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Vector control (motor)

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Vector control, also called field-oriented control (FOC), is a variable frequency drive (VFD) control method which controls three-phase AC electric motor output by means of three controllable VFD inverter output variables:[1][2]

  • Voltage magnitude
  • Voltage angle
  • Frequency.

FOC or vector control and its closely related flux vector control method[3] are math-intensive control techniques used in brushless DC and AC induction motor applications[3]developed for high-performance applications that have the potential for reducing motor's size, cost and power consumption for lower performance applications.[4]

Not only is FOC very common in induction motor control applications due to its traditional superiority in high-performance applications but the expectation is that it will eventually nearly universally displace single-variable, scalar V/Hz control.[5][6]

Indirect field-oriented control (IFOC) is a variant of FOC that measures the field position within the induction motor indirectly via slip calculations using a mathematical model of the motor (rather than measuring the field directly).

Development history

Block diagram from Blaschke's 1971 US patent application

Technical University Darmstadt's K. Hasse and Siemens' F. Blaschke pioneered vector control of AC motors starting in 1968 and into the early 1970s, Hasse in terms of proposing indirect vector control, Blaschke in terms of proposing direct vector control.[7][2], Technical University Braunschweig's Werner Leonhard further developed FOC techniques and was instrumental in opening up opportunities for AC drives to be a competitive alternative to DC drives.[8][9]

Yet it was not until sometime after the commercialization of microprocessors, that is in the early 1980s, that general purpose AC drives became available.[10][11] The barriers to use of FOC for AC drive applications included higher cost and complexity and lower maintainability compared to DC drives, FOC having until then required many electronic components in terms of sensors, amplifiers and so on..[12]

Technical overview

The stator current of an induction motor can be broken down into the torque and field component currents. When decoupled, this means the torque and field of a motor can be controlled independently using these current components. Using the stator's axis as a point of origin, the torque and field currents can be related to the stator in a way allowing for control algorithm development.[13]

For DC motors, the field and torque currents are equivalent to the field and armature currents and can be controlled that way, while AC motors require a microprocessor to calculate the corresponding currents.[13]

Inverters can be implemented as open-loop sensorless FOC 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.[8]


Indirect field-oriented control

Method

The stator phase currents are measured and converted into a corresponding complex (space) vector. This current vector is then transformed to a coordinate system rotating with the rotor of the machine. For this the rotor position has to be known. Thus at least speed measurement is required, the position can then be obtained by integrating the speed.

Then the rotor flux linkage vector is estimated by multiplying the stator current vector with magnetizing inductance Lm and low-pass filtering the result with the rotor no-load time constant Lr/Rr, that is the ratio of the rotor inductance to rotor resistance.

Using this rotor flux linkage vector the stator current vector is further transformed into a coordinate system where the real x-axis is aligned with the rotor flux linkage vector.

Now the real x-axis component of the stator current vector in this rotor flux oriented coordinate system can be used to control the rotor flux linkage and the imaginary y-axis component can be used to control the motor torque.

Typically PI-controllers are used to control these currents to their reference values. However, bang-bang type current control, that gives better dynamics, is also possible.

With PI-controllers the outputs of the controllers are the x-y components of the voltage reference vector for the stator. Usually due to the cross coupling between the x- and y-axes a decoupling term is further added to the controller output to improve control performance when big and rapid changes in speed, current and flux linkage occur. Usually the PI-controller also needs low-pass filtering of either the input or output of the controller to prevent the current ripple due to transistor switching from being amplified excessively and unstabilizing the control. Unfortunately, the filtering also limits the dynamics of the control system. Thus quite high switching frequency (typically more than 10 kHz) is required to allow only minimum filtering for high performance drives such as servo drives.

Next the voltage references are first transformed to the stationary coordinate system (usually through rotor d-q coordinates) and then fed into a modulator that using one of the many Pulse Width Modulation (PWM) algorithms defines the required pulse widths of the stator phase voltages and controls the transistors (usually IGBTs) of the inverter according to these.

This control method implies the following properties of the control:

  • Speed or position measurement or some sort of estimation is needed
  • Torque and flux can be changed reasonably fast, in less than 5-10 milliseconds, by changing the references
  • The step response has some overshoot if PI control is used
  • The switching frequency of the transistors is usually constant and set by the modulator
  • The accuracy of the torque depends on the accuracy of the motor parameters used in the control. Thus large errors due to for example rotor temperature changes often are encountered.
  • Reasonable processor performance is required, typically the control algorithm has to be calculated at least every millisecond.

Although the vector control algorithm is more complicated than the Direct Torque Control (DTC), the algorithm is not needed to be calculated as frequently as the DTC algorithm. Also the current sensors need not be the best in the market. Thus the cost of the processor and other control hardware is lower making it suitable for applications where the ultimate performance of DTC is not required.

References

  1. ^ Zambada, Jorge (Nov. 8, 2007). "Field-oriented control for motors". MachineDesign.com. {{cite web}}: Check date values in: |date= (help)
  2. ^ a b Yano, Masao; et al. "History of Power Electronics for Motor Drives in Japan" (PDF). p. 6, Fig 13. Retrieved 18 April 2012. {{cite web}}: Explicit use of et al. in: |first= (help)
  3. ^ a b Lewin, Chuck (April 10, 2006). "New Developments in Commutation and Motor Control Techniques". DesignNews.com.
  4. ^ Godbole, Kedar (Sept 23, 2006). "Field oriented control reduces motor size, cost and power consumption in industrial applications". Texas Instruments. {{cite web}}: Check date values in: |date= (help)
  5. ^ Bose, Bimal K. (June 2009). "The Past, Present, and Future of Power Electronics". Industrial Electronics Magazine, IEEE. 3 (2): 11. doi:10.1109/MIE.2009.932709.
  6. ^ Murray, Aengus (Sept. 27, 2007). "Transforming motion: Field-oriented control of ac motors". EDN. Retrieved 11 May 2012. {{cite web}}: Check date values in: |date= (help)
  7. ^ Rafiq, Md Abdur (2006). "Fast Speed Response Field-Orientation Control of Induction Motor Drive with Adaptive Neural Integator". Journal of Electrical and Electronics Engineering. 6 (2). University of Istanbul: 229.
  8. ^ a b Drury, Bill (2009). The Control Techniques Drives and Controls Handbook (2nd ed.). Stevenage, Herts, UK: Institution of Engineering and Technology. p. xxx. ISBN 978-1-84919-101-2. Cite error: The named reference "Drury (2009)" was defined multiple times with different content (see the help page).
  9. ^ Bose, Bimal K. (2006). Power Electronics and Motor Drives : Advances and Trends. Amsterdam: Academic. p. 22. ISBN 978-0-12-088405-6.
  10. ^ "The Development of Vector Control Drive".
  11. ^ Bose (2006), p. 605
  12. ^ Gabriel, R. (March/April 1980). "Field Oriented Control of Standard AC Motors Using Microprocessors". Trans. on Industry Applications. IA-16 (2): 188. {{cite journal}}: Check date values in: |date= (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)
  13. ^ a b Sinha, Naresh Kumar (1986). Microprocessor-based control systems. D. Reidel Publishing. pp. 161 & 175. ISBN 90-277-2287-0.

See also

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