-Ber. VDI Reihe 21, Nr. 35. VDI-Verlag 1988. ISBN 3-18-143521-X
The first commercial application was in traction. At the end of 80's DSC was tried in German diesel-electric locomotives
Diesel locomotive
A diesel locomotive is a type of railroad locomotive in which the prime mover is a diesel engine, a reciprocating engine operating on the Diesel cycle as invented by Dr. Rudolf Diesel...
DE502 http://commons.wikimedia.org/wiki/Category:MaK_DE_502http://www.loks-aus-kiel.de/index.php?nav=1400726&lang=1 and DE1003 http://www.loks-aus-kiel.de/index.php?nav=1400728 by ABB, please see paper:
- Jänecke, M., Kremer, R., Steuerwald, G.: Direct Self-Control (DSC), A Novel Method Of Controlling Asynchronous Machines In Traction Applications. Proceedings of EPE 1989, October 9–12, 1989 Aachen, Germany, Vol. 1, pp. 75–81.
The first major commercial application was, however, the ACS600 variable speed drive
Adjustable Speed Drive
Adjustable speed drive or variable-speed drive describes equipment used to control the speed of machinery. Many industrial processes such as assembly lines must operate at different speeds for different products...
by ABB that saw the daylight in 1995. ACS600 has later been replaced with ACS800 http://www.abb.com/product/us/9AAC133421.aspx. A good presentation of ACS600 and a full theoretical treatment of direct torque control can be found in the book:
- Peter Vas: Sensorless Vector and Direct Torque Control, Oxford University Press, 1998, ISBN 0-19-856465-1
Also dealing with DTC and ACS600 are:
- Tiitinen, P., Pohjalainen, P., Lalu, J.: The Next Generation Motor Control Method: Direct Torque Control (DTC). EPE Journal, Vol. 5., no 1, March 1995, pp. 14–18 http://www.epe-association.org/epe/documents.detail.php?documents_id=3237.
- Nash, J.: Direct Torque Control, Induction Motor Vector Control Without an Encoder. IEEE Tr. on Industry Applications, Vol. 33, No. 2, March/April 1997 http://ieeexplore.ieee.org/search/srchabstract.jsp?arnumber=567792&isnumber=12339&punumber=28&k2dockey=567792@ieeejrns&query=%28induction+motor+vector+control+without+an+encoder%29%3Cin%3Emetadata&pos=0.
DTC has also been applied to three-phase
Three-phase
In electrical engineering, three-phase electric power systems have at least three conductors carrying voltage waveforms that are radians offset in time...
grid side converter
Voltage converter
A voltage converter changes the voltage of an electrical power source. It may be combined with other components to create a power supply.-AC and DC:...
control . Grid side converter is identical in structure to the transistor
Transistor
A transistor is a semiconductor device used to amplify and switch electronic signals and power. It is composed of a semiconductor material with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor's terminals changes the current...
inverter
Inverter (electrical)
An inverter is an electrical device that converts direct current to alternating current ; the converted AC can be at any required voltage and frequency with the use of appropriate transformers, switching, and control circuits....
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
Sine wave
The sine wave or sinusoid is a mathematical function that describes a smooth repetitive oscillation. It occurs often in pure mathematics, as well as physics, signal processing, electrical engineering and many other fields...
and power factor
Power factor
The power factor of an AC electric power system is defined as the ratio of the real power flowing to the load over the apparent power in the circuit, and is a dimensionless number between 0 and 1 . Real power is the capacity of the circuit for performing work in a particular time...
can be adjusted as desired. In the grid side converter DTC version the grid is considered to be a big electric machine (which, actually, there are many in the grid!). A paper dealing with grid side converter DTC was presented already in 1995:
- Manninen, V.: Application of Direct Torque Control Modulation to a Line Converter. Proc. of EPE 1995, Sept. 19-21, 1995, Sevilla, Spain, Proceedings pp. 1.292-1.296.
In the late 1990s DTC techniques for the Interior Permanent Magnet
Magnet
A magnet is a material or object that produces a magnetic field. This magnetic field is invisible but is responsible for the most notable property of a magnet: a force that pulls on other ferromagnetic materials, such as iron, and attracts or repels other magnets.A permanent magnet is an object...
Synchronous
Synchronization
Synchronization is timekeeping which requires the coordination of events to operate a system in unison. The familiar conductor of an orchestra serves to keep the orchestra in time....
Machine (IPMSM) appeared.
Further, in the beginning of 2000's DTC was applied to doubly fed machine
Doubly-fed electric machine
Doubly fed electric machines are electric motors or electric generators that have windings on both stationary and rotating parts, where both windings transfer significant power between shaft and electrical system...
control . Doubly fed generators are today commonly used in wind turbine
Wind turbine
A wind turbine is a device that converts kinetic energy from the wind into mechanical energy. If the mechanical energy is used to produce electricity, the device may be called a wind generator or wind charger. If the mechanical energy is used to drive machinery, such as for grinding grain or...
applications.
Thinking of the outstanding torque control dynamics of the DTC it was somewhat surprising that the first servo drive using DTC, ABB's ACSM1,http://www.abb.com/product/seitp322/1b619d05b5719186c125734b0023cc8c.aspx?productLanguage=us&country=00 was introduced quite late, in 2007.
During 2000's several papers have been published about DTC. Also several modifications such as space vector modulated DTC that has constant switching frequency, has been presented.
Due to the expiring of the DTC patent by Depenbrock in 2004 it is to be expected that other companies than ABB will also start to sell drives using DTC.