Direct torque control

The term direct self-regulation (DSR ) is understood in the power converter technology, especially in electrical drive technology, a control method for electronic control of induction machines, induction machines specifically.

The direct self-regulation is a form of direct regulation because they are the sizes of the machine flux and torque directly and separately regulates each other - in contrast to the field-oriented control, in which the engine speed is always at the center of the scheme.

Motivation

As with any converter, the shape of the output variables is formed by rapid switching of semiconductor switches from a constant DC link variable (current or voltage). Especially in high-power converters with DC link voltages of several kilovolts the maximum switching frequency of the semiconductor switch is severely limited. More are smoothing capacitors for high voltage large and expensive, why are usually used only small capacities. The DC link voltage can thus no longer be regarded as constant.

This has the consequence that can not be conventional modulation schemes such as space vector modulation for such power converters optimum use.

Operation

For the electronic control of rotating field machine three half-bridges are generally required to memorize in each phase of the machine a continuous current. Thus arise, as in the case of the space vector modulation, six switch positions which each represent an active voltage space vector and two passive zero voltage space vector.

The basic principle of direct self control now is to lead using the six active voltage space vector of the stator flux of the induction motors on a defined trajectory. By injecting a voltage space vector to the machine itself an adequate over Statorflussraumzeiger sets. If now the voltage space vector continuously changed chronologically ( lead on a hexagonal trajectory ), the Statorflussraumzeiger turns consequently after accordingly. Thus, the magnetization state of the asynchronous machines is fixed. The torque of the engine is determined by the speed at which Statorflussraumzeiger moves on this trajectory, which in turn depends on the magnitude of the voltage space vector. With the remaining two zero voltage space vectors can be determined by cyclic switching to one of these space vector of the magnitude of the resultant space vector.

In conventional modulation types, this voltage space vector now be cycled after a specified time to the machine. For direct self- control the indexing of voltage space vector is, however, now determined by Statorflussraumzeiger. Exceeds the component of the respective β - axis ( depending on the current switch position ) of the Statorflussraumzeigers a certain amount, it changes to the next voltage space vector. This therefore gives the flux of the machine before the indexing of the voltage space vector. A hysteresis controller ( flow regulator ) compares this to the stator with the set desired flow. Thus, there is no modulator realizes a predetermined (switching) frequency, as for example for vector control in the case.

In parallel, compares another hysteresis (torque control) the actual torque of the machine with a desired predetermined value, and switches when needed instead of the voltage space vector chosen by the flow regulator one of the two zero voltage space vector to the machine.

Because the half-bridges only have to be switched so as soon as a deviation occurs, the switching frequency, in contrast to conventional modulation method is switched where permanent, are significantly reduced. The switching frequency thus depends primarily on the machine itself. In addition, another controller can be used that changes the hysteresis of the torque controller in dependence on the current operating frequency. Thus, for a given maximum switching frequency optimal control of the torque is always achieved.

With respect to the function of direct self control therefore is the knowledge of the current machine flow as well as the instantaneous value of the torque. To obtain these parameters, a suitable mathematical model of the machine is required, by means of which the signal processor by using the measured parameters, such as stator current and stator voltage, calculates the required sizes.

Application

Common applications for the direct self-regulation mainly in electric drive vehicles, since considerable voltage fluctuations due to temporary connection interruptions can occur between pantograph and overhead contact line adjacent to the high voltage. Since the direct self-regulation is extremely robust against fluctuating DC voltage, this control method is particularly appropriate.