Vector control (motor)
Vector control is a control concept in which sinusoidal - or as a largely sinusoidal adopted - alternating quantities ( for example, alternating voltages and alternating currents ) are not regulated directly in their temporal instantaneous value, but in a by the phase angle within the period adjusted instantaneous value. For this purpose, the detected change quantities are transmitted in a rotating with the frequency of the alternating sizes coordinate system. Within the rotating coordinate system are then obtained from the alternating quantities equal sizes, can be applied to all the usual methods of control engineering.
General
For practical reasons, always a standing with two perpendicular axes d and q is practically chosen in the control of electrical parameters for the rotating coordinate system. This has the advantage that it flows and the space vector representation (english space vector) of AC voltages and their relation identical to each other, which can be used the corresponding models of electrical machines directly.
By this vector control, which is called with respect to the rotating field of an electrical machine also field-oriented control, reached a frequency inverter for electric motors an extended speed and positioning accuracy compared to a system that provides filtered only through a low pass effective values of currents and voltages of one or even several period ( n ) is used.
The application of vector control is not limited to actuators, even for converters to be fed into networks, the principle is applicable. Here, frequency and phase can be defined either from the network, they will then, as with the drive control also measured, or the power converter are, for example, in an off-grid, these variables even before.
Vector control is the limit of its applicability, if to be controlled, the variables to be measured, and are no longer sufficiently sinusoidal. In this case, the transmission in the rotating coordinate system to eliminate only the influence of the fundamental, the harmonics remain, and can not be distinguished from application of disturbance by the controller. For practical applications in drive technology, but the filtering of harmonics is usually enough. For applications for converters that do not act on mechanical drives or not supported by this, but for example turn to other converters work directly (eg battery power converter in an isolated power supply 230V ~ compact fluorescent lamps ), but this must be considered.
Vector control for synchronous machines
Are stator flux and stator current in the rotating DQ field in a synchronous motor in parallel, so the torque is zero. The rotor- related d / q- system is calculated using the Clarke transformation, followed by d / q transformation ( Park transformation ) from the three-phase stator- system. The D-and Q vectors are orthogonal, the Q value is the torque, and the d value of the magnetic flux density decreases, and can be modeled similar to a direct-current machine with a PI controller. By externally predetermined reference value q, the torque of the engine can be influenced. When the rotor permanent-magnet synchronous motors, typically this is in the so-called brushless DC (BLDC) motors, the d- reference value in the basic speed range zero, provided that the d-and q- inductances are equal, so there is no Reluktanzbeitrag to the torque. In the field weakening range, a negative d- reference value is used to limit the induced voltage. With an inverse transformation and subsequent space vector modulation (English Space Vector PWM) the control signals for the three-phase four-quadrant running are formed.
For regulation of the space vector at a right angle, a control circuit is necessary which indicates the position of the magnet wheel with a feedback to the machine. This feedback was implemented in synchronous machines usually with three Hall sensors. However, since these are very error-prone and expensive, mostly encoder (resolver, optical incremental and absolute encoder or inductive sensors) are used nowadays. Sensorless controllers can be implemented with block commutation by returning measuring the induced voltage in the engine back. However, this feedback has increasing disadvantages, especially at low speeds.
Another way to operate a synchronous machine for sensorless vector control means, based on a mathematical calculation of the required control parameters. The main factor here is a realistic and accurate as possible software model of the machine. A digital signal processor "estimates " using this machine model required to control parameters such as the rotor angle and speed. The only required in this method, measurements are the three stator of the machine. Since the neutral point of the machine (if it exists) is not commonly connected, the measurement of two stator currents, which can be necessarily indicative of the third stator is sufficient.
Vector control for induction motors
Vector control of induction machines was invented in 1968 by K. Hasse at the TH Darmstadt and independently also in the PhD thesis of Felix Blaschke 1973 at the Technical University of Braunschweig, entitled described " The method of the field orientation to regulate the induction machine ".
In the approach of field-oriented control has been recognized that the magnetic air gap field is crucial for the performance of the asynchronous machine. The magnetizing current must be kept independent of the speed constant in the case of the basic speed range here.
Asynchronous machines can also be controlled to improve performance with vector modulated frequency. In this case the active and reactive current Iw and Iu are interesting. The reactive current provides a magnetization of the stator, the effective current for the torque. Since the reactance X of the stand with the frequency changes, the ohmic resistance R remains constant with frequency changes, the V / f characteristic is not really linear in an asynchronous motor. In particular at low frequencies, the voltage drop across R is such that no full reactive current to flow through more X and the machine loses in torque due to insufficient magnetization. This is also the reason why one can compensate very small velocities with asynchronous only bad and the asynchronous motor for accurate positioning is inappropriate.
To compensate for these disadvantages at low speeds, offer many frequency converters, which operate according to the V / f characteristic curve, for small velocities a "boost " mode on, in which the R to the stator voltage drop is added to the grid. However, such a fixed boost factors are merely a compromise for a medium output torque, since the torque also causes an active current. This active current has just such a voltage drop in the equivalent circuit of the stator R being the reason why the desired reactive power is not optimal in turn. Event of a faulty reactive current so that the torque is either too small, or the machine has to implement a high iron loss in heat in case of magnetization. In addition, the effective resistance R of the engine changes when heated to a particularly low speed by no more negligible value.
At this point engages the vector modulation and leads the rotating field vector in a closed control loop according to which, ideally, take into account any interference of the system. In practice, this zero almost ideal servo characteristics can be achieved with asynchronous machines for positioning up to speed. In different frequency, it is state of the art, that the regulation of vector modulation is adaptive. Unknown machine models themselves are learned and automatically readjusted application-specific load steps.