Direct Torque Control with Space Vector Modulation (DTC-SVM) of ...

Introduction
vector control, which is based on relation valid for dynamics states, not just magnitude and
frequency (angular speed), but also instantaneous position of voltage, current and flux space
vectors are controlled. Thus, the control system adjust the position of the space vectors and
guarantee their correct orientation for both steady states and transients.
The scalar constant V/Hz control for PMSM without damper winding (squire cage) is not
simple as for induction motor. It requires additional stabilization control loop, which can be
provide by feedback from: rotor velocity perturbation, active power or DC-link current
perturbation [9].
The most popular vector control method developed in 70s, known as field oriented control
(FOC) [31] gives the permanent magnet synchronous motor high performance. In this method
the motor equation are transformed in a coordinate system that rotates in synchronism with
permanent magnet flux. It allows separately and indirectly control flux and torque quantities
by using current control loop with PI controllers like in well known DC machine control [3].
In search of a simpler and more robust high performance control system in 80s new vector
control called direct torque control (DTC) was developed [50]. It was innovative studies at
this time and completely different approach which depart from the idea of coordinate
transformation and the analogy with DC motor control. It allows direct control flux and torque
quantities without inner current control loops. Using bang-bang hysteresis controllers for flux
and torque control loops made this control concept very fast and not complicated. However,
the main disadvantage of DTC is fast sampling time required and variable switching
frequency, because of hysteresis based control loops. In order to eliminate above
disadvantages and kept basic control rules of classical DTC, at the beginning of 90’s a new
developed control technique called direct torque control with space vector modulator (DTC-
SVM) has been introduced [54,55]. However, from the formal consideration this method can
also be viewed as stator flux oriented control (SFOC). This control employed instead of
hysteresis controller as for classical DTC, the PI controllers and space vector modulator
(SVM). It allows to achieve fixed switching frequency, what considerably reduce switching
losses as well as torque and current ripples. Also requirement of very fast sampling time is
eliminated [113,115,117]. Therefore, this new method is subject of this thesis. In spite of
many control strategies there is no one which may be considered as standard solution.
5