page_title: Vector control (motor)
section_title: Technical overview
hierarchical_section_title: Vector control (motor) / Technical overview
caption_attribution_description: English: Sensorless vector control block diagram http://www.appliancemagazine.com/editorial.php?article=1571&amp;zone=1&amp;first=1 http://www.ema.uni-wuppertal.de/paper/upload/paper_36.pdf
context_page_description: Vector control, also called field-oriented control, is a variable-frequency drive control method in which the stator currents of a three-phase AC electric motor are identified as two orthogonal components that can be visualized with a vector. One component defines the magnetic flux of the motor, the other the torque. The control system of the drive calculates the corresponding current component references from the flux and torque references given by the drive's speed control. Typically proportional-integral controllers are used to keep the measured current components at their reference values. The pulse-width modulation of the variable-frequency drive defines the transistor switching according to the stator voltage references that are the output of the PI current controllers.
  FOC is used to control AC synchronous and induction motors. It was originally developed for high-performance motor applications that are required to operate smoothly over the full speed range, generate full torque at zero speed, and have high dynamic performance including fast acceleration and deceleration.
context_section_description: Overview of key competing VFD control platforms:
   
  While the analysis of AC drive controls can be technically quite involved ("See also" section), such analysis invariably starts with modeling of the drive-motor circuit involved along the lines of accompanying signal flow graph and equations.
  In vector control, an AC induction or synchronous motor is controlled under all operating conditions like a separately excited DC motor. That is, the AC motor behaves like a DC motor in which the field flux linkage and armature flux linkage created by the respective field and armature (or torque component) currents are orthogonally aligned such that, when torque is controlled, the field flux linkage is not affected, hence enabling dynamic torque response.
  Vector control accordingly generates a three-phase PWM motor voltage output derived from a complex voltage vector to control a complex current vector derived from motor's three-phase stator current input through projections or rotations back and forth between the three-phase speed and time dependent system and these vectors' rotating reference-frame two-coordinate time invariant system.
  Such complex stator current space vector can be defined in a (d,q) coordinate system with orthogonal components along d (direct) and q (quadrature) axes such that field flux linkage component of current is aligned along the d axis and torque component of current is aligned along the q axis. The induction motor's (d,q) coordinate system can be superimposed to the motor's instantaneous (a,b,c) three-phase sinusoidal system as shown in accompanying image (phases b & c not shown for clarity). Components of the (d,q) system current vector allow conventional control such as proportional and integral, or PI, control, as with a DC motor.
  Projections associated with the (d,q) coordinate system typically involve:
  Forward projection from instantaneous currents to (a,b,c) complex stator current space vector representation of the three-phase sinusoidal system.
  Forward three-to-two phase, (a,b,c)-to-(,) projection using the Clarke transformation. Vector control implementations usually assume ungrounded motor with balanced three-phase currents such that only two motor current phases need to be sensed. Also, backward two-to-three phase, (,)-to-(a,b,c) projection uses space vector PWM modulator or inverse Clarke transformation and one of the other PWM modulators.
  Forward and backward two-to-two phase,(,)-to-(d,q) and (d,q)-to-(,) projections using the Park and inverse Park transformations, respectively.
  However, it is not uncommon for sources to use three-to-two, (a,b,c)-to-(d,q) and inverse projections.
  While (d,q) coordinate system rotation can arbitrarily be set to any speed, there are three preferred speeds or reference frames:
  Stationary reference frame where (d,q) coordinate system does not rotate;
  Synchronously rotating reference frame where (d,q) coordinate system rotates at synchronous speed;
  Rotor reference frame where (d,q) coordinate system rotates at rotor speed.
  Decoupled torque and field currents can thus be derived from raw stator current inputs for control algorithm development.
  Whereas magnetic field and torque components in DC motors can be operated relatively simply by separately controlling the respective field and armature currents, economical control of AC motors in variable speed application has required development of microprocessor-based controls with all AC drives now using powerful DSP (digital signal processing) technology.
  Inverters can be implemented as either open-loop sensorless or closed-loop FOC, the key limitation of open-loop operation being minimum speed possible at 100% torque, namely, about 0.8 Hz compared to standstill for closed-loop operation.
  There are two vector control methods, direct or feedback vector control (DFOC) and indirect or feedforward vector control (IFOC), IFOC being more commonly used because in closed-loop mode such drives more easily operate throughout the speed range from zero speed to high-speed field-weakening. In DFOC, flux magnitude and angle feedback
caption: Sensorless FOC Block Diagram [19][20]
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