# What's the point of vector control for ac machines?

for what I understood vector control controls the rotor's flux and the torque (both in magnitude) by changing the stator currents in the qd0 reference frame.

let's say we are dealing with direct vector control for simplicity:

in DC machines the rotor magnetic field is always orthogonal to the stator, and this is the goal in PM machines control too (to maximise the torque with using the minimum current, basically the Max torque per Amp problem).

Is this the goal here too? One thing I don't really get is how the flux has a phase if it's a scalar (and this phase seems to be on the D axis, so the same as the rotor's magnetic field), but maybe is an abuse of notation?

I believe my question could be translated in: given a desired torque and speed, will the control drive the rotor flux to be orthogonal (if it even makes sense for fluxes) to the stator's, or this may not be the case since the goal of this type of control is another?

## 1 Answer

Your question seems to come down to this: Why would we want to control the rotor flux?

Leaving it as normal (at maximum flux and running with it orthogonal) would give maximum efficiency, but maximum flux also gives you maximum back-EMF. Without vector control, your speed will be limited to where your BEMF approaches your rail voltage.

Vector control allows you to do "field weakening". By reducing the strength of the flux, you also reduce the BEMF and can then run the motor faster than without vector control.

There are other advantages as well, but I don't understand them all.

• this is a good answer. do you know by any chance if the stator and rotor magnetic fields are always orthogonal even with no control,when the motor is running? thinking about it it makes sense, but I'm not really sure. – user3149593 Jul 12 '16 at 23:13
• My understanding is that it is the deviation from orthogonal that allows you to weaken the flux. – Mark Jul 13 '16 at 1:07