Field-oriented Control - Application Recap

Application Recap

1. Stator phase currents are measured, converted to complex space vector in (a,b,c) coordinate system.

2. Current vector is converted to (, ) coordinate system. transformed to a coordinate system rotating in rotor reference frame, rotor position being derived by integrating the speed by means of speed measurement sensor.

3. Rotor flux linkage vector is estimated by multiplying the stator current vector with magnetizing inductance L_m and low-pass filtering the result with the rotor no-load time constant L_r/R_r, namely, the rotor inductance to rotor resistance ratio.

4. Current vector is converted to (d,q) coordinate system.

5. d-axis component of the stator current vector is used to control the rotor flux linkage and the imaginary q-axis component is used to control the motor torque. While PI controllers can be used to control these currents, bang-bang type current control provides better dynamic performance.

6. PI controllers provide (d,q) coordinate voltage components. A decoupling term is sometimes added to the controller output to improve control performance to mitigate cross coupling or big and rapid changes in speed, current and flux linkage. PI-controller also sometimes need low-pass filtering at the input or output to prevent the current ripple due to transistor switching from being amplified excessively and destabilizing the control. However, such filtering also limits the dynamic control system performance. High switching frequency (typically more than 10 kHz) is typically required to minimize filtering requirements for high-performance drives such as servo drives.

7. Voltage components are transformed from (d,q) coordinate system to (, ) coordinate system.

8. Voltage components are transformed from (, ) coordinate system to (a,b,c) coordinate system or fed in Pulse Width Modulation (PWM) modulator, or both, for signaling to the power inverter section.

Significant aspects of vector control application:

• 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.