04-07-2013, 04:23 PM
Vector Control of Induction Machines
Vector Control.ppt (Size: 1.08 MB / Downloads: 423)
Introduction
The traditional way to control the speed of induction motors is the V/Hz-control
Low dynamic performance
In applications like servo drives and rolling mills quick torque response is required.
Desire to replace dc drives led to vector control
Braunschweig, Leonhard, Blaschke, Hasse,late 70-ies
What is vector control?
Vector control implies that an ac motor is forced to behave dynamically as a dc motor by the use of feedback control.
Always consider the stator frequency to be a variable quantity.
Think in synchronous coordinates.
Rotor flux orientation
Difficult to find the transformation angle since the direction of the flux must be known
Flux measurement is required
Flux sensors (and fitting) are expensive and unreliable
Rotor position measurement does not tell the flux position
The solution is flux estimation
Remarks on indirect field orientation
Does not directly involve flux estimation (superscript f dropped)
Not ”flux coordinates” but ”synchronous coordinates”
Since the slip relation is used instead of flux estimation, the method is called indirect field orientation
Hysteresis control(Tolerance band control)
Measure each line current and subtract from the reference. The result is fed to a comparator with hysteresis.
Pulse width modulation is achieved directly by the current control
The switching frequency is chosen by means of the width of the tolerance band.
No tuning is required.
Very quick response
Drawbacks of hysteresis control
The switching frequency is not constant.
The actual tolerance band is twice the chosen one.
Sometimes a series of fast switchings occur.
Suitable for analog implementation. Digital implementation requires a very high sampling frequency.
Realistic choice of bandwidth
Care must be taken that the bandwidth of the speed controller is not unnecessarily high.
In fact this should be decided during the first steps in the design process of a drive system
The bandwidth is directly connected to the current rating of the inverter.