Vector Control of Induction Motor: Technical Overview and Application

Vector Control of Induction Motor is called also (FOC) field-oriented control. It is (VFD) variable frequency drive controlling way. In this stator current of 3 phase AC electric motor is identified as 2 components orthogonally being visualized with vectors. 1 component defines motor’s magnetic flux. Other component defines torque. Control systems of drive calculate from torque and flux reference. This is provided by speed controls of drives which correspond to references of current components. (PI) Proportional-integral controllers typically have been used for keeping current measured components at reference values. Pulse width modulations of variable frequency drives thereby defines transistor switching as per stator reference voltage which are output of PI current controller. FOC has been used for controlling induction motors and AC synchronous motors. It was developed originally for high performance motor applications. It is needed for smooth operation over full speeded ranges, generating full torques at zero speeds and having high dynamic performances. This includes deceleration and fast acceleration. It is, however, increasingly becoming attractive for low performance applications. It is due to FOC’s power consumption, cost and motor size superiority reduction. It has been expected that together with increased computational microprocessor powers it shall nearly eventually universally displace scalar single variable volts-per-hertz (V/f) controls.

Overview technically: Whilst analysis of AC driving controls is technically involved, such analysis starts invariably with modelling of drive motor circuits being involved through lines of signal flow graphs and equation accompaniment. In vector controls of Induction Motor the motor have been controlled in operating conditions like separate excited DC motors. AC motors behave like DC motors where armature flux linkage and field flux linkage are created by respective fields. Armature currents or torque components are aligned orthogonally. When torque has been controlled, field flux linkages are not affected. This enables dynamic torque responses. Vector controls generate accordingly 3 phase PWM motor output voltages being derived from complex voltage vectors for controlling complex current vectors. This is derived from induction motor’s 3 phase stator current inputs by means of rotations or projections forth and back. This is in between 3 phase time and speed dependent systems. There are vectors rotating reference frames 2 coordinate time invariant system. Complex stator space current vectors of this kind are defined in (d, q) coordinate systems with component orthogonally through q (quadrature) and d (direct) axes. Field flux linkages current components are aligned along d axis. Torque current components are aligned along q axis. Induction motor’s (d, q) coordinate systems are superimposed to instantaneous (a, b, c) 3 phase sinusoidal system of motor. Components of (d, q) system current vectors in turn allow control conventionally like PI control or proportional and integral motor, as with DC motors. There are 2 vector controlling ways: Direct or Feedback vector control (DFOC) and Indirect or Feedforward vector control (IFOC). More commonly being used is IFOC, since in closed loop modes; drives have very easy operation throughout speed ranging from 0 speeds to high speed field weakening. In DFOC, angle feedback and flux magnitude signals are calculated directly by using so called current or voltage models. In IFOC angle feedforward flux magnitude and flux space signals 1st measure stator rotor speeds and currents. It is then deriving of flux spaces angling proper by summing rotor angles. This corresponds to rotor speeds and reference value calculated of slip angles which correspond to slip frequencies.

Application: Phase stator currents are measured and converted to complex space vectors in (a, b, c) coordinate systems. Current vectors are converted to (alpha, beta) coordinate systems.

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