Vector Control Frequency Inverters (Adjustable Speed Drives, AFDs) (part 3)
Continued from the previous two articles: Vector Control Frequency Inverters (Adjustable Speed Drives, AFDs) (part 1) , Vector Control Frequency Inverters (Adjustable Speed Drives, AFDs) (part 2) , we still introduce vector control for frequency converters (frequency changers, ac drives).
One common technique for estimating the rotor flux is based on the slip relation. This technique requires the measurement of the rotor position and the stator current. With current and position sensors, this method performs reasonably well over the whole speed range. The most high-performance variable speed drives (VSDs, vvvf drives) in operation today employ indirect field orientation based on the slip relation.
The main disadvantage of this method is the need of the rotor position information, using the shaft mounted encoder. This means additional wiring and component cost. It increases the size of the motor. When the ac drive (VSD) and the motor are far apart, the additional wiring poses a challenge.
To overcome the sensor/encoder problem, today's main research focus is in the area of a sensorless approach. The advantages of the vector control are to better the torque response, compared to the scalar control (V/F control), full-load torque close to zero speed, accurate speed control and performance approaching direct current (DC) drive, among others. But this requires a complex algorithm for speed calculation in real-time. Due to feedback devices, vector control becomes costly compared to the scalar control (v/f control).
