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-phaseAC 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. Method

Statorflux linkage is estimated by integrating the stator voltages. Torque is estimated as a cross product of estimated stator flux linkagevector and measured motor currentvector. 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 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:

?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


?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 and voltage measuring devices have to be high quality ones without noise and low-pass filtering, because noise and slow response ruins the hysteresis control ?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. 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.


Direct torque control was patented by Manfred Depenbrock in U.S. Patent 4,678,248 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 。

DTC has also been applied to three-phasegrid side converter control (U.S. Patent 5,940,286). Grid side converter is identical in structure to the transistorinverter 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!).

In the late 1990s DTC techniques for the Interior Permanent MagnetSynchronous Machine (IPMSM) appeared.

Further, in the beginning of 2000's DTC was applied to doubly fed machine control (U.S. Patent 6,448,735). Doubly fed generators are today commonly used in wind turbine applications.

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.





通过定子磁通估计与定子电压相结合。转矩为定子磁链估计矢量的叉积和测量电机电流矢量,估计磁通和转矩的大小,然后比较其参考价值。如果不是从参考更多流量或转矩的估计超过允许公差偏离,变频驱动晶体管处于关闭状态,并以这样一种方式,将磁链和磁通的公差带尽可能快的带回到标准值。因此,直接转矩控制是一种滞后或Bang – Bang的控制形式。



?高效率,低损失- 损失最小化的开关。因为只有当晶体管需要保持在其滞后带转矩和磁通内的滞后阶段的时候才会被改变










?数字控制设备的速度是非常快的,以便能够防止公差带偏离远通量和扭矩。通常情况下,控制算法要进行10 - 30微秒或更短的时间间隔。然而,由于该算法简





如果在包括零频率运行速度低,需要连续作业,速度或位置传感器可以被添加到DTC 系统。随着传感器的转矩和速度控制精度的提高,可以在整个范围内将速度保持下去。











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