Field-oriented Control - Technical Overview

Technical Overview

Overview of key competing VFD control platforms:

While the analysis of AC drive controls can be technically quite involved (refer to "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 motor 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 motor 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 a & b 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 mimimum 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 signals are directly calculated using so-called voltage or current models. In IFOC, flux space angle feedforward and flux magnitude signals first measure stator currents and rotor speed for then deriving flux space angle proper by summing the rotor angle corresponding to the rotor speed and the calculated reference value of slip angle corresponding to the slip frequency.

Sensorless control (see Sensorless FOC Block Diagram) of AC drives is attractive for cost and reliability considerations. Sensorless control requires derivation of rotor speed information from measured stator voltage and currents in combination with open-loop estimators or closed-loop observers.