Direct self-regulation
The term direct self-regulation ( DSR ) is understood in converter technology , especially in electrical drive technology , to be a regulation method for the electronic control of induction machines , especially asynchronous machines .
The direct self-regulation is a form of the direct regulation, because it regulates the parameters machine flow and torque directly and separately from each other - in contrast to the field-oriented regulation , in which the motor speed is always the focus of the regulation.
motivation
As with any power converter, the output variables are formed from a constant intermediate circuit variable ( current or voltage ) by rapidly switching semiconductor switches . However, especially in the case of high-performance converters with intermediate circuit voltages of several kilovolts , the maximum permissible switching frequency of the semiconductor switches is severely limited. Furthermore, smoothing capacitors for high voltages are large and expensive, which is why only small capacitances are usually used. The intermediate circuit voltage can no longer be regarded as constant.
The consequence of this is that conventional modulation methods such as space vector modulation cannot be used optimally for such converters.
functionality
For the electronic control of induction machines, three half bridges are generally required in order to be able to impress a continuous current in every phase of the machine. As with space vector modulation, this results in six switch positions, each representing an active voltage space vector and two passive zero voltage space vectors.
The basic principle of direct self-regulation is to use the six active voltage space vectors to guide the stator flux of the asynchronous machines on a defined trajectory. By connecting a voltage space vector to the machine, a corresponding stator flux space vector is set. If the voltage space vector is now continuously changed chronologically (lead on a hexagonal trajectory), the stator flux space vector rotates accordingly accordingly. This defines the magnetization state of the asynchronous machines. The torque of the machine is determined by the speed at which the stator flux space vector moves on this trajectory, which in turn depends on the magnitude of the voltage space vector. With the two remaining zero-voltage space vectors, the absolute value of the resulting space vector can be determined by cyclically switching to one of these space vectors.
With conventional types of modulation, these voltage space vectors are now switched to the machine cyclically after a specified time. With direct self-regulation, however, the switching of the voltage space vector is now determined by the stator flux space vector. If the component of the respective β-axis (depending on the current switch position) of the stator flux space vector exceeds a certain amount, the system switches to the next voltage space vector. As a result, the flow of the machine dictates the switching of the voltage space vector. A hysteresis controller (flux controller) compares the stator flux with the specified desired flux. So there is no modulator that realizes a predetermined (switching) frequency, as is the case with e.g. B. is the case with vector control .
At the same time, a further hysteresis controller (torque controller) compares the current torque of the machine with a desired specified value and, if necessary, switches one of the two zero-voltage space vectors to the machine instead of the voltage space vector selected by the flow controller.
Since the half bridges only have to be switched as soon as a deviation occurs, the switching frequency can be significantly reduced in contrast to conventional modulation methods, where switching is permanent. The switching frequency therefore depends largely on the machine itself. In addition, another controller can be used that changes the hysteresis of the torque controller depending on the current switching frequency. In this way, optimum torque control is always achieved at a given maximum switching frequency.
Knowledge of the current machine flow and the instantaneous value of the torque is decisive for the function of the direct self-regulation. In order to obtain these parameters, a suitable mathematical model of the machine is required, which the signal processor uses to calculate the desired values using measured parameters such as stator current and stator voltage.
application
The direct self-regulation is mainly used in electric traction vehicles , since here, in addition to the high voltage, considerable voltage fluctuations can occur due to brief interruptions in the connection between pantograph and overhead line . Since the direct self-regulation is extremely robust against fluctuating intermediate circuit voltage, this regulation method is particularly suitable.
literature
- Dierk Schröder: Electrical drives - control of drive systems 3rd edition, Springer Berlin Heidelberg, Berlin, Heidelberg, 2009, ISBN 978-3-540-89612-8
- Felix Jenni, Dieter Wüest: Control procedure for self-commutated converters 1st edition, BG Teubner, Stuttgart, 1995, ISBN 3-519-06176-7