The V/F, voltage/frequency, Control method – also called Volts-per-Hertz control is the most common inverter control method. Requiring no feedback device, it is suitable for general purpose and multiple motor applications.
V/F control is the standard operating method and is more than sufficient for most drive tasks. Here, the motor voltage (V) is adjusted proportionally to the output frequency (f). Accordingly, the magnetizing current of the motor is kept practically constant in a wide adjustment range.
This means a practically constant torque for the connected motor across the entire speed adjustment range. A reduced torque is only normally to be expected in the lower speed range due to the physical properties of the asynchronous motor.
A control method that enables preventing reductions in the power factor or efficiency of a motor in a wide range of variable speed operation for changes in the frequency for speed control by outputting a voltage (V/f characteristic) corresponding to the frequency set by a parameter in an Inverter. The revolution speed of an induction motor is proportional to the frequency, which can reduce the power factor and efficiency of a motor even with a variable frequency because changes to the frequency cause the internal impedance of a motor to change. Therefore you must change the voltage corresponding to the frequency. V/f control reduces the torque in low-speed operation with the primary resistance voltage drop even through it attempts to keep the torque stable regardless of the frequency. A torque boost can increase the torque somewhat in low-speed operation, but it never produces optimized control, causing the current-torque ratio to drop and resulting in an inability to get the same torque as the base frequency. It also requies the troublesome boost adjustment.
V/F Control with PG Feedback gives the better speed regulation of a closed loop system.
Vector control
For highest demands on the drive behaviour of the three-phase motor, the SLV method is practically extended by the feed back of the actual motor speed. However, a sensor such as an incremental encoder on the motor shaft is necessary, which measures the actual motor speed and feeds it back to the control loop. The following advantage result compared to the extra effort in addition to the SLV process:
Shortest reaction time with setpoint changes
Shortest response times with load changes
Highest torque, even at speeds = 0 Hz (static torque)
Highest torque, even at speeds = 0 Hz (static torque)
Vector control, also called field-oriented control (FOC), is a variable frequency drive (VFD) control method which controls three-phase AC electric motor output by means of two controllable VFD inverter output variables:
Voltage magnitude
Frequency.
(Voltage angle, or phase, is only indirectly controlled)
FOC is a control technique used in AC synchronous and induction motor applications that was originally developed for high-performance motor applications which can operate smoothly over the full speed range, can generate full torque at zero speed, and is capable of fast acceleration and deceleration but that is becoming increasingly attractive for lower performance applications as well due to FOC's motor size, cost and power consumption reduction superiority.
Not only is FOC very common in induction motor control applications due to its traditional superiority in high-performance applications, but the expectation is that it will eventually nearly universally displace single-variable scalar volts-per-Hertz (V/f) control.
Development history
Yet it was not until after the commercialization of microprocessors, that is in the early 1980s, that general purpose AC drives became available.Barriers to use of FOC for AC drive applications included higher cost and complexity and lower maintainability compared to DC drives, FOC having until then required many electronic components in terms of sensors, amplifiers and so on.Technical University Darmstadt's K. Hasse and Siemens' F. Blaschke pioneered vector control of AC motors starting in 1968 and in the early 1970s, Hasse in terms of proposing indirect vector control, Blaschke in terms of proposing direct vector control., Technical University Braunschweig's Werner Leonhard further developed FOC techniques and was instrumental in opening up opportunities for AC drives to be a competitive alternative to DC drives.
The Park transformation has long been widely used in the analysis and study of synchronous and induction machines. The transformation is by far the single most important concept needed for an understanding of how FOC works, the concept having been first conceptualized in a 1929 paper authored by Robert H. Park. Park's paper was ranked second most important in terms of impact from among all power engineering related papers ever published in the twentieth century. The novelty of Park's work involves his ability to transform any related machine's linear differential equation set from one with time varying coefficients to another with time invariant coefficients.
Technical overview
Overview of key competing VFD control platforms:
| VFD, with sensor or sensorless |
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In vector control, an AC induction or synchronous motor is controlled under all operating conditions like a separately excited DC motor. That is, the AC motor behaves like a DC motor in which the field flux linkage andar mature flux linkage created by the respective field and armature (or torque component) currents are orthogonally aligned such that, when torque is controlled, the field flux linkage is not affected, hence enabling dynamic torque response.
Vector control accordingly generates a three-phase PWM motor voltage output derived from a complex voltage vector to control a complex current vector derived from motor's three-phase motor stator current input through projections or rotations back and forth between the three-phase speed and time dependent system and these vectors' rotating reference-frame two-coordinate time invariant system.
Such complex stator motor current space vector can be defined in a (d,q) coordinate system with orthogonal components along d (direct) and q (quadrature) axes such that field flux linkage component of current is aligned along the d axis and torque component of current is aligned along the q axis. The induction motor's (d,q) coordinate system can be superimposed to the motor's instantaneous (a,b,c) three-phase sinusoidal system as shown in accompanying image (phases a & b not shown for clarity). Components of the (d,q) system current vector, allow conventional control such as proportional and integral, or PI, control, as with a DC motor.
Projections associated with the (d,q) coordinate system typically involve:
Forward projection from instantaneous currents to (a,b,c) complex stator current space vector representation of the three-phase sinusoidal system.
Forward three-to-two phase, (a,b,c)-to-(,) projection using the Clarke transformation. Vector control implementations usually assume ungrounded motor with balanced three-phase currents such that only two motor current phases need to be sensed. Also, backward two-to-three phase, (,)-to-(a,b,c) projection uses space vector PWM modulator or inverse Clarke transformation and one of the other PWM modulators.
Forward and backward two-to-two phase,(,)-to-(d,q) and (d,q)-to-(,) projections using the Park and inverse Park transformations, respectively.
However, it is not uncommon for sources to use three-to-two, (a,b,c)-to-(d,q) and inverse projections.
While (d,q) coordinate system rotation can arbitrarily be set to any speed, there are three preferred speeds or reference frames:
Stationary reference frame where (d,q) coordinate system does not rotate;
Synchronously rotating reference frame where (d,q) coordinate system rotates at synchronous speed;
Rotor reference frame where (d,q) coordinate system rotates at rotor speed.
Decoupled torque and field currents can thus be derived from raw stator current inputs for control algorithm development.
Whereas magnetic field and torque components in DC motors can be operated relatively simply by separately controlling the respective field and armature currents, economical control of AC motors in variable speed application has required development of microprocessor-based controls with all AC drives now using powerful DSP (digital signal processing) technology.
Inverters can be implemented as either open-loop sensorless or closed-loop FOC, the key limitation of open-loop operation being mimimum speed possible at 100% torque, namely, about 0.8 Hz compared to standstill for closed-loop operation.
There are two vector control methods, direct or feedback vector control (DFOC) and indirect or feedforward vector control (IFOC), IFOC being more commonly used because in closed-loop mode such drives more easily operate throughout the speed range from zero speed to high-speed field-weakening. In DFOC, flux magnitude and angle feedback signals are directly calculated using so-called voltage or current models. In IFOC, flux space angle feedforward and flux magnitude signals first measure stator currents and rotor speed for then deriving flux space angle proper by summing the rotor angle corresponding to the rotor speed and the calculated reference value of slip angle corresponding to the slip frequency.
Sensorless control (see Sensorless FOC Block Diagram) of AC drives is attractive for cost and reliability considerations. Sensorless control requires derivation of rotor speed information from measured stator voltage and currents in combination with open-loop estimators or closed-loop observers.
A standard VFD (lets call it a Scalar Drive) puts out a PWM pattern designed to maintain a constant V/Hz pattern to the motor under ideal conditions. How the motor reacts to that PWM pattern is very dependent upon the load conditions. The Scalar drive knows nothing about that, it only tells the motor what to do. If for example it provides 43Hz to the motor, and the motor spins at a speed equivalent to 40Hz, the Scalar Drive doesn't know. You can't do true torque control with a scalar drive because it has no way of knowing what the motor output torque is.
These problems associated with the scalar VFDs inability to alter it's output with changes in the load gets worse as the speed reference goes down, so the "rule of thumb" in determining the need for which technology to use is that scalar drives work OK at speed ranges between 5:1 (50Hz applications) or 6:1 (60Hz applications). So if your application will need accurate control below 10Hz, scalar may not work for you.
A Vector Drive uses feedback of various real world information (more on that later) to further modify the PWM pattern to maintain more precise control of the desired operating parameter, be it speed or torque. Using a more powerful and faster microprocessor, it uses the feedback information to calculate the exact vector of voltage and frequency to attain the goal. In a true closed loop fashion, it goes on to constantly update that vector to maintain it. It tells the motor what to do, then checks to see if it did it, then changes its command to correct for any error. Vector drives come in 2 types, Open Loop and Closed Loop, based upon the way they get their feedback information.
A true Closed Loop Vector Drive uses a shaft encoder on the motor to give positive shaft position indication back to the microprocessor (mP). So when the mP says move x radians, the encoder says "it only moved x-2 radians". The mP then alters the PWM signature on the fly to make up for the error. For torque control, the feedback allows the mP to adjust the pattern so that a constant level of torque can be maintained regardless of speed, i.e. a winder application where diameters are constantly changing. If the shaft moves one way or the other too much, the torque requirement is wrong and the error is corrected. A true closed Loop Vector Drive can also make an AC motor develop continuous full torque at zero speed, something that previously only DC drives were capable of. That makes them suitable for crane and hoist applications where the motor must produce full torque before the brake is released or else the load begins dropping and it can't be stopped. Closed Loop is also so close to being a servo drive that some people use them as such. The shaft encoder can be used to provide precise travel feedback by counting pulses. (Note: See Addendum below for additional information)
Open Loop is actually a misnomer because it is actually a closed loop system, but the feedback loop comes from within the VFD itself instead of an external encoder. For this reason there is a trend to refer to them as "Sensorless Vector" drives. The mP creates a mathematical "model" of the motor operating parameters and keeps it in memory. As the motor operates, the mP monitors the output current (mainly), compares it to the model and determines from experience what the different current effects mean in terms of the motor performance. Then the mP executes the necessary error corrections just as the closed Loop Vector Drive does. The only drawback is that as the motor gets slower, the ability of the mP to detect the subtle changes in magnetics becomes more difficult. At zero speed it is generally accepted that an Open loop Vector Drive is not reliable enough to use on cranes and hoists. For most other applications though it is just fine.
This is all done at very high speeds, that is why you did not see Vector Drives as available earlier on. The cost of the high speed mP technology has now come down to every day availability.
jraef is absolutely correct in his "crash course" on VFD. One might add that the "vector" that pops up in the description and the name of this drive technology is the rotating space vector that describes the flux in the motor. Since flux and current are in phase, it also describes the current in the stator.
An induction motor is very similar to a DC motor. It needs a magnetizing current and a torque producing current. In a DC motor, these two currents are fed to two different windings; the field winding and the armature winding. In an induction motor, there is only one set of windings: the stator winding. So the vector drive has to separate the two components some other way.
It does this by keeping in mind that magnetizing current always lags (inductive) the voltage by 90 degrees and that the torque producing current is always in phase with the voltage. It controls the magnetizing current (usually named Id) in one control loop and the torque producing current (Iq) in another control loop. The two vectors Id and Iq, which are always 90 degrees apart, are then added (vector sum) and sent to the modulator, which turns the vector information into a rotating PWM modulated three-phase system with the correct frequency and voltage.
As soon as a deviation from correct speed or torque or magnetizing current is detected by the control loops the corresponding variable will be changed by the controller to correct the variable.
If - for example - the speed is wrong, the output frequency will be corrected and also the voltage so that the correct magnetizing current is maintained. And correspondingly, if the stator winding heats up the magnetizing current would go down if the decrease wasn't detected and corrected by the controller. The action in this latter case is that the voltage goes up (PWM adjusted), but not the frequency (the speed was already correct.
Vector drives are among the most complex standard equipments that exist. But keeping in mind that there are always two control loops, one for magnetizing current and one for speed/torque will help thinking about them.
Follow up: It might be worth noting that microprocessor technology is rapidly making scalar drives obsolete except in the smallest of sizes. The low cost for processing power has made the issue of having and maintaining separate designs untenable for many manufacturers.
Addendum regarding an additional important issue on the importance of encoder feedback with Closed loop Vector drives by member DickDV on 4/18/08:
When talking about speed control and error in a drive/motor system, it is important to understand that there are several different aspects to the issue.
First, speed error is generally due to changes in torque demand. In an induction motor, this error is mostly slip. So the question becomes, how well does the drive compensate for torque induced slip speed changes. With a good vector drive, this can get down in the range of one-tenth of motor slip without an encoder. If you need better than that, an encoder is required. Note here that the error is a result of torque changes. If your torque doesn't change, you won't have much speed error to start with.
Second, in some applications, especially those involving web products and tension control, cumulative error is just as important as actual error. For example, even if you are very accurate with actual error, if it is all negative or all positive, eventually you are going to have too much or too little tension. No encoderless system will assure non-cumulative error. For that you need an encoder.
Third, speed reference error is often overlooked. That is error either in the speed signal going into the drive or error in the drive translating the input command into an actual output speed. Usually, the majority of this error is due to the analog input terminal analog-to-digital conversion. A 10 bit resolution A/D input will not be nearly as accurate as a 14 bit resolution input. This is a matter of purchasing a drive with the input resolution adequate for the intended purpose.
Sensorless vector control (SLV) (Sensorless vector control, sometimes called open loop vector, utilizes a more complex control algorithm to give precision speed control, quick response and higher torque at low speed.)
SLV control is a further technological development of V/f control. If increased demands are placed on the three-phase motor, it generally requires a control which uses a sensor, e.g. incremental encoder on the motor shaft, so that the measured motor speed is fed back to the control loop. However, SLV control does not need a sensor. This is where the term sensorless originates.
However, it is just as high-performance as vector control which requires sensor feed back. The motor speed is calculated in the frequency inverter using a mathematical motor model. The downstream control provides the correct torque in the respective operating state.
SLV control has the following advantages over V/f control:
Short reaction times with setpoint value changes
Short response times with load changes
High torque, even at low speeds (e.g. DV51 start torque > 200 % at approx. 1 Hz)
High speed stability with load change
Closed-Loop Vector Torque Control
Closed Loop Vector or Flux Vector requires encoder feedback and gives precise speed and full rated torque control over a wide speed range – sometimes even at zero RPM.
Inverters and their motors can also be operated in a “Constant Horsepower” profile where motor speed can be extended beyond the base speed rating with torque capacity de-rating.
As we have seen, sinewave commutation ensures that the static torque produced by the motor does not vary based upon the shaft’s position. But the SSt servo drive’s closed-loop vector torque control goes further to ensure that the torque produced under dynamic conditions is highly accurate and efficient.
A big step beyond simple sinewave commutation
So let’s take a look at why a simple sinewave commutated servo amplifier is less accurate than the SSt servo drive’s closed-loop vector torque control. The main difference between these two techniques is that the closed-loop vector torque control constantly measures the amount of torque (and extraneous heating) that is produced, and continuously works the servo torque to the command value while driving the extraneous heating to zero. In contrast, a sinewave amplifier calculates what the currents should be to produce the required torque, passes these to three separate current loops and assumes that the proper torque will be produced without ever checking. Although each individual current amp is closed-loop, the torque control is open-loop! The difference is shown in the diagrams below:
- Figure 1: Block diagram of a Sinewave Amplifier
The first thing you’ll notice is that the current loops in a sinewave amplifier operate in an uncoordinated fashion. However, the phases themselves are connected together, so the currents are dependent upon each other.
Let’s explore a simple analogy that describes the dilemma this causes: Imagine you are in a house with two bathrooms. You and your spouse are both taking showers at the same time (in separate bathrooms) and you decide you want the water a little hotter. So you, like the independent current loop, open up your hot water valve a little more. You get hotter water, but because you are "connected" through the house’s water heater, your spouse’s water cools down. So your spouse adjusts to maintain the desired water temperature. This goes on for a few moments with both of you adjusting your water valves until you find an equilibrium that suits both of you. Now imagine what would happen if you were trying to constantly change the water temperature along some arbitrary profile, like the sine angles trying to follow the arbitrary position of the rotor? It would be nearly impossible for you and your spouse to do this accurately without communicating and coordinating. Add a little noise or imbalance (someone washing the dishes or flushing a toilet) and it gets even harder. So because the current sensors have some noise, the rotor’s back-EMF is not a true sinewave, and the amplifier may run out of voltage headroom, etc., a sinewave amplifier will naturally induce torque fluctuations as the independent current loops seek equilibrium.
The second failing of the simple sinewave-commutated amplifier is that the current loop response delay forces the currents to lead or lag (depending upon where within the four quadrants of operation the motor is operating—motoring, braking, accelerating, etc.). As the speed becomes higher, the rate of change of the sinewave demand gets faster and the phase error becomes more significant. This lead or lag in time directly affects the angle of the magnetic vector produced in the motor, moving it off the ninety degree mark, causing torque to be reduced (and inaccurate relative to the position/velocity compensator’s command) and motor heating to be increased.
The third problem is caused by limited voltage headroom as the motor approaches rated speed. This is a little harder to explain, but suffice it to say that without active measurement and control of the magnetic vector, these amplifiers run out of headroom earlier as speed is increased, drastically affecting their Torque Response Time. (If you’re an electrical engineer, this is analogous to having reduced large signal bandwidth.) So, just when you need a rapid reversal in torque (say at the peak speed of a triangle move) your sinewave amplifier will delay its response, affecting your tracking accuracy and settling time. This reduced "large signal response" typically also limits your ability to turn up the position/velocity compensator gains, which further limits your stiffness, tracking accuracy and settling time.
So what is Closed-Loop Vector Torque Control?
In contrast to the simple sinewave amplifier, which uses the sinewave references as commands to uncoordinated current loops, the SSt servo drive’s closed-loop vector torque controller uses the sinewave references in mathematical transforms to "de-rotate" the amplitude and angle of the electromagnetic vector from the measured currents. Then, after the torque loop calculates the voltage amplitude and angle, another transform "re-rotates" these into the three simultaneous voltages provided to the motor. See figure 2.
- Figure 2: Vector closed-loop torque control
O.K., that’s a big mouthful—let’s imagine you are trying to shoot a bank robber as he flees in a getaway car, swerving, braking and accelerating all the while. The difference just described above is like trying to shoot him from the sidewalk as he erratically speeds by, as compared to shooting him from the back seat of the getaway car. In the latter case, your reference frame is the same as his, making your objective quite easy. In the same way, the uncoordinated loops of the sinewave amplifier attempt to pump current in and out of the phases as they see the rotor move past, hopelessly trying to hit the magnetic vector target which is constantly moving. (By the time they "shoot" at it, it has moved). The closed-loop vector controller, however, has at its input, currents already "de-rotated" into the magnetic vector referred to the rotor. Therefore the torque controller sees the magnetic vector (and thus the real torque) as if the controller was sitting on top of the spinning rotor. This, and the re-rotation of the controller’s voltage outputs, essentially removes the moving target effect; putting the controller "in the back seat". So the SSt servo drive’s closed-loop vector torque control is extremely accurate and swift under all conditions of speed, acceleration, deceleration, rate of change of torque demand, etc.
Again, let’s re-emphasize, even if the torque calculated by the position/velocity compensator is perfectly accurate, the servo control will be inferior if the actual torque at the motor shaft is not swiftly and accurately produced. The SSt servo drive not only calculates the optimum torque command, but also quickly and accurately produces it at the motor shaft.
Direct Torque Control
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.
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 from the reference more than allowed 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 control platform
Direct Torque Control
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.
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 from the reference more than allowed 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 control platform
| VFD |
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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:
| 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 for continuous braking | Good with position or speed sensor |
| Controlled variables | torque & stator flux | rotor flux, torque current iq & rotor flux current id vector components |
| Steady-state torque/current/flux ripple & distortion | Low (requires high quality current sensors) | Low |
| Parameter sensitivity, sensorless | Stator resistance | d, q inductances, rotor resistance |
| Parameter sensitivity, closed-loop | d, q inductances, flux (near zero 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 frequency | Varies widely around average frequency | Constant |
| Switching losses | Lower (requires high quality current sensors) | Low |
| Audible noise | spread spectrum sizzling noise | 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 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 and in Germany, 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 and in an IEEE paper published in late 1986. 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. (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.
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 and the 1995 launch of the ACS600 drives family. ACS600 drives has since been replaced by ACS800 drives. Vas, Nash and Tiitinen provide a good treatment of ACS600 and DTC.
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.
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 and in Germany, 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 and in an IEEE paper published in late 1986. 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. (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.
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 and the 1995 launch of the ACS600 drives family. ACS600 drives has since been replaced by ACS800 drives. Vas, Nash and Tiitinen provide a good treatment of ACS600 and DTC.
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.
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.
DTC techniques for the interior permanent magnet synchronous machine (IPMSM) were introduced in the late 1990s.
DTC was applied to doubly fed machine control in the early 2000s, doubly-fed generators now commonly being used in 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.
During the 2000s several papers have been published about DTC including in terms of space vector modulation, which offers constant switching frequency, and DTC drive compatibility of synchronous reluctance motors (SynRM) in addition to induction machines (IM) and permanent magnet machines (PMM).
In light of the mid-2000s expiration of Depenbrock's key DTC patents, it is to be expected that companies other than ABB will develop DTC-controlled drives.
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